* [PATCH v2 RFC 0/7] KFuzzTest: a new kernel fuzzing framework
@ 2025-09-01 16:42 Ethan Graham
2025-09-01 16:42 ` [PATCH v2 RFC 1/7] mm/kasan: implement kasan_poison_range Ethan Graham
` (6 more replies)
0 siblings, 7 replies; 15+ messages in thread
From: Ethan Graham @ 2025-09-01 16:42 UTC (permalink / raw)
To: ethangraham, glider
Cc: andreyknvl, brendan.higgins, davidgow, dvyukov, jannh, elver,
rmoar, shuah, tarasmadan, kasan-dev, kunit-dev, linux-kernel,
linux-mm, dhowells, lukas, ignat, herbert, davem, linux-crypto
From: Ethan Graham <ethangraham@google.com>
This patch series introduces KFuzzTest, a lightweight framework for
creating in-kernel fuzz targets for internal kernel functions.
The primary motivation for KFuzzTest is to simplify the fuzzing of
low-level, relatively stateless functions (e.g., data parsers, format
converters) that are difficult to exercise effectively from the syscall
boundary. It is intended for in-situ fuzzing of kernel code without
requiring that it be built as a separate userspace library or that its
dependencies be stubbed out. Using a simple macro-based API, developers
can add a new fuzz target with minimal boilerplate code.
The core design consists of three main parts:
1. A `FUZZ_TEST(name, struct_type)` macro that allows developers to
easily define a fuzz test.
2. A binary input format that allows a userspace fuzzer to serialize
complex, pointer-rich C structures into a single buffer.
3. Metadata for test targets, constraints, and annotations, which is
emitted into dedicated ELF sections to allow for discovery and
inspection by userspace tools. These are found in
".kfuzztest_{targets, constraints, annotations}".
To demonstrate this framework's viability, support for KFuzzTest has been
prototyped in a development fork of syzkaller, enabling coverage-guided
fuzzing. To validate its end-to-end effectiveness, we performed an
experiment by manually introducing an off-by-one buffer over-read into
pkcs7_parse_message, like so:
-ret = asn1_ber_decoder(&pkcs7_decoder, ctx, data, datalen);
+ret = asn1_ber_decoder(&pkcs7_decoder, ctx, data, datalen + 1);
A syzkaller instance fuzzing the new test_pkcs7_parse_message target
introduced in patch 7 successfully triggered the bug inside of
asn1_ber_decoder in under a 30 seconds from a cold start.
This RFC continues to seek feedback on the overall design of KFuzzTest
and the minor changes made in V2. We are particularly interested in
comments on:
- The ergonomics of the API for defining fuzz targets.
- The overall workflow and usability for a developer adding and running
a new in-kernel fuzz target.
- The high-level architecture.
The patch series is structured as follows:
- Patch 1 adds and exposes a new KASAN function needed by KFuzzTest.
- Patch 2 introduces the core KFuzzTest API and data structures.
- Patch 3 adds the runtime implementation for the framework.
- Patch 4 adds a tool for sending structured inputs into a fuzz target.
- Patch 5 adds documentation.
- Patch 6 provides example fuzz targets.
- Patch 7 defines fuzz targets for real kernel functions.
Changes in v2:
- Per feedback from Eric Biggers and Ignat Korchagin, move the /crypto
fuzz target samples into a new /crypto/tests directory to separate
them from the functional source code.
- Per feedback from David Gow and Marco Elver, add the kfuzztest-bridge
tool to generate structured inputs for fuzz targets. The tool can
populate parts of the input structure with data from a file, enabling
both simple randomized fuzzing (e.g, using /dev/urandom) and
targeted testing with file-based inputs.
We would like to thank David Gow for his detailed feedback regarding the
potential integration with KUnit. The v1 discussion highlighted three
potential paths: making KFuzzTests a special case of KUnit tests, sharing
implementation details in a common library, or keeping the frameworks
separate while ensuring API familiarity.
Following a productive conversation with David, we are moving forward
with the third option for now. While tighter integration is an
attractive long-term goal, we believe the most practical first step is
to establish KFuzzTest as a valuable, standalone framework. This avoids
premature abstraction (e.g., creating a shared library with only one
user) and allows KFuzzTest's design to stabilize based on its specific
focus: fuzzing with complex, structured inputs.
Ethan Graham (7):
mm/kasan: implement kasan_poison_range
kfuzztest: add user-facing API and data structures
kfuzztest: implement core module and input processing
tools: add kfuzztest-bridge utility
kfuzztest: add ReST documentation
kfuzztest: add KFuzzTest sample fuzz targets
crypto: implement KFuzzTest targets for PKCS7 and RSA parsing
Documentation/dev-tools/index.rst | 1 +
Documentation/dev-tools/kfuzztest.rst | 371 +++++++++++++
arch/x86/kernel/vmlinux.lds.S | 22 +
crypto/asymmetric_keys/Kconfig | 15 +
crypto/asymmetric_keys/Makefile | 2 +
crypto/asymmetric_keys/tests/Makefile | 2 +
crypto/asymmetric_keys/tests/pkcs7_kfuzz.c | 22 +
.../asymmetric_keys/tests/rsa_helper_kfuzz.c | 38 ++
include/linux/kasan.h | 16 +
include/linux/kfuzztest.h | 508 ++++++++++++++++++
lib/Kconfig.debug | 1 +
lib/Makefile | 2 +
lib/kfuzztest/Kconfig | 20 +
lib/kfuzztest/Makefile | 4 +
lib/kfuzztest/main.c | 163 ++++++
lib/kfuzztest/parse.c | 208 +++++++
mm/kasan/shadow.c | 31 ++
samples/Kconfig | 7 +
samples/Makefile | 1 +
samples/kfuzztest/Makefile | 3 +
samples/kfuzztest/overflow_on_nested_buffer.c | 52 ++
samples/kfuzztest/underflow_on_buffer.c | 41 ++
tools/Makefile | 15 +-
tools/kfuzztest-bridge/.gitignore | 2 +
tools/kfuzztest-bridge/Build | 6 +
tools/kfuzztest-bridge/Makefile | 48 ++
tools/kfuzztest-bridge/bridge.c | 93 ++++
tools/kfuzztest-bridge/byte_buffer.c | 87 +++
tools/kfuzztest-bridge/byte_buffer.h | 31 ++
tools/kfuzztest-bridge/encoder.c | 356 ++++++++++++
tools/kfuzztest-bridge/encoder.h | 16 +
tools/kfuzztest-bridge/input_lexer.c | 243 +++++++++
tools/kfuzztest-bridge/input_lexer.h | 57 ++
tools/kfuzztest-bridge/input_parser.c | 373 +++++++++++++
tools/kfuzztest-bridge/input_parser.h | 79 +++
tools/kfuzztest-bridge/rand_stream.c | 61 +++
tools/kfuzztest-bridge/rand_stream.h | 46 ++
37 files changed, 3037 insertions(+), 6 deletions(-)
create mode 100644 Documentation/dev-tools/kfuzztest.rst
create mode 100644 crypto/asymmetric_keys/tests/Makefile
create mode 100644 crypto/asymmetric_keys/tests/pkcs7_kfuzz.c
create mode 100644 crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c
create mode 100644 include/linux/kfuzztest.h
create mode 100644 lib/kfuzztest/Kconfig
create mode 100644 lib/kfuzztest/Makefile
create mode 100644 lib/kfuzztest/main.c
create mode 100644 lib/kfuzztest/parse.c
create mode 100644 samples/kfuzztest/Makefile
create mode 100644 samples/kfuzztest/overflow_on_nested_buffer.c
create mode 100644 samples/kfuzztest/underflow_on_buffer.c
create mode 100644 tools/kfuzztest-bridge/.gitignore
create mode 100644 tools/kfuzztest-bridge/Build
create mode 100644 tools/kfuzztest-bridge/Makefile
create mode 100644 tools/kfuzztest-bridge/bridge.c
create mode 100644 tools/kfuzztest-bridge/byte_buffer.c
create mode 100644 tools/kfuzztest-bridge/byte_buffer.h
create mode 100644 tools/kfuzztest-bridge/encoder.c
create mode 100644 tools/kfuzztest-bridge/encoder.h
create mode 100644 tools/kfuzztest-bridge/input_lexer.c
create mode 100644 tools/kfuzztest-bridge/input_lexer.h
create mode 100644 tools/kfuzztest-bridge/input_parser.c
create mode 100644 tools/kfuzztest-bridge/input_parser.h
create mode 100644 tools/kfuzztest-bridge/rand_stream.c
create mode 100644 tools/kfuzztest-bridge/rand_stream.h
--
2.51.0.318.gd7df087d1a-goog
^ permalink raw reply [flat|nested] 15+ messages in thread
* [PATCH v2 RFC 1/7] mm/kasan: implement kasan_poison_range
2025-09-01 16:42 [PATCH v2 RFC 0/7] KFuzzTest: a new kernel fuzzing framework Ethan Graham
@ 2025-09-01 16:42 ` Ethan Graham
2025-09-01 16:42 ` [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures Ethan Graham
` (5 subsequent siblings)
6 siblings, 0 replies; 15+ messages in thread
From: Ethan Graham @ 2025-09-01 16:42 UTC (permalink / raw)
To: ethangraham, glider
Cc: andreyknvl, brendan.higgins, davidgow, dvyukov, jannh, elver,
rmoar, shuah, tarasmadan, kasan-dev, kunit-dev, linux-kernel,
linux-mm, dhowells, lukas, ignat, herbert, davem, linux-crypto
From: Ethan Graham <ethangraham@google.com>
Introduce a new helper function, kasan_poison_range(), to encapsulate
the logic for poisoning an arbitrary memory range of a given size, and
expose it publically in <include/linux/kasan.h>.
This is a preparatory change for the upcoming KFuzzTest patches, which
requires the ability to poison the inter-region padding in its input
buffers.
No functional change to any other subsystem is intended by this commit.
Signed-off-by: Ethan Graham <ethangraham@google.com>
---
include/linux/kasan.h | 16 ++++++++++++++++
mm/kasan/shadow.c | 31 +++++++++++++++++++++++++++++++
2 files changed, 47 insertions(+)
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 890011071f2b..09baeb6c9f4d 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -102,6 +102,21 @@ static inline bool kasan_has_integrated_init(void)
}
#ifdef CONFIG_KASAN
+
+/**
+ * kasan_poison_range - poison the memory range [start, start + size)
+ *
+ * The exact behavior is subject to alignment with KASAN_GRANULE_SIZE, defined
+ * in <mm/kasan/kasan.h>.
+ *
+ * - If @start is unaligned, the initial partial granule at the beginning
+ * of the range is only poisoned if CONFIG_KASAN_GENERIC is enabled.
+ * - The poisoning of the range only extends up to the last full granule before
+ * the end of the range. Any remaining bytes in a final partial granule are
+ * ignored.
+ */
+void kasan_poison_range(const void *start, size_t size);
+
void __kasan_unpoison_range(const void *addr, size_t size);
static __always_inline void kasan_unpoison_range(const void *addr, size_t size)
{
@@ -402,6 +417,7 @@ static __always_inline bool kasan_check_byte(const void *addr)
#else /* CONFIG_KASAN */
+static inline void kasan_poison_range(const void *start, size_t size) {}
static inline void kasan_unpoison_range(const void *address, size_t size) {}
static inline void kasan_poison_pages(struct page *page, unsigned int order,
bool init) {}
diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index d2c70cd2afb1..a1b6bfb35f07 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -147,6 +147,37 @@ void kasan_poison(const void *addr, size_t size, u8 value, bool init)
}
EXPORT_SYMBOL_GPL(kasan_poison);
+void kasan_poison_range(const void *start, size_t size)
+{
+ void *end = (char *)start + size;
+ uintptr_t start_addr = (uintptr_t)start;
+ uintptr_t head_granule_start;
+ uintptr_t poison_body_start;
+ uintptr_t poison_body_end;
+ size_t head_prefix_size;
+ uintptr_t end_addr;
+
+ end_addr = ALIGN_DOWN((uintptr_t)end, KASAN_GRANULE_SIZE);
+ if (start_addr >= end_addr)
+ return;
+
+ head_granule_start = ALIGN_DOWN(start_addr, KASAN_GRANULE_SIZE);
+ head_prefix_size = start_addr - head_granule_start;
+
+ if (IS_ENABLED(CONFIG_KASAN_GENERIC) && head_prefix_size > 0)
+ kasan_poison_last_granule((void *)head_granule_start,
+ head_prefix_size);
+
+ poison_body_start = ALIGN(start_addr, KASAN_GRANULE_SIZE);
+ poison_body_end = ALIGN_DOWN(end_addr, KASAN_GRANULE_SIZE);
+
+ if (poison_body_start < poison_body_end)
+ kasan_poison((void *)poison_body_start,
+ poison_body_end - poison_body_start,
+ KASAN_SLAB_REDZONE, false);
+}
+EXPORT_SYMBOL(kasan_poison_range);
+
#ifdef CONFIG_KASAN_GENERIC
void kasan_poison_last_granule(const void *addr, size_t size)
{
--
2.51.0.318.gd7df087d1a-goog
^ permalink raw reply related [flat|nested] 15+ messages in thread
* [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures
2025-09-01 16:42 [PATCH v2 RFC 0/7] KFuzzTest: a new kernel fuzzing framework Ethan Graham
2025-09-01 16:42 ` [PATCH v2 RFC 1/7] mm/kasan: implement kasan_poison_range Ethan Graham
@ 2025-09-01 16:42 ` Ethan Graham
2025-09-02 10:37 ` Marco Elver
` (3 more replies)
2025-09-01 16:42 ` [PATCH v2 RFC 3/7] kfuzztest: implement core module and input processing Ethan Graham
` (4 subsequent siblings)
6 siblings, 4 replies; 15+ messages in thread
From: Ethan Graham @ 2025-09-01 16:42 UTC (permalink / raw)
To: ethangraham, glider
Cc: andreyknvl, brendan.higgins, davidgow, dvyukov, jannh, elver,
rmoar, shuah, tarasmadan, kasan-dev, kunit-dev, linux-kernel,
linux-mm, dhowells, lukas, ignat, herbert, davem, linux-crypto
From: Ethan Graham <ethangraham@google.com>
Add the foundational user-facing components for the KFuzzTest framework.
This includes the main API header <linux/kfuzztest.h>, the Kconfig
option to enable the feature, and the required linker script changes
which introduce three new ELF sections in vmlinux.
Note that KFuzzTest is intended strictly for debug builds only, and
should never be enabled in a production build. The fact that it exposes
internal kernel functions and state directly to userspace may constitute
a serious security vulnerability if used for any reason other than
testing.
The header defines:
- The FUZZ_TEST() macro for creating test targets.
- The data structures required for the binary serialization format,
which allows passing complex inputs from userspace.
- The metadata structures for test targets, constraints and annotations,
which are placed in dedicated ELF sections (.kfuzztest_*) for discovery.
This patch only adds the public interface and build integration; no
runtime logic is included.
Signed-off-by: Ethan Graham <ethangraham@google.com>
---
arch/x86/kernel/vmlinux.lds.S | 22 ++
include/linux/kfuzztest.h | 508 ++++++++++++++++++++++++++++++++++
lib/Kconfig.debug | 1 +
lib/kfuzztest/Kconfig | 20 ++
4 files changed, 551 insertions(+)
create mode 100644 include/linux/kfuzztest.h
create mode 100644 lib/kfuzztest/Kconfig
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 4fa0be732af1..484e3e1ffb9f 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -112,6 +112,26 @@ ASSERT(__relocate_kernel_end - __relocate_kernel_start <= KEXEC_CONTROL_CODE_MAX
#else
#define KEXEC_RELOCATE_KERNEL
#endif
+
+#ifdef CONFIG_KFUZZTEST
+#define KFUZZTEST_TABLE \
+ . = ALIGN(PAGE_SIZE); \
+ __kfuzztest_targets_start = .; \
+ KEEP(*(.kfuzztest_target)); \
+ __kfuzztest_targets_end = .; \
+ . = ALIGN(PAGE_SIZE); \
+ __kfuzztest_constraints_start = .; \
+ KEEP(*(.kfuzztest_constraint)); \
+ __kfuzztest_constraints_end = .; \
+ . = ALIGN(PAGE_SIZE); \
+ __kfuzztest_annotations_start = .; \
+ KEEP(*(.kfuzztest_annotation)); \
+ __kfuzztest_annotations_end = .;
+
+#else /* CONFIG_KFUZZTEST */
+#define KFUZZTEST_TABLE
+#endif /* CONFIG_KFUZZTEST */
+
PHDRS {
text PT_LOAD FLAGS(5); /* R_E */
data PT_LOAD FLAGS(6); /* RW_ */
@@ -199,6 +219,8 @@ SECTIONS
CONSTRUCTORS
KEXEC_RELOCATE_KERNEL
+ KFUZZTEST_TABLE
+
/* rarely changed data like cpu maps */
READ_MOSTLY_DATA(INTERNODE_CACHE_BYTES)
diff --git a/include/linux/kfuzztest.h b/include/linux/kfuzztest.h
new file mode 100644
index 000000000000..11a647c1d925
--- /dev/null
+++ b/include/linux/kfuzztest.h
@@ -0,0 +1,508 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * The Kernel Fuzz Testing Framework (KFuzzTest) API for defining fuzz targets
+ * for internal kernel functions.
+ *
+ * For more information please see Documentation/dev-tools/kfuzztest.rst.
+ *
+ * Copyright 2025 Google LLC
+ */
+#ifndef KFUZZTEST_H
+#define KFUZZTEST_H
+
+#include <linux/fs.h>
+#include <linux/printk.h>
+#include <linux/types.h>
+
+#define KFUZZTEST_HEADER_MAGIC (0xBFACE)
+#define KFUZZTEST_V0 (0)
+
+/**
+ * @brief The KFuzzTest Input Serialization Format
+ *
+ * KFuzzTest receives its input from userspace as a single binary blob. This
+ * format allows for the serialization of complex, pointer-rich C structures
+ * into a flat buffer that can be safely passed into the kernel. This format
+ * requires only a single copy from userspace into a kenrel buffer, and no
+ * further kernel allocations. Pointers are patched internally using a "region"
+ * system where each region corresponds to some pointed-to data.
+ *
+ * Regions should be padded to respect alignment constraints of their underlying
+ * types, and should be followed by at least 8 bytes of padding. These padded
+ * regions are poisoned by KFuzzTest to ensure that KASAN catches OOB accesses.
+ *
+ * The format consists of a prefix and three main components:
+ * 1. An 8-byte header: Contains KFUZZTEST_MAGIC in the first 4 bytes, and the
+ * version number in the subsequent 4 bytes. This ensures backwards
+ * compatibility in the event of future format changes.
+ * 2. A reloc_region_array: Defines the memory layout of the target structure
+ * by partitioning the payload into logical regions. Each logical region
+ * should contain the byte representation of the type that it represents,
+ * including any necessary padding. The region descriptors should be
+ * ordered by offset ascending.
+ * 3. A reloc_table: Provides "linking" instructions that tell the kernel how
+ * to patch pointer fields to point to the correct regions. By design,
+ * the first region (index 0) is passed as input into a FUZZ_TEST.
+ * 4. A Payload: The raw binary data for the structure and its associated
+ * buffers. This should be aligned to the maximum alignment of all
+ * regions to satisfy alignment requirements of the input types, but this
+ * isn't checked by the parser.
+ *
+ * For a detailed specification of the binary layout see the full documentation
+ * at: Documentation/dev-tools/kfuzztest.rst
+ */
+
+/**
+ * struct reloc_region - single contiguous memory region in the payload
+ *
+ * @offset: The byte offset of this region from the start of the payload, which
+ * should be aligned to the alignment requirements of the region's
+ * underlying type.
+ * @size: The size of this region in bytes.
+ */
+struct reloc_region {
+ uint32_t offset;
+ uint32_t size;
+};
+
+/**
+ * struct reloc_region_array - array of regions in an input
+ * @num_regions: The total number of regions defined.
+ * @regions: A flexible array of `num_regions` region descriptors.
+ */
+struct reloc_region_array {
+ uint32_t num_regions;
+ struct reloc_region regions[];
+};
+
+/**
+ * struct reloc_entry - a single pointer to be patched in an input
+ *
+ * @region_id: The index of the region in the `reloc_region_array` that
+ * contains the pointer.
+ * @region_offset: The start offset of the pointer inside of the region.
+ * @value: contains the index of the pointee region, or KFUZZTEST_REGIONID_NULL
+ * if the pointer is NULL.
+ */
+struct reloc_entry {
+ uint32_t region_id;
+ uint32_t region_offset;
+ uint32_t value;
+};
+
+/**
+ * struct reloc_entry - array of relocations required by an input
+ *
+ * @num_entries: the number of pointer relocations.
+ * @padding_size: the number of padded bytes between the last relocation in
+ * entries, and the start of the payload data. This should be at least
+ * 8 bytes, as it is used for poisoning.
+ * @entries: array of relocations.
+ */
+struct reloc_table {
+ uint32_t num_entries;
+ uint32_t padding_size;
+ struct reloc_entry entries[];
+};
+
+/**
+ * kfuzztest_parse_and_relocate - validate and relocate a KFuzzTest input
+ *
+ * @input: A buffer containing the serialized input for a fuzz target.
+ * @input_size: the size in bytes of the @input buffer.
+ * @arg_ret: return pointer for the test case's input structure.
+ */
+int kfuzztest_parse_and_relocate(void *input, size_t input_size, void **arg_ret);
+
+/*
+ * Dump some information on the parsed headers and payload. Can be useful for
+ * debugging inputs when writing an encoder for the KFuzzTest input format.
+ */
+__attribute__((unused)) static inline void kfuzztest_debug_header(struct reloc_region_array *regions,
+ struct reloc_table *rt, void *payload_start,
+ void *payload_end)
+{
+ uint32_t i;
+
+ pr_info("regions: { num_regions = %u } @ %px", regions->num_regions, regions);
+ for (i = 0; i < regions->num_regions; i++) {
+ pr_info(" region_%u: { start: 0x%x, size: 0x%x }", i, regions->regions[i].offset,
+ regions->regions[i].size);
+ }
+
+ pr_info("reloc_table: { num_entries = %u, padding = %u } @ offset 0x%lx", rt->num_entries, rt->padding_size,
+ (char *)rt - (char *)regions);
+ for (i = 0; i < rt->num_entries; i++) {
+ pr_info(" reloc_%u: { src: %u, offset: 0x%x, dst: %u }", i, rt->entries[i].region_id,
+ rt->entries[i].region_offset, rt->entries[i].value);
+ }
+
+ pr_info("payload: [0x%lx, 0x%lx)", (char *)payload_start - (char *)regions,
+ (char *)payload_end - (char *)regions);
+}
+
+struct kfuzztest_target {
+ const char *name;
+ const char *arg_type_name;
+ ssize_t (*write_input_cb)(struct file *filp, const char __user *buf, size_t len, loff_t *off);
+} __aligned(32);
+
+/**
+ * FUZZ_TEST - defines a KFuzzTest target
+ *
+ * @test_name: The unique identifier for the fuzz test, which is used to name
+ * the debugfs entry, e.g., /sys/kernel/debug/kfuzztest/@test_name.
+ * @test_arg_type: The struct type that defines the inputs for the test. This
+ * must be the full struct type (e.g., "struct my_inputs"), not a typedef.
+ *
+ * Context:
+ * This macro is the primary entry point for the KFuzzTest framework. It
+ * generates all the necessary boilerplate for a fuzz test, including:
+ * - A static `struct kfuzztest_target` instance that is placed in a
+ * dedicated ELF section for discovery by userspace tools.
+ * - A `debugfs` write callback that handles receiving serialized data from
+ * a fuzzer, parsing it, and "hydrating" it into a valid C struct.
+ * - A function stub where the developer places the test logic.
+ *
+ * User-Provided Logic:
+ * The developer must provide the body of the fuzz test logic within the curly
+ * braces following the macro invocation. Within this scope, the framework
+ * provides the following variables:
+ *
+ * - `arg`: A pointer of type `@test_arg_type *` to the fully hydrated input
+ * structure. All pointer fields within this struct have been relocated
+ * and are valid kernel pointers. This is the primary variable to use
+ * for accessing fuzzing inputs.
+ *
+ * - `regions`: A pointer of type `struct reloc_region_array *`. This is an
+ * advanced feature that allows access to the raw region metadata, which
+ * can be useful for checking the actual allocated size of a buffer via
+ * `KFUZZTEST_REGION_SIZE(n)`.
+ *
+ * Example Usage:
+ *
+ * // 1. The kernel function we want to fuzz.
+ * int process_data(const char *data, size_t len);
+ *
+ * // 2. Define a struct to hold all inputs for the function.
+ * struct process_data_inputs {
+ * const char *data;
+ * size_t len;
+ * };
+ *
+ * // 3. Define the fuzz test using the FUZZ_TEST macro.
+ * FUZZ_TEST(process_data_fuzzer, struct process_data_inputs)
+ * {
+ * int ret;
+ * // Use KFUZZTEST_EXPECT_* to enforce preconditions.
+ * // The test will exit early if data is NULL.
+ * KFUZZTEST_EXPECT_NOT_NULL(process_data_inputs, data);
+ *
+ * // Use KFUZZTEST_ANNOTATE_* to provide hints to the fuzzer.
+ * // This links the 'len' field to the 'data' buffer.
+ * KFUZZTEST_ANNOTATE_LEN(process_data_inputs, len, data);
+ *
+ * // Call the function under test using the 'arg' variable. OOB memory
+ * // accesses will be caught by KASAN, but the user can also choose to
+ * // validate the return value and log any failures.
+ * ret = process_data(arg->data, arg->len);
+ * }
+ */
+#define FUZZ_TEST(test_name, test_arg_type) \
+ static ssize_t kfuzztest_write_cb_##test_name(struct file *filp, const char __user *buf, size_t len, \
+ loff_t *off); \
+ static void kfuzztest_logic_##test_name(test_arg_type *arg); \
+ const struct kfuzztest_target __fuzz_test__##test_name __section(".kfuzztest_target") __used = { \
+ .name = #test_name, \
+ .arg_type_name = #test_arg_type, \
+ .write_input_cb = kfuzztest_write_cb_##test_name, \
+ }; \
+ static ssize_t kfuzztest_write_cb_##test_name(struct file *filp, const char __user *buf, size_t len, \
+ loff_t *off) \
+ { \
+ test_arg_type *arg; \
+ void *buffer; \
+ int ret; \
+ \
+ buffer = kmalloc(len, GFP_KERNEL); \
+ if (!buffer) \
+ return -ENOMEM; \
+ ret = simple_write_to_buffer(buffer, len, off, buf, len); \
+ if (ret < 0) \
+ goto out; \
+ ret = kfuzztest_parse_and_relocate(buffer, len, (void **)&arg); \
+ if (ret < 0) \
+ goto out; \
+ kfuzztest_logic_##test_name(arg); \
+ ret = len; \
+out: \
+ kfree(buffer); \
+ return ret; \
+ } \
+ static void kfuzztest_logic_##test_name(test_arg_type *arg)
+
+enum kfuzztest_constraint_type {
+ EXPECT_EQ,
+ EXPECT_NE,
+ EXPECT_LT,
+ EXPECT_LE,
+ EXPECT_GT,
+ EXPECT_GE,
+ EXPECT_IN_RANGE,
+};
+
+/**
+ * struct kfuzztest_constraint - a metadata record for a domain constraint
+ *
+ * Domain constraints are rules about the input data that must be satisfied for
+ * a fuzz test to proceed. While they are enforced in the kernel with a runtime
+ * check, they are primarily intended as a discoverable contract for userspace
+ * fuzzers.
+ *
+ * Instances of this struct are generated by the KFUZZTEST_EXPECT_* macros
+ * and placed into the read-only ".kfuzztest_constraint" ELF section of the
+ * vmlinux binary. A fuzzer can parse this section to learn about the
+ * constraints and generate valid inputs more intelligently.
+ *
+ * For an example of how these constraints are used within a fuzz test, see the
+ * documentation for the FUZZ_TEST() macro.
+ *
+ * @input_type: The name of the input struct type, without the leading
+ * "struct ".
+ * @field_name: The name of the field within the struct that this constraint
+ * applies to.
+ * @value1: The primary value used in the comparison (e.g., the upper
+ * bound for EXPECT_LE).
+ * @value2: The secondary value, used only for multi-value comparisons
+ * (e.g., the upper bound for EXPECT_IN_RANGE).
+ * @type: The type of the constraint.
+ */
+struct kfuzztest_constraint {
+ const char *input_type;
+ const char *field_name;
+ uintptr_t value1;
+ uintptr_t value2;
+ enum kfuzztest_constraint_type type;
+} __aligned(64);
+
+#define __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val1, val2, tpe) \
+ static struct kfuzztest_constraint __constraint_##arg_type##_##field __section(".kfuzztest_constraint") \
+ __used = { \
+ .input_type = "struct " #arg_type, \
+ .field_name = #field, \
+ .value1 = (uintptr_t)val1, \
+ .value2 = (uintptr_t)val2, \
+ .type = tpe, \
+ }
+
+/**
+ * KFUZZTEST_EXPECT_EQ - constrain a field to be equal to a value
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable
+ * @val: a value of the same type as @arg_type.@field
+ */
+#define KFUZZTEST_EXPECT_EQ(arg_type, field, val) \
+ do { \
+ if (arg->field != val) \
+ return; \
+ __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_EQ); \
+ } while (0)
+
+/**
+ * KFUZZTEST_EXPECT_NE - constrain a field to be not equal to a value
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @val: a value of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_NE(arg_type, field, val) \
+ do { \
+ if (arg->field == val) \
+ return; \
+ __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_NE); \
+ } while (0)
+
+/**
+ * KFUZZTEST_EXPECT_LT - constrain a field to be less than a value
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @val: a value of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_LT(arg_type, field, val) \
+ do { \
+ if (arg->field >= val) \
+ return; \
+ __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_LT); \
+ } while (0)
+
+/**
+ * KFUZZTEST_EXPECT_LE - constrain a field to be less than or equal to a value
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @val: a value of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_LE(arg_type, field, val) \
+ do { \
+ if (arg->field > val) \
+ return; \
+ __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_LE); \
+ } while (0)
+
+/**
+ * KFUZZTEST_EXPECT_GT - constrain a field to be greater than a value
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @val: a value of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_GT(arg_type, field, val) \
+ do { \
+ if (arg->field <= val) \
+ return; \
+ __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_GT) \
+ } while (0)
+
+/**
+ * KFUZZTEST_EXPECT_GE - constrain a field to be greater than or equal to a value
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @val: a value of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_GE(arg_type, field, val) \
+ do { \
+ if (arg->field < val) \
+ return; \
+ __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_GE)` \
+ } while (0)
+
+/**
+ * KFUZZTEST_EXPECT_GE - constrain a pointer field to be non-NULL
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @val: a value of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_NOT_NULL(arg_type, field) KFUZZTEST_EXPECT_NE(arg_type, field, NULL)
+
+/**
+ * KFUZZTEST_EXPECT_IN_RANGE - constrain a field to be within a range
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @lower_bound: a lower bound of the same type as @arg_type.@field.
+ * @upper_bound: an upper bound of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_IN_RANGE(arg_type, field, lower_bound, upper_bound) \
+ do { \
+ if (arg->field < lower_bound || arg->field > upper_bound) \
+ return; \
+ __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, lower_bound, upper_bound, EXPECT_IN_RANGE) \
+ } while (0)
+
+/**
+ * Annotations express attributes about structure fields that can't be easily
+ * or safely verified at runtime. They are intended as hints to the fuzzing
+ * engine to help it generate more semantically correct and effective inputs.
+ * Unlike constraints, annotations do not add any runtime checks and do not
+ * cause a test to exit early.
+ *
+ * For example, a `char *` field could be a raw byte buffer or a C-style
+ * null-terminated string. A fuzzer that is aware of this distinction can avoid
+ * creating inputs that would cause trivial, uninteresting crashes from reading
+ * past the end of a non-null-terminated buffer.
+ */
+enum kfuzztest_annotation_attribute : uint8_t {
+ ATTRIBUTE_LEN,
+ ATTRIBUTE_STRING,
+ ATTRIBUTE_ARRAY,
+};
+
+/**
+ * struct kfuzztest_annotation - a metadata record for a fuzzer hint
+ *
+ * This struct captures a single hint about a field in the input structure.
+ * Instances are generated by the KFUZZTEST_ANNOTATE_* macros and are placed
+ * into the read-only ".kfuzztest_annotation" ELF section of the vmlinux binary.
+ *
+ * A userspace fuzzer can parse this section to understand the semantic
+ * relationships between fields (e.g., which field is a length for which
+ * buffer) and the expected format of the data (e.g., a null-terminated
+ * string). This allows the fuzzer to be much more intelligent during input
+ * generation and mutation.
+ *
+ * For an example of how annotations are used within a fuzz test, see the
+ * documentation for the FUZZ_TEST() macro.
+ *
+ * @input_type: The name of the input struct type.
+ * @field_name: The name of the field being annotated (e.g., the data
+ * buffer field).
+ * @linked_field_name: For annotations that link two fields (like
+ * ATTRIBUTE_LEN), this is the name of the related field (e.g., the
+ * length field). For others, this may be unused.
+ * @attrib: The type of the annotation hint.
+ */
+struct kfuzztest_annotation {
+ const char *input_type;
+ const char *field_name;
+ const char *linked_field_name;
+ enum kfuzztest_annotation_attribute attrib;
+} __aligned(32);
+
+#define __KFUZZTEST_ANNOTATE(arg_type, field, linked_field, attribute) \
+ static struct kfuzztest_annotation __annotation_##arg_type##_##field __section(".kfuzztest_annotation") \
+ __used = { \
+ .input_type = "struct " #arg_type, \
+ .field_name = #field, \
+ .linked_field_name = #linked_field, \
+ .attrib = attribute, \
+ }
+
+/**
+ * KFUZZTEST_ANNOTATE_STRING - annotate a char* field as a C string
+ *
+ * We define a C string as a sequence of non-zero characters followed by exactly
+ * one null terminator.
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: the name of the field to annotate.
+ */
+#define KFUZZTEST_ANNOTATE_STRING(arg_type, field) __KFUZZTEST_ANNOTATE(arg_type, field, NULL, ATTRIBUTE_STRING)
+
+/**
+ * KFUZZTEST_ANNOTATE_ARRAY - annotate a pointer as an array
+ *
+ * We define an array as a contiguous memory region containing zero or more
+ * elements of the same type.
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: the name of the field to annotate.
+ */
+#define KFUZZTEST_ANNOTATE_ARRAY(arg_type, field) __KFUZZTEST_ANNOTATE(arg_type, field, NULL, ATTRIBUTE_ARRAY)
+
+/**
+ * KFUZZTEST_ANNOTATE_LEN - annotate a field as the length of another
+ *
+ * This expresses the relationship `arg_type.field == len(linked_field)`, where
+ * `linked_field` is an array.
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: the name of the field to annotate.
+ * @linked_field: the name of an array field with length @field.
+ */
+#define KFUZZTEST_ANNOTATE_LEN(arg_type, field, linked_field) \
+ __KFUZZTEST_ANNOTATE(arg_type, field, linked_field, ATTRIBUTE_LEN)
+
+#define KFUZZTEST_REGIONID_NULL U32_MAX
+
+/**
+ * The end of the input should be padded by at least this number of bytes as
+ * it is poisoned to detect out of bounds accesses at the end of the last
+ * region.
+ */
+#define KFUZZTEST_POISON_SIZE 0x8
+
+#endif /* KFUZZTEST_H */
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index ebe33181b6e6..3542e94204c8 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1947,6 +1947,7 @@ endmenu
menu "Kernel Testing and Coverage"
source "lib/kunit/Kconfig"
+source "lib/kfuzztest/Kconfig"
config NOTIFIER_ERROR_INJECTION
tristate "Notifier error injection"
diff --git a/lib/kfuzztest/Kconfig b/lib/kfuzztest/Kconfig
new file mode 100644
index 000000000000..f9fb5abf8d27
--- /dev/null
+++ b/lib/kfuzztest/Kconfig
@@ -0,0 +1,20 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+config KFUZZTEST
+ bool "KFuzzTest - enable support for internal fuzz targets"
+ depends on DEBUG_FS && DEBUG_KERNEL
+ help
+ Enables support for the kernel fuzz testing framework (KFuzzTest), an
+ interface for exposing internal kernel functions to a userspace fuzzing
+ engine. KFuzzTest targets are exposed via a debugfs interface that
+ accepts serialized userspace inputs, and is designed to make it easier
+ to fuzz deeply nested kernel code that is hard to reach from the system
+ call boundary. Using a simple macro-based API, developers can add a new
+ fuzz target with minimal boilerplate code.
+
+ It is strongly recommended to also enable CONFIG_KASAN for byte-accurate
+ out-of-bounds detection, as KFuzzTest was designed with this in mind. It
+ is also recommended to enable CONFIG_KCOV for coverage guided fuzzing.
+
+ WARNING: This exposes internal kernel functions directly to userspace
+ and must NEVER be enabled in production builds.
--
2.51.0.318.gd7df087d1a-goog
^ permalink raw reply related [flat|nested] 15+ messages in thread
* [PATCH v2 RFC 3/7] kfuzztest: implement core module and input processing
2025-09-01 16:42 [PATCH v2 RFC 0/7] KFuzzTest: a new kernel fuzzing framework Ethan Graham
2025-09-01 16:42 ` [PATCH v2 RFC 1/7] mm/kasan: implement kasan_poison_range Ethan Graham
2025-09-01 16:42 ` [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures Ethan Graham
@ 2025-09-01 16:42 ` Ethan Graham
2025-09-03 9:53 ` Alexander Potapenko
2025-09-01 16:42 ` [PATCH v2 RFC 4/7] tools: add kfuzztest-bridge utility Ethan Graham
` (3 subsequent siblings)
6 siblings, 1 reply; 15+ messages in thread
From: Ethan Graham @ 2025-09-01 16:42 UTC (permalink / raw)
To: ethangraham, glider
Cc: andreyknvl, brendan.higgins, davidgow, dvyukov, jannh, elver,
rmoar, shuah, tarasmadan, kasan-dev, kunit-dev, linux-kernel,
linux-mm, dhowells, lukas, ignat, herbert, davem, linux-crypto
From: Ethan Graham <ethangraham@google.com>
Add the core runtime implementation for KFuzzTest. This includes the
module initialization, and the logic for receiving and processing
user-provided inputs through debugfs.
On module load, the framework discovers all test targets by iterating
over the .kfuzztest_target section, creating a corresponding debugfs
directory with a write-only 'input' file for each of them.
Writing to an 'input' file triggers the main fuzzing sequence:
1. The serialized input is copied from userspace into a kernel buffer.
2. The buffer is parsed to validate the region array and relocation
table.
3. Pointers are patched based on the relocation entries, and in KASAN
builds the inter-region padding is poisoned.
4. The resulting struct is passed to the user-defined test logic.
Signed-off-by: Ethan Graham <ethangraham@google.com>
---
v2:
- The module's init function now taints the kernel with TAINT_TEST.
---
---
lib/Makefile | 2 +
lib/kfuzztest/Makefile | 4 +
lib/kfuzztest/main.c | 163 ++++++++++++++++++++++++++++++++
lib/kfuzztest/parse.c | 208 +++++++++++++++++++++++++++++++++++++++++
4 files changed, 377 insertions(+)
create mode 100644 lib/kfuzztest/Makefile
create mode 100644 lib/kfuzztest/main.c
create mode 100644 lib/kfuzztest/parse.c
diff --git a/lib/Makefile b/lib/Makefile
index c38582f187dd..511c44ef4b19 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -354,6 +354,8 @@ obj-$(CONFIG_GENERIC_LIB_CMPDI2) += cmpdi2.o
obj-$(CONFIG_GENERIC_LIB_UCMPDI2) += ucmpdi2.o
obj-$(CONFIG_OBJAGG) += objagg.o
+obj-$(CONFIG_KFUZZTEST) += kfuzztest/
+
# pldmfw library
obj-$(CONFIG_PLDMFW) += pldmfw/
diff --git a/lib/kfuzztest/Makefile b/lib/kfuzztest/Makefile
new file mode 100644
index 000000000000..142d16007eea
--- /dev/null
+++ b/lib/kfuzztest/Makefile
@@ -0,0 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0
+
+obj-$(CONFIG_KFUZZTEST) += kfuzztest.o
+kfuzztest-objs := main.o parse.o
diff --git a/lib/kfuzztest/main.c b/lib/kfuzztest/main.c
new file mode 100644
index 000000000000..c24350eb1fca
--- /dev/null
+++ b/lib/kfuzztest/main.c
@@ -0,0 +1,163 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KFuzzTest core module initialization and debugfs interface.
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <linux/debugfs.h>
+#include <linux/fs.h>
+#include <linux/kfuzztest.h>
+#include <linux/module.h>
+#include <linux/printk.h>
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Ethan Graham <ethangraham@google.com>");
+MODULE_DESCRIPTION("Kernel Fuzz Testing Framework (KFuzzTest)");
+
+extern const struct kfuzztest_target __kfuzztest_targets_start[];
+extern const struct kfuzztest_target __kfuzztest_targets_end[];
+
+/**
+ * struct kfuzztest_dentry - A container for a debugfs dentry and its fops.
+ * @dentry: Pointer to the created debugfs dentry.
+ * @fops: The file_operations struct associated with this dentry.
+ *
+ * This simplifies state management by keeping a file's dentry and its
+ * operations bundled together.
+ */
+struct kfuzztest_dentry {
+ struct dentry *dentry;
+ struct file_operations fops;
+};
+
+/**
+ * struct kfuzztest_debugfs_state - Per-test-case debugfs state.
+ * @test_dir: The top-level debugfs directory for a single test case, e.g.,
+ * /sys/kernel/debug/kfuzztest/<test-name>/.
+ * @input_dentry: The state for the "input" file, which is write-only.
+ *
+ * Wraps all debugfs components created for a single test case.
+ */
+struct kfuzztest_debugfs_state {
+ struct dentry *target_dir;
+ struct kfuzztest_dentry input_dentry;
+};
+
+/**
+ * struct kfuzztest_simple_fuzzer_state - Global state for the KFTF module.
+ * @kfuzztest_dir: The root debugfs directory, /sys/kernel/debug/kfuzztest/.
+ * @debugfs_state: A statically sized array holding the state for each
+ * registered test case.
+ */
+struct kfuzztest_state {
+ struct file_operations fops;
+ struct dentry *kfuzztest_dir;
+ struct kfuzztest_debugfs_state *debugfs_state;
+};
+
+/* Global static variable to hold all state for the module. */
+static struct kfuzztest_state state;
+
+const umode_t KFUZZTEST_INPUT_PERMS = 0222;
+
+/**
+ * kfuzztest_init - Initializes the debug filesystem for KFuzzTest.
+ *
+ * Each registered test in the ".kfuzztest" section gets its own subdirectory
+ * under "/sys/kernel/debug/kfuzztest/<test-name>" with one files:
+ * - input: write-only file to send input to the fuzz driver
+ *
+ * Returns:
+ * 0 on success.
+ * -ENODEV or other error codes if debugfs creation fails.
+ */
+static int __init kfuzztest_init(void)
+{
+ const struct kfuzztest_target *targ;
+ int ret = 0;
+ int i = 0;
+ size_t num_test_cases;
+
+ num_test_cases = __kfuzztest_targets_end - __kfuzztest_targets_start;
+
+ state.debugfs_state =
+ kzalloc(num_test_cases * sizeof(struct kfuzztest_debugfs_state),
+ GFP_KERNEL);
+ if (!state.debugfs_state)
+ return -ENOMEM;
+
+ /* Create the main "kfuzztest" directory in /sys/kernel/debug. */
+ state.kfuzztest_dir = debugfs_create_dir("kfuzztest", NULL);
+ if (!state.kfuzztest_dir) {
+ pr_warn("KFuzzTest: could not create debugfs");
+ return -ENODEV;
+ }
+
+ if (IS_ERR(state.kfuzztest_dir)) {
+ state.kfuzztest_dir = NULL;
+ return PTR_ERR(state.kfuzztest_dir);
+ }
+
+ for (targ = __kfuzztest_targets_start; targ < __kfuzztest_targets_end;
+ targ++, i++) {
+ /* Create debugfs directory for the target. */
+ state.debugfs_state[i].target_dir =
+ debugfs_create_dir(targ->name, state.kfuzztest_dir);
+
+ if (!state.debugfs_state[i].target_dir) {
+ ret = -ENOMEM;
+ goto cleanup_failure;
+ } else if (IS_ERR(state.debugfs_state[i].target_dir)) {
+ ret = PTR_ERR(state.debugfs_state[i].target_dir);
+ goto cleanup_failure;
+ }
+
+ /* Create an input file under the target's directory. */
+ state.debugfs_state[i].input_dentry.fops =
+ (struct file_operations){
+ .owner = THIS_MODULE,
+ .write = targ->write_input_cb,
+ };
+ state.debugfs_state[i].input_dentry.dentry =
+ debugfs_create_file(
+ "input", KFUZZTEST_INPUT_PERMS,
+ state.debugfs_state[i].target_dir, NULL,
+ &state.debugfs_state[i].input_dentry.fops);
+ if (!state.debugfs_state[i].input_dentry.dentry) {
+ ret = -ENOMEM;
+ goto cleanup_failure;
+ } else if (IS_ERR(state.debugfs_state[i].input_dentry.dentry)) {
+ ret = PTR_ERR(
+ state.debugfs_state[i].input_dentry.dentry);
+ goto cleanup_failure;
+ }
+
+ pr_info("KFuzzTest: registered target %s", targ->name);
+ }
+
+ /* Taint the kernel after successfully creating the debugfs entries. */
+ add_taint(TAINT_TEST, LOCKDEP_STILL_OK);
+ return 0;
+
+cleanup_failure:
+ debugfs_remove_recursive(state.kfuzztest_dir);
+ return ret;
+}
+
+static void __exit kfuzztest_exit(void)
+{
+ pr_info("KFuzzTest: exiting");
+ if (!state.kfuzztest_dir)
+ return;
+
+ debugfs_remove_recursive(state.kfuzztest_dir);
+ state.kfuzztest_dir = NULL;
+
+ if (state.debugfs_state) {
+ kfree(state.debugfs_state);
+ state.debugfs_state = NULL;
+ }
+}
+
+module_init(kfuzztest_init);
+module_exit(kfuzztest_exit);
diff --git a/lib/kfuzztest/parse.c b/lib/kfuzztest/parse.c
new file mode 100644
index 000000000000..6010171190ad
--- /dev/null
+++ b/lib/kfuzztest/parse.c
@@ -0,0 +1,208 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * KFuzzTest input parsing and validation.
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <linux/kfuzztest.h>
+#include <linux/kasan.h>
+
+/*
+ * Enforce a fixed struct size to ensure a consistent stride when iterating over
+ * the array of these structs in the dedicated ELF section.
+ */
+static_assert(sizeof(struct kfuzztest_target) == 32, "struct kfuzztest_target should have size 32");
+static_assert(sizeof(struct kfuzztest_constraint) == 64, "struct kfuzztest_constraint should have size 64");
+static_assert(sizeof(struct kfuzztest_annotation) == 32, "struct kfuzztest_annotation should have size 32");
+
+static int kfuzztest_relocate_v0(struct reloc_region_array *regions, struct reloc_table *rt, void *payload_start,
+ void *payload_end)
+{
+ struct reloc_region reg, src, dst;
+ void *poison_start, *poison_end;
+ uintptr_t *ptr_location;
+ struct reloc_entry re;
+ size_t i;
+
+ /* Patch pointers. */
+ for (i = 0; i < rt->num_entries; i++) {
+ re = rt->entries[i];
+ src = regions->regions[re.region_id];
+ ptr_location = (uintptr_t *)((char *)payload_start + src.offset + re.region_offset);
+ if (re.value == KFUZZTEST_REGIONID_NULL)
+ *ptr_location = (uintptr_t)NULL;
+ else if (re.value < regions->num_regions) {
+ dst = regions->regions[re.value];
+ *ptr_location = (uintptr_t)((char *)payload_start + dst.offset);
+ } else
+ return -EINVAL;
+ }
+
+ /* Poison the padding between regions. */
+ for (i = 0; i < regions->num_regions; i++) {
+ reg = regions->regions[i];
+
+ /* Points to the beginning of the inter-region padding */
+ poison_start = payload_start + reg.offset + reg.size;
+ if (i < regions->num_regions - 1)
+ poison_end = payload_start + regions->regions[i + 1].offset;
+ else
+ poison_end = payload_end;
+
+ if ((char *)poison_end > (char *)payload_end)
+ return -EINVAL;
+
+ kasan_poison_range(poison_start, poison_end - poison_start);
+ }
+
+ /* Poison the padded area preceding the payload. */
+ kasan_poison_range((char *)payload_start - rt->padding_size, rt->padding_size);
+ return 0;
+}
+
+static bool kfuzztest_input_is_valid(struct reloc_region_array *regions, struct reloc_table *rt, void *payload_start,
+ void *payload_end)
+{
+ size_t payload_size = (char *)payload_end - (char *)payload_start;
+ struct reloc_region reg, next_reg;
+ size_t usable_payload_size;
+ uint32_t region_end_offset;
+ struct reloc_entry reloc;
+ uint32_t i;
+
+ if ((char *)payload_start > (char *)payload_end)
+ return false;
+ if (payload_size < KFUZZTEST_POISON_SIZE)
+ return false;
+ usable_payload_size = payload_size - KFUZZTEST_POISON_SIZE;
+
+ for (i = 0; i < regions->num_regions; i++) {
+ reg = regions->regions[i];
+ if (check_add_overflow(reg.offset, reg.size, ®ion_end_offset))
+ return false;
+ if ((size_t)region_end_offset > usable_payload_size)
+ return false;
+
+ if (i < regions->num_regions - 1) {
+ next_reg = regions->regions[i + 1];
+ if (reg.offset > next_reg.offset)
+ return false;
+ /*
+ * Enforce the minimum poisonable gap between
+ * consecutive regions.
+ */
+ if (reg.offset + reg.size + KFUZZTEST_POISON_SIZE > next_reg.offset)
+ return false;
+ }
+ }
+
+ if (rt->padding_size < KFUZZTEST_POISON_SIZE) {
+ pr_info("validation failed because rt->padding_size = %u", rt->padding_size);
+ return false;
+ }
+
+ for (i = 0; i < rt->num_entries; i++) {
+ reloc = rt->entries[i];
+ if (reloc.region_id >= regions->num_regions)
+ return false;
+ if (reloc.value != KFUZZTEST_REGIONID_NULL && reloc.value >= regions->num_regions)
+ return false;
+
+ reg = regions->regions[reloc.region_id];
+ if (reloc.region_offset % (sizeof(uintptr_t)) || reloc.region_offset + sizeof(uintptr_t) > reg.size)
+ return false;
+ }
+
+ return true;
+}
+
+static int kfuzztest_parse_input_v0(void *input, size_t input_size, struct reloc_region_array **ret_regions,
+ struct reloc_table **ret_reloc_table, void **ret_payload_start,
+ void **ret_payload_end)
+{
+ size_t reloc_entries_size, reloc_regions_size;
+ size_t reloc_table_size, regions_size;
+ struct reloc_region_array *regions;
+ void *payload_end, *payload_start;
+ struct reloc_table *rt;
+ size_t curr_offset = 0;
+
+ if (input_size < sizeof(struct reloc_region_array) + sizeof(struct reloc_table))
+ return -EINVAL;
+
+ regions = input;
+ if (check_mul_overflow(regions->num_regions, sizeof(struct reloc_region), &reloc_regions_size))
+ return -EINVAL;
+ if (check_add_overflow(sizeof(*regions), reloc_regions_size, ®ions_size))
+ return -EINVAL;
+
+ curr_offset = regions_size;
+ if (curr_offset > input_size)
+ return -EINVAL;
+ if (input_size - curr_offset < sizeof(struct reloc_table))
+ return -EINVAL;
+
+ rt = (struct reloc_table *)((char *)input + curr_offset);
+
+ if (check_mul_overflow((size_t)rt->num_entries, sizeof(struct reloc_entry), &reloc_entries_size))
+ return -EINVAL;
+ if (check_add_overflow(sizeof(*rt), reloc_entries_size, &reloc_table_size))
+ return -EINVAL;
+ if (check_add_overflow(reloc_table_size, rt->padding_size, &reloc_table_size))
+ return -EINVAL;
+
+ if (check_add_overflow(curr_offset, reloc_table_size, &curr_offset))
+ return -EINVAL;
+ if (curr_offset > input_size)
+ return -EINVAL;
+
+ payload_start = (char *)input + curr_offset;
+ payload_end = (char *)input + input_size;
+
+ if (!kfuzztest_input_is_valid(regions, rt, payload_start, payload_end))
+ return -EINVAL;
+
+ *ret_regions = regions;
+ *ret_reloc_table = rt;
+ *ret_payload_start = payload_start;
+ *ret_payload_end = payload_end;
+ return 0;
+}
+
+static int kfuzztest_parse_and_relocate_v0(void *input, size_t input_size, void **arg_ret)
+{
+ struct reloc_region_array *regions;
+ void *payload_start, *payload_end;
+ struct reloc_table *reloc_table;
+ int ret;
+
+ ret = kfuzztest_parse_input_v0(input, input_size, ®ions, &reloc_table, &payload_start, &payload_end);
+ if (ret < 0)
+ return ret;
+
+ ret = kfuzztest_relocate_v0(regions, reloc_table, payload_start, payload_end);
+ if (ret < 0)
+ return ret;
+ *arg_ret = payload_start;
+ return 0;
+}
+
+int kfuzztest_parse_and_relocate(void *input, size_t input_size, void **arg_ret)
+{
+ u32 version, magic;
+
+ if (input_size < sizeof(u32) + sizeof(u32))
+ return -EINVAL;
+
+ magic = *(u32 *)input;
+ if (magic != KFUZZTEST_HEADER_MAGIC)
+ return -EINVAL;
+
+ version = *(u32 *)((char *)input + sizeof(u32));
+ switch (version) {
+ case KFUZZTEST_V0:
+ return kfuzztest_parse_and_relocate_v0(input + sizeof(u64), input_size - sizeof(u64), arg_ret);
+ }
+
+ return -EINVAL;
+}
--
2.51.0.318.gd7df087d1a-goog
^ permalink raw reply related [flat|nested] 15+ messages in thread
* [PATCH v2 RFC 4/7] tools: add kfuzztest-bridge utility
2025-09-01 16:42 [PATCH v2 RFC 0/7] KFuzzTest: a new kernel fuzzing framework Ethan Graham
` (2 preceding siblings ...)
2025-09-01 16:42 ` [PATCH v2 RFC 3/7] kfuzztest: implement core module and input processing Ethan Graham
@ 2025-09-01 16:42 ` Ethan Graham
2025-09-03 14:07 ` Alexander Potapenko
2025-09-01 16:42 ` [PATCH v2 RFC 5/7] kfuzztest: add ReST documentation Ethan Graham
` (2 subsequent siblings)
6 siblings, 1 reply; 15+ messages in thread
From: Ethan Graham @ 2025-09-01 16:42 UTC (permalink / raw)
To: ethangraham, glider
Cc: andreyknvl, brendan.higgins, davidgow, dvyukov, jannh, elver,
rmoar, shuah, tarasmadan, kasan-dev, kunit-dev, linux-kernel,
linux-mm, dhowells, lukas, ignat, herbert, davem, linux-crypto
From: Ethan Graham <ethangraham@google.com>
Introduce the kfuzztest-bridge tool, a userspace utility for sending
structured inputs to KFuzzTest harnesses via debugfs.
The bridge takes a textual description of the expected input format, a
file containing random bytes, and the name of the target fuzz test. It
parses the description, encodes the random data into the binary format
expected by the kernel, and writes the result to the corresponding
debugfs entry.
This allows for both simple manual testing and integration with
userspace fuzzing engines. For example, it can be used for smoke testing
by providing data from /dev/urandom, or act as a bridge for blob-based
fuzzers (e.g., AFL) to target KFuzzTest harnesses.
Signed-off-by: Ethan Graham <ethangraham@google.com>
---
tools/Makefile | 15 +-
tools/kfuzztest-bridge/.gitignore | 2 +
tools/kfuzztest-bridge/Build | 6 +
tools/kfuzztest-bridge/Makefile | 48 ++++
tools/kfuzztest-bridge/bridge.c | 93 +++++++
tools/kfuzztest-bridge/byte_buffer.c | 87 ++++++
tools/kfuzztest-bridge/byte_buffer.h | 31 +++
tools/kfuzztest-bridge/encoder.c | 356 ++++++++++++++++++++++++
tools/kfuzztest-bridge/encoder.h | 16 ++
tools/kfuzztest-bridge/input_lexer.c | 243 +++++++++++++++++
tools/kfuzztest-bridge/input_lexer.h | 57 ++++
tools/kfuzztest-bridge/input_parser.c | 373 ++++++++++++++++++++++++++
tools/kfuzztest-bridge/input_parser.h | 79 ++++++
tools/kfuzztest-bridge/rand_stream.c | 61 +++++
tools/kfuzztest-bridge/rand_stream.h | 46 ++++
15 files changed, 1507 insertions(+), 6 deletions(-)
create mode 100644 tools/kfuzztest-bridge/.gitignore
create mode 100644 tools/kfuzztest-bridge/Build
create mode 100644 tools/kfuzztest-bridge/Makefile
create mode 100644 tools/kfuzztest-bridge/bridge.c
create mode 100644 tools/kfuzztest-bridge/byte_buffer.c
create mode 100644 tools/kfuzztest-bridge/byte_buffer.h
create mode 100644 tools/kfuzztest-bridge/encoder.c
create mode 100644 tools/kfuzztest-bridge/encoder.h
create mode 100644 tools/kfuzztest-bridge/input_lexer.c
create mode 100644 tools/kfuzztest-bridge/input_lexer.h
create mode 100644 tools/kfuzztest-bridge/input_parser.c
create mode 100644 tools/kfuzztest-bridge/input_parser.h
create mode 100644 tools/kfuzztest-bridge/rand_stream.c
create mode 100644 tools/kfuzztest-bridge/rand_stream.h
diff --git a/tools/Makefile b/tools/Makefile
index c31cbbd12c45..7f1dfe022045 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -21,6 +21,7 @@ help:
@echo ' hv - tools used when in Hyper-V clients'
@echo ' iio - IIO tools'
@echo ' intel-speed-select - Intel Speed Select tool'
+ @echo ' kfuzztest-bridge - KFuzzTest userspace utility'
@echo ' kvm_stat - top-like utility for displaying kvm statistics'
@echo ' leds - LEDs tools'
@echo ' nolibc - nolibc headers testing and installation'
@@ -69,7 +70,7 @@ acpi: FORCE
cpupower: FORCE
$(call descend,power/$@)
-counter firewire hv guest bootconfig spi usb virtio mm bpf iio gpio objtool leds wmi firmware debugging tracing: FORCE
+counter firewire hv guest bootconfig spi usb virtio mm bpf iio gpio objtool leds wmi firmware debugging tracing kfuzztest-bridge: FORCE
$(call descend,$@)
bpf/%: FORCE
@@ -126,7 +127,8 @@ all: acpi counter cpupower gpio hv firewire \
perf selftests bootconfig spi turbostat usb \
virtio mm bpf x86_energy_perf_policy \
tmon freefall iio objtool kvm_stat wmi \
- debugging tracing thermal thermometer thermal-engine ynl
+ debugging tracing thermal thermometer thermal-engine ynl \
+ kfuzztest-bridge
acpi_install:
$(call descend,power/$(@:_install=),install)
@@ -134,7 +136,7 @@ acpi_install:
cpupower_install:
$(call descend,power/$(@:_install=),install)
-counter_install firewire_install gpio_install hv_install iio_install perf_install bootconfig_install spi_install usb_install virtio_install mm_install bpf_install objtool_install wmi_install debugging_install tracing_install:
+counter_install firewire_install gpio_install hv_install iio_install perf_install bootconfig_install spi_install usb_install virtio_install mm_install bpf_install objtool_install wmi_install debugging_install tracing_install kfuzztest-bridge_install:
$(call descend,$(@:_install=),install)
selftests_install:
@@ -170,7 +172,8 @@ install: acpi_install counter_install cpupower_install gpio_install \
virtio_install mm_install bpf_install x86_energy_perf_policy_install \
tmon_install freefall_install objtool_install kvm_stat_install \
wmi_install debugging_install intel-speed-select_install \
- tracing_install thermometer_install thermal-engine_install ynl_install
+ tracing_install thermometer_install thermal-engine_install ynl_install \
+ kfuzztest-bridge_install
acpi_clean:
$(call descend,power/acpi,clean)
@@ -178,7 +181,7 @@ acpi_clean:
cpupower_clean:
$(call descend,power/cpupower,clean)
-counter_clean hv_clean firewire_clean bootconfig_clean spi_clean usb_clean virtio_clean mm_clean wmi_clean bpf_clean iio_clean gpio_clean objtool_clean leds_clean firmware_clean debugging_clean tracing_clean:
+counter_clean hv_clean firewire_clean bootconfig_clean spi_clean usb_clean virtio_clean mm_clean wmi_clean bpf_clean iio_clean gpio_clean objtool_clean leds_clean firmware_clean debugging_clean tracing_clean kfuzztest-bridge_clean:
$(call descend,$(@:_clean=),clean)
libapi_clean:
@@ -230,6 +233,6 @@ clean: acpi_clean counter_clean cpupower_clean hv_clean firewire_clean \
freefall_clean build_clean libbpf_clean libsubcmd_clean \
gpio_clean objtool_clean leds_clean wmi_clean firmware_clean debugging_clean \
intel-speed-select_clean tracing_clean thermal_clean thermometer_clean thermal-engine_clean \
- sched_ext_clean ynl_clean
+ sched_ext_clean ynl_clean kfuzztest-bridge_clean
.PHONY: FORCE
diff --git a/tools/kfuzztest-bridge/.gitignore b/tools/kfuzztest-bridge/.gitignore
new file mode 100644
index 000000000000..4aa9fb0d44e2
--- /dev/null
+++ b/tools/kfuzztest-bridge/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+kfuzztest-bridge
diff --git a/tools/kfuzztest-bridge/Build b/tools/kfuzztest-bridge/Build
new file mode 100644
index 000000000000..d07341a226d6
--- /dev/null
+++ b/tools/kfuzztest-bridge/Build
@@ -0,0 +1,6 @@
+kfuzztest-bridge-y += bridge.o
+kfuzztest-bridge-y += byte_buffer.o
+kfuzztest-bridge-y += encoder.o
+kfuzztest-bridge-y += input_lexer.o
+kfuzztest-bridge-y += input_parser.o
+kfuzztest-bridge-y += rand_stream.o
diff --git a/tools/kfuzztest-bridge/Makefile b/tools/kfuzztest-bridge/Makefile
new file mode 100644
index 000000000000..3a4437fb0d3f
--- /dev/null
+++ b/tools/kfuzztest-bridge/Makefile
@@ -0,0 +1,48 @@
+# SPDX-License-Identifier: GPL-2.0
+# Makefile for KFuzzTest-Bridge
+include ../scripts/Makefile.include
+
+bindir ?= /usr/bin
+
+ifeq ($(srctree),)
+srctree := $(patsubst %/,%,$(dir $(CURDIR)))
+srctree := $(patsubst %/,%,$(dir $(srctree)))
+endif
+
+MAKEFLAGS += -r
+
+override CFLAGS += -O2 -g
+override CFLAGS += -Wall -Wextra
+override CFLAGS += -D_GNU_SOURCE
+override CFLAGS += -I$(OUTPUT)include -I$(srctree)/tools/include
+
+ALL_TARGETS := kfuzztest-bridge
+ALL_PROGRAMS := $(patsubst %,$(OUTPUT)%,$(ALL_TARGETS))
+
+KFUZZTEST_BRIDGE_IN := $(OUTPUT)kfuzztest-bridge-in.o
+KFUZZTEST_BRIDGE := $(OUTPUT)kfuzztest-bridge
+
+all: $(ALL_PROGRAMS)
+
+export srctree OUTPUT CC LD CFLAGS
+include $(srctree)/tools/build/Makefile.include
+
+$(KFUZZTEST_BRIDGE_IN): FORCE
+ $(Q)$(MAKE) $(build)=kfuzztest-bridge
+
+$(KFUZZTEST_BRIDGE): $(KFUZZTEST_BRIDGE_IN)
+ $(QUIET_LINK)$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
+
+clean:
+ rm -f $(ALL_PROGRAMS)
+ find $(or $(OUTPUT),.) -name '*.o' -delete -o -name '\.*.d' -delete -o -name '\.*.o.cmd' -delete
+
+install: $(ALL_PROGRAMS)
+ install -d -m 755 $(DESTDIR)$(bindir); \
+ for program in $(ALL_PROGRAMS); do \
+ install $$program $(DESTDIR)$(bindir); \
+ done
+
+FORCE:
+
+.PHONY: all install clean FORCE prepare
diff --git a/tools/kfuzztest-bridge/bridge.c b/tools/kfuzztest-bridge/bridge.c
new file mode 100644
index 000000000000..3815cb8fd6e7
--- /dev/null
+++ b/tools/kfuzztest-bridge/bridge.c
@@ -0,0 +1,93 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KFuzzTest tool for sending inputs into a KFuzzTest harness
+ *
+ * Copyright 2025 Google LLC
+ */
+
+#include <asm-generic/errno-base.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "byte_buffer.h"
+#include "encoder.h"
+#include "input_lexer.h"
+#include "input_parser.h"
+#include "rand_stream.h"
+
+static int invoke_kfuzztest_target(const char *target_name, const char *data, size_t data_size)
+{
+ ssize_t bytes_written;
+ char buf[256];
+ int ret;
+ int fd;
+
+ ret = snprintf(buf, sizeof(buf), "/sys/kernel/debug/kfuzztest/%s/input", target_name);
+ if (ret < 0)
+ return ret;
+
+ fd = openat(AT_FDCWD, buf, O_WRONLY, 0);
+ if (fd < 0)
+ return fd;
+
+ bytes_written = write(fd, (void *)data, data_size);
+ if (bytes_written < 0) {
+ close(fd);
+ return bytes_written;
+ }
+
+ if (close(fd) != 0)
+ return 1;
+ return 0;
+}
+
+static int invoke_one(const char *input_fmt, const char *fuzz_target, const char *input_filepath)
+{
+ struct ast_node *ast_prog;
+ struct byte_buffer *bb;
+ struct rand_stream *rs;
+ struct token **tokens;
+ size_t num_tokens;
+ size_t num_bytes;
+ int err;
+
+ err = tokenize(input_fmt, &tokens, &num_tokens);
+ if (err) {
+ printf("tokenization failed: %s\n", strerror(-err));
+ return err;
+ }
+
+ err = parse(tokens, num_tokens, &ast_prog);
+ if (err) {
+ printf("parsing failed: %s\n", strerror(-err));
+ return err;
+ }
+
+ rs = new_rand_stream(input_filepath, 1024);
+ err = encode(ast_prog, rs, &num_bytes, &bb);
+ if (err) {
+ printf("encoding failed: %s\n", strerror(-err));
+ return err;
+ }
+
+ err = invoke_kfuzztest_target(fuzz_target, bb->buffer, num_bytes);
+ if (err) {
+ printf("invocation failed: %s\n", strerror(-err));
+ return err;
+ }
+ destroy_byte_buffer(bb);
+ return err;
+}
+
+int main(int argc, char *argv[])
+{
+ if (argc != 4) {
+ printf("Usage: %s <input-description> <fuzz-target-name> <input-file>\n", argv[0]);
+ printf("For more detailed information see /Documentation/dev-tools/kfuzztest.rst\n");
+ return 1;
+ }
+
+ return invoke_one(argv[1], argv[2], argv[3]);
+}
diff --git a/tools/kfuzztest-bridge/byte_buffer.c b/tools/kfuzztest-bridge/byte_buffer.c
new file mode 100644
index 000000000000..fafb036fbeb1
--- /dev/null
+++ b/tools/kfuzztest-bridge/byte_buffer.c
@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A simple byte buffer implementation for encoding binary data
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <asm-generic/errno-base.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "byte_buffer.h"
+
+struct byte_buffer *new_byte_buffer(size_t initial_size)
+{
+ struct byte_buffer *ret;
+ size_t alloc_size = initial_size >= 8 ? initial_size : 8;
+
+ ret = malloc(sizeof(*ret));
+ if (!ret)
+ return NULL;
+
+ ret->alloc_size = alloc_size;
+ ret->buffer = malloc(alloc_size);
+ if (!ret->buffer) {
+ free(ret);
+ return NULL;
+ }
+ ret->num_bytes = 0;
+ return ret;
+}
+
+void destroy_byte_buffer(struct byte_buffer *buf)
+{
+ free(buf->buffer);
+ free(buf);
+}
+
+int append_bytes(struct byte_buffer *buf, const char *bytes, size_t num_bytes)
+{
+ size_t req_size;
+ size_t new_size;
+ char *new_ptr;
+
+ req_size = buf->num_bytes + num_bytes;
+ new_size = buf->alloc_size;
+
+ while (req_size > new_size)
+ new_size *= 2;
+ if (new_size != buf->alloc_size) {
+ new_ptr = realloc(buf->buffer, new_size);
+ if (!buf->buffer)
+ return -ENOMEM;
+ buf->buffer = new_ptr;
+ buf->alloc_size = new_size;
+ }
+ memcpy(buf->buffer + buf->num_bytes, bytes, num_bytes);
+ buf->num_bytes += num_bytes;
+ return 0;
+}
+
+int append_byte(struct byte_buffer *buf, char c)
+{
+ return append_bytes(buf, &c, 1);
+}
+
+int encode_le(struct byte_buffer *buf, uint64_t value, size_t byte_width)
+{
+ size_t i;
+ int ret;
+
+ for (i = 0; i < byte_width; ++i) {
+ if ((ret = append_byte(buf, (uint8_t)((value >> (i * 8)) & 0xFF)))) {
+ return ret;
+ }
+ }
+ return 0;
+}
+
+int pad(struct byte_buffer *buf, size_t num_padding)
+{
+ int ret;
+ size_t i;
+ for (i = 0; i < num_padding; i++)
+ if ((ret = append_byte(buf, 0)))
+ return ret;
+ return 0;
+}
diff --git a/tools/kfuzztest-bridge/byte_buffer.h b/tools/kfuzztest-bridge/byte_buffer.h
new file mode 100644
index 000000000000..6a31bfb5e78f
--- /dev/null
+++ b/tools/kfuzztest-bridge/byte_buffer.h
@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A simple byte buffer implementation for encoding binary data
+ *
+ * Copyright 2025 Google LLC
+ */
+#ifndef KFUZZTEST_BRIDGE_BYTE_BUFFER_H
+#define KFUZZTEST_BRIDGE_BYTE_BUFFER_H
+
+#include <stdint.h>
+#include <stdlib.h>
+
+struct byte_buffer {
+ char *buffer;
+ size_t num_bytes;
+ size_t alloc_size;
+};
+
+struct byte_buffer *new_byte_buffer(size_t initial_size);
+
+void destroy_byte_buffer(struct byte_buffer *buf);
+
+int append_bytes(struct byte_buffer *buf, const char *bytes, size_t num_bytes);
+
+int append_byte(struct byte_buffer *buf, char c);
+
+int encode_le(struct byte_buffer *buf, uint64_t value, size_t byte_width);
+
+int pad(struct byte_buffer *buf, size_t num_padding);
+
+#endif /* KFUZZTEST_BRIDGE_BYTE_BUFFER_H */
diff --git a/tools/kfuzztest-bridge/encoder.c b/tools/kfuzztest-bridge/encoder.c
new file mode 100644
index 000000000000..ceddd78ddbb5
--- /dev/null
+++ b/tools/kfuzztest-bridge/encoder.c
@@ -0,0 +1,356 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Encoder for KFuzzTest binary input format
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <asm-generic/errno-base.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "byte_buffer.h"
+#include "input_parser.h"
+#include "rand_stream.h"
+
+#define KFUZZTEST_MAGIC 0xBFACE
+#define KFUZZTEST_PROTO_VERSION 0
+/*
+ * The KFuzzTest binary input format requires at least 8 bytes of padding
+ * at the head and tail of every region.
+ */
+#define KFUZZTEST_POISON_SIZE 8
+
+#define BUFSIZE_SMALL 32
+#define BUFSIZE_LARGE 128
+
+struct region_info {
+ const char *name;
+ uint32_t offset;
+ uint32_t size;
+};
+
+struct reloc_info {
+ uint32_t src_reg;
+ uint32_t offset;
+ uint32_t dst_reg;
+};
+
+struct encoder_ctx {
+ struct byte_buffer *payload;
+ struct rand_stream *rand;
+
+ struct region_info *regions;
+ size_t num_regions;
+
+ struct reloc_info *relocations;
+ size_t num_relocations;
+
+ size_t reg_offset;
+ int curr_reg;
+};
+
+static void cleanup_ctx(struct encoder_ctx *ctx)
+{
+ if (ctx->regions)
+ free(ctx->regions);
+ if (ctx->relocations)
+ free(ctx->relocations);
+ if (ctx->payload)
+ destroy_byte_buffer(ctx->payload);
+}
+
+static int pad_payload(struct encoder_ctx *ctx, size_t amount)
+{
+ int ret;
+
+ if ((ret = pad(ctx->payload, amount)))
+ return ret;
+ ctx->reg_offset += amount;
+ return 0;
+}
+
+static int align_payload(struct encoder_ctx *ctx, size_t alignment)
+{
+ size_t pad_amount = ROUND_UP_TO_MULTIPLE(ctx->payload->num_bytes, alignment) - ctx->payload->num_bytes;
+ return pad_payload(ctx, pad_amount);
+}
+
+static int lookup_reg(struct encoder_ctx *ctx, const char *name)
+{
+ size_t i;
+
+ for (i = 0; i < ctx->num_regions; i++) {
+ if (strcmp(ctx->regions[i].name, name) == 0)
+ return i;
+ }
+ return -ENOENT;
+}
+
+static int add_reloc(struct encoder_ctx *ctx, struct reloc_info reloc)
+{
+ void *new_ptr = realloc(ctx->relocations, (ctx->num_relocations + 1) * sizeof(struct reloc_info));
+ if (!new_ptr)
+ return -ENOMEM;
+
+ ctx->relocations = new_ptr;
+ ctx->relocations[ctx->num_relocations] = reloc;
+ ctx->num_relocations++;
+ return 0;
+}
+
+static int build_region_map(struct encoder_ctx *ctx, struct ast_node *top_level)
+{
+ struct ast_program *prog;
+ struct ast_node *reg;
+ size_t i;
+
+ if (top_level->type != NODE_PROGRAM)
+ return -EINVAL;
+
+ prog = &top_level->data.program;
+ ctx->regions = malloc(prog->num_members * sizeof(struct region_info));
+ if (!ctx->regions)
+ return -ENOMEM;
+
+ ctx->num_regions = prog->num_members;
+ for (i = 0; i < ctx->num_regions; i++) {
+ reg = prog->members[i];
+ /* Offset can only be determined after the second pass. */
+ ctx->regions[i] = (struct region_info){
+ .name = reg->data.region.name,
+ .size = node_size(reg),
+ };
+ }
+ return 0;
+}
+/**
+ * Encodes a value node as little-endian. A value node is one that has no
+ * children, and can therefore be directly written into the payload.
+ */
+static int encode_value_le(struct encoder_ctx *ctx, struct ast_node *node)
+{
+ size_t array_size;
+ char rand_char;
+ size_t length;
+ size_t i;
+ int reg;
+ int ret;
+
+ switch (node->type) {
+ case NODE_ARRAY:
+ array_size = node->data.array.num_elems * node->data.array.elem_size;
+ for (i = 0; i < array_size; i++) {
+ if ((ret = next_byte(ctx->rand, &rand_char)))
+ return ret;
+ if ((ret = append_byte(ctx->payload, rand_char)))
+ return ret;
+ }
+ ctx->reg_offset += array_size;
+ if (node->data.array.null_terminated) {
+ if ((ret = pad_payload(ctx, 1)))
+ return ret;
+ ctx->reg_offset++;
+ }
+ break;
+ case NODE_LENGTH:
+ reg = lookup_reg(ctx, node->data.length.length_of);
+ if (reg < 0)
+ return reg;
+ length = ctx->regions[reg].size;
+ if ((ret = encode_le(ctx->payload, length, node->data.length.byte_width)))
+ return ret;
+ ctx->reg_offset += node->data.length.byte_width;
+ break;
+ case NODE_PRIMITIVE:
+ for (i = 0; i < node->data.primitive.byte_width; i++) {
+ if ((ret = next_byte(ctx->rand, &rand_char)))
+ return ret;
+ if ((ret = append_byte(ctx->payload, rand_char)))
+ return ret;
+ }
+ ctx->reg_offset += node->data.primitive.byte_width;
+ break;
+ case NODE_POINTER:
+ reg = lookup_reg(ctx, node->data.pointer.points_to);
+ if (reg < 0)
+ return reg;
+ if ((ret = add_reloc(ctx, (struct reloc_info){ .src_reg = ctx->curr_reg,
+ .offset = ctx->reg_offset,
+ .dst_reg = reg })))
+ return ret;
+ /* Placeholder pointer value, as pointers are patched by KFuzzTest anyways. */
+ if ((ret = encode_le(ctx->payload, UINTPTR_MAX, sizeof(uintptr_t))))
+ return ret;
+ ctx->reg_offset += sizeof(uintptr_t);
+ break;
+ case NODE_PROGRAM:
+ case NODE_REGION:
+ default:
+ return -EINVAL;
+ }
+ return 0;
+}
+
+static int encode_region(struct encoder_ctx *ctx, struct ast_region *reg)
+{
+ struct ast_node *child;
+ size_t i;
+ int ret;
+
+ ctx->reg_offset = 0;
+ for (i = 0; i < reg->num_members; i++) {
+ child = reg->members[i];
+ if ((ret = align_payload(ctx, node_alignment(child))))
+ return ret;
+ if ((ret = encode_value_le(ctx, child)))
+ return ret;
+ }
+ return 0;
+}
+
+static int encode_payload(struct encoder_ctx *ctx, struct ast_node *top_level)
+{
+ struct ast_node *reg;
+ size_t i;
+ int ret;
+
+ for (i = 0; i < ctx->num_regions; i++) {
+ reg = top_level->data.program.members[i];
+ if ((ret = align_payload(ctx, node_alignment(reg))))
+ return ret;
+
+ ctx->curr_reg = i;
+ ctx->regions[i].offset = ctx->payload->num_bytes;
+ if ((ret = encode_region(ctx, ®->data.region)))
+ return ret;
+ if ((ret = pad_payload(ctx, KFUZZTEST_POISON_SIZE)))
+ return ret;
+ }
+ return 0;
+}
+
+static int encode_region_array(struct encoder_ctx *ctx, struct byte_buffer **ret)
+{
+ struct byte_buffer *reg_array;
+ struct region_info info;
+ int retcode;
+ size_t i;
+
+ reg_array = new_byte_buffer(BUFSIZE_SMALL);
+ if (!reg_array)
+ return -ENOMEM;
+
+ if ((retcode = encode_le(reg_array, ctx->num_regions, sizeof(uint32_t))))
+ goto fail;
+
+ for (i = 0; i < ctx->num_regions; i++) {
+ info = ctx->regions[i];
+ if ((retcode = encode_le(reg_array, info.offset, sizeof(uint32_t))))
+ goto fail;
+ if ((retcode = encode_le(reg_array, info.size, sizeof(uint32_t))))
+ goto fail;
+ }
+ *ret = reg_array;
+ return 0;
+
+fail:
+ destroy_byte_buffer(reg_array);
+ return retcode;
+}
+
+static int encode_reloc_table(struct encoder_ctx *ctx, size_t padding_amount, struct byte_buffer **ret)
+{
+ struct byte_buffer *reloc_table;
+ struct reloc_info info;
+ int retcode;
+ size_t i;
+
+ reloc_table = new_byte_buffer(BUFSIZE_SMALL);
+ if (!reloc_table)
+ return -ENOMEM;
+
+ if ((retcode = encode_le(reloc_table, ctx->num_relocations, sizeof(uint32_t))) ||
+ (retcode = encode_le(reloc_table, padding_amount, sizeof(uint32_t))))
+ goto fail;
+
+ for (i = 0; i < ctx->num_relocations; i++) {
+ info = ctx->relocations[i];
+ if ((retcode = encode_le(reloc_table, info.src_reg, sizeof(uint32_t))) ||
+ (retcode = encode_le(reloc_table, info.offset, sizeof(uint32_t))) ||
+ (retcode = encode_le(reloc_table, info.dst_reg, sizeof(uint32_t))))
+ goto fail;
+ }
+ pad(reloc_table, padding_amount);
+ *ret = reloc_table;
+ return 0;
+
+fail:
+ destroy_byte_buffer(reloc_table);
+ return retcode;
+}
+
+static size_t reloc_table_size(struct encoder_ctx *ctx)
+{
+ return 2 * sizeof(uint32_t) + 3 * ctx->num_relocations * sizeof(uint32_t);
+}
+
+int encode(struct ast_node *top_level, struct rand_stream *r, size_t *num_bytes, struct byte_buffer **ret)
+{
+ struct byte_buffer *region_array = NULL;
+ struct byte_buffer *final_buffer = NULL;
+ struct byte_buffer *reloc_table = NULL;
+ size_t header_size;
+ int alignment;
+ int retcode;
+
+ struct encoder_ctx ctx = { 0 };
+ if ((retcode = build_region_map(&ctx, top_level)))
+ goto fail;
+
+ ctx.rand = r;
+ ctx.payload = new_byte_buffer(32);
+ if (!ctx.payload) {
+ retcode = -ENOMEM;
+ goto fail;
+ }
+ if ((retcode = encode_payload(&ctx, top_level)))
+ goto fail;
+
+ if ((retcode = encode_region_array(&ctx, ®ion_array)))
+ goto fail;
+
+ header_size = sizeof(uint64_t) + region_array->num_bytes + reloc_table_size(&ctx);
+ alignment = node_alignment(top_level);
+ if ((retcode = encode_reloc_table(
+ &ctx, ROUND_UP_TO_MULTIPLE(header_size + KFUZZTEST_POISON_SIZE, alignment) - header_size,
+ &reloc_table)))
+ goto fail;
+
+ final_buffer = new_byte_buffer(BUFSIZE_LARGE);
+ if (!final_buffer) {
+ retcode = -ENOMEM;
+ goto fail;
+ }
+
+ if ((retcode = encode_le(final_buffer, KFUZZTEST_MAGIC, sizeof(uint32_t))) ||
+ (retcode = encode_le(final_buffer, KFUZZTEST_PROTO_VERSION, sizeof(uint32_t))) ||
+ (retcode = append_bytes(final_buffer, region_array->buffer, region_array->num_bytes)) ||
+ (retcode = append_bytes(final_buffer, reloc_table->buffer, reloc_table->num_bytes)) ||
+ (retcode = append_bytes(final_buffer, ctx.payload->buffer, ctx.payload->num_bytes))) {
+ destroy_byte_buffer(final_buffer);
+ goto fail;
+ }
+
+ *num_bytes = final_buffer->num_bytes;
+ *ret = final_buffer;
+
+fail:
+ if (region_array)
+ destroy_byte_buffer(region_array);
+ if (reloc_table)
+ destroy_byte_buffer(reloc_table);
+ cleanup_ctx(&ctx);
+ return retcode;
+}
diff --git a/tools/kfuzztest-bridge/encoder.h b/tools/kfuzztest-bridge/encoder.h
new file mode 100644
index 000000000000..73f8c4b7893c
--- /dev/null
+++ b/tools/kfuzztest-bridge/encoder.h
@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Encoder for KFuzzTest binary input format
+ *
+ * Copyright 2025 Google LLC
+ */
+#ifndef KFUZZTEST_BRIDGE_ENCODER_H
+#define KFUZZTEST_BRIDGE_ENCODER_H
+
+#include "input_parser.h"
+#include "rand_stream.h"
+#include "byte_buffer.h"
+
+int encode(struct ast_node *top_level, struct rand_stream *r, size_t *num_bytes, struct byte_buffer **ret);
+
+#endif /* KFUZZTEST_BRIDGE_ENCODER_H */
diff --git a/tools/kfuzztest-bridge/input_lexer.c b/tools/kfuzztest-bridge/input_lexer.c
new file mode 100644
index 000000000000..19b6678b0a0e
--- /dev/null
+++ b/tools/kfuzztest-bridge/input_lexer.c
@@ -0,0 +1,243 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Parser for KFuzzTest textual input format
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <asm-generic/errno-base.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "input_lexer.h"
+
+struct keyword_map {
+ const char *keyword;
+ enum token_type type;
+};
+
+static struct keyword_map keywords[] = {
+ { "ptr", TOKEN_KEYWORD_PTR }, { "arr", TOKEN_KEYWORD_ARR }, { "len", TOKEN_KEYWORD_LEN },
+ { "str", TOKEN_KEYWORD_STR }, { "u8", TOKEN_KEYWORD_U8 }, { "u16", TOKEN_KEYWORD_U16 },
+ { "u32", TOKEN_KEYWORD_U32 }, { "u64", TOKEN_KEYWORD_U64 },
+};
+
+static struct token *make_token(enum token_type type)
+{
+ struct token *ret = calloc(1, sizeof(*ret));
+ ret->type = type;
+ return ret;
+}
+
+struct lexer {
+ const char *start;
+ const char *current;
+};
+
+static char advance(struct lexer *l)
+{
+ l->current++;
+ return l->current[-1];
+}
+
+static void retreat(struct lexer *l)
+{
+ l->current--;
+}
+
+static char peek(struct lexer *l)
+{
+ return *l->current;
+}
+
+static bool is_digit(char c)
+{
+ return c >= '0' && c <= '9';
+}
+
+static bool is_alpha(char c)
+{
+ return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z');
+}
+
+static bool is_whitespace(char c)
+{
+ switch (c) {
+ case ' ':
+ case '\r':
+ case '\t':
+ case '\n':
+ return true;
+ default:
+ return false;
+ }
+}
+
+static void skip_whitespace(struct lexer *l)
+{
+ for (;;) {
+ if (is_whitespace(peek(l))) {
+ advance(l);
+ } else {
+ return;
+ }
+ }
+}
+
+static struct token *number(struct lexer *l)
+{
+ struct token *tok;
+ uint64_t value;
+ while (is_digit(peek(l)))
+ advance(l);
+ value = strtoull(l->start, NULL, 10);
+ tok = make_token(TOKEN_INTEGER);
+ tok->data.integer = value;
+ return tok;
+}
+
+static enum token_type check_keyword(struct lexer *l, const char *keyword, enum token_type type)
+{
+ size_t len = strlen(keyword);
+
+ if (((size_t)(l->current - l->start) == len) && strncmp(l->start, keyword, len) == 0)
+ return type;
+ return TOKEN_IDENTIFIER;
+}
+
+static struct token *identifier(struct lexer *l)
+{
+ enum token_type type = TOKEN_IDENTIFIER;
+ struct token *tok;
+ size_t i;
+
+ while (is_digit(peek(l)) || is_alpha(peek(l)) || peek(l) == '_')
+ advance(l);
+
+ for (i = 0; i < ARRAY_SIZE(keywords); i++) {
+ if (check_keyword(l, keywords[i].keyword, keywords[i].type) != TOKEN_IDENTIFIER) {
+ type = keywords[i].type;
+ break;
+ }
+ }
+
+ tok = make_token(type);
+ if (!tok)
+ return NULL;
+ if (type == TOKEN_IDENTIFIER) {
+ tok->data.identifier.start = l->start;
+ tok->data.identifier.length = l->current - l->start;
+ }
+ return tok;
+}
+
+static struct token *scan_token(struct lexer *l)
+{
+ char c;
+ skip_whitespace(l);
+
+ l->start = l->current;
+ c = peek(l);
+
+ if (c == '\0')
+ return make_token(TOKEN_EOF);
+
+ advance(l);
+ switch (c) {
+ case '{':
+ return make_token(TOKEN_LBRACE);
+ case '}':
+ return make_token(TOKEN_RBRACE);
+ case '[':
+ return make_token(TOKEN_LBRACKET);
+ case ']':
+ return make_token(TOKEN_RBRACKET);
+ case ',':
+ return make_token(TOKEN_COMMA);
+ case ';':
+ return make_token(TOKEN_SEMICOLON);
+ default:
+ retreat(l);
+ if (is_digit(c))
+ return number(l);
+ if (is_alpha(c) || c == '_')
+ return identifier(l);
+ return make_token(TOKEN_ERROR);
+ }
+}
+
+int primitive_byte_width(enum token_type type)
+{
+ switch (type) {
+ case TOKEN_KEYWORD_U8:
+ return 1;
+ case TOKEN_KEYWORD_U16:
+ return 2;
+ case TOKEN_KEYWORD_U32:
+ return 4;
+ case TOKEN_KEYWORD_U64:
+ return 8;
+ default:
+ return 0;
+ }
+}
+
+int tokenize(const char *input, struct token ***tokens, size_t *num_tokens)
+{
+ struct lexer l = { .start = input, .current = input };
+ struct token **ret_tokens;
+ size_t token_arr_size;
+ size_t token_count;
+ struct token *tok;
+ void *tmp;
+ size_t i;
+ int err;
+
+ token_arr_size = 128;
+ ret_tokens = calloc(token_arr_size, sizeof(struct token *));
+ if (!ret_tokens)
+ return -ENOMEM;
+
+ token_count = 0;
+ do {
+ tok = scan_token(&l);
+ if (!tok) {
+ err = -ENOMEM;
+ goto failure;
+ }
+
+ if (token_count == token_arr_size) {
+ token_arr_size *= 2;
+ tmp = realloc(ret_tokens, token_arr_size);
+ if (!tmp) {
+ err = -ENOMEM;
+ goto failure;
+ }
+ ret_tokens = tmp;
+ }
+
+ ret_tokens[token_count] = tok;
+ if (tok->type == TOKEN_ERROR) {
+ err = -EINVAL;
+ goto failure;
+ }
+ token_count++;
+ } while (tok->type != TOKEN_EOF);
+
+ *tokens = ret_tokens;
+ *num_tokens = token_count;
+ return 0;
+
+failure:
+ for (i = 0; i < token_count; i++)
+ free(ret_tokens[i]);
+ free(ret_tokens);
+ return err;
+}
+
+bool is_primitive(struct token *tok)
+{
+ return tok->type >= TOKEN_KEYWORD_U8 && tok->type <= TOKEN_KEYWORD_U64;
+}
diff --git a/tools/kfuzztest-bridge/input_lexer.h b/tools/kfuzztest-bridge/input_lexer.h
new file mode 100644
index 000000000000..bdc55e08a3eb
--- /dev/null
+++ b/tools/kfuzztest-bridge/input_lexer.h
@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Lexer for KFuzzTest textual input format
+ *
+ * Copyright 2025 Google LLC
+ */
+#ifndef KFUZZTEST_BRIDGE_INPUT_LEXER_H
+#define KFUZZTEST_BRIDGE_INPUT_LEXER_H
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof(x[0]))
+
+enum token_type {
+ TOKEN_LBRACE,
+ TOKEN_RBRACE,
+ TOKEN_LBRACKET,
+ TOKEN_RBRACKET,
+ TOKEN_COMMA,
+ TOKEN_SEMICOLON,
+
+ TOKEN_KEYWORD_PTR,
+ TOKEN_KEYWORD_ARR,
+ TOKEN_KEYWORD_LEN,
+ TOKEN_KEYWORD_STR,
+ TOKEN_KEYWORD_U8,
+ TOKEN_KEYWORD_U16,
+ TOKEN_KEYWORD_U32,
+ TOKEN_KEYWORD_U64,
+
+ TOKEN_IDENTIFIER,
+ TOKEN_INTEGER,
+
+ TOKEN_EOF,
+ TOKEN_ERROR,
+};
+
+struct token {
+ enum token_type type;
+ union {
+ uint64_t integer;
+ struct {
+ const char *start;
+ size_t length;
+ } identifier;
+ } data;
+ int position;
+};
+
+int tokenize(const char *input, struct token ***tokens, size_t *num_tokens);
+
+bool is_primitive(struct token *tok);
+int primitive_byte_width(enum token_type type);
+
+#endif /* KFUZZTEST_BRIDGE_INPUT_LEXER_H */
diff --git a/tools/kfuzztest-bridge/input_parser.c b/tools/kfuzztest-bridge/input_parser.c
new file mode 100644
index 000000000000..7320dac8aa17
--- /dev/null
+++ b/tools/kfuzztest-bridge/input_parser.c
@@ -0,0 +1,373 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Parser for KFuzzTest textual input format
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <asm-generic/errno-base.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "input_lexer.h"
+#include "input_parser.h"
+
+#define MAX(a, b) ((a) > (b) ? (a) : (b))
+
+static struct token *peek(struct parser *p)
+{
+ return p->tokens[p->curr_token];
+}
+
+static struct token *advance(struct parser *p)
+{
+ struct token *tok = peek(p);
+ p->curr_token++;
+ return tok;
+}
+
+static struct token *consume(struct parser *p, enum token_type type, const char *err_msg)
+{
+ if (peek(p)->type != type) {
+ printf("parser failure: %s\n", err_msg);
+ return NULL;
+ }
+ return advance(p);
+}
+
+static bool match(struct parser *p, enum token_type t)
+{
+ struct token *tok = peek(p);
+ return tok->type == t;
+}
+
+static int parse_primitive(struct parser *p, struct ast_node **node_ret)
+{
+ struct ast_node *ret;
+ struct token *tok;
+ int byte_width;
+
+ tok = advance(p);
+ byte_width = primitive_byte_width(tok->type);
+ if (!byte_width)
+ return -EINVAL;
+
+ ret = malloc(sizeof(*ret));
+ if (!ret)
+ return -ENOMEM;
+
+ ret->type = NODE_PRIMITIVE;
+ ret->data.primitive.byte_width = byte_width;
+ *node_ret = ret;
+ return 0;
+}
+
+static int parse_ptr(struct parser *p, struct ast_node **node_ret)
+{
+ const char *points_to;
+ struct ast_node *ret;
+ struct token *tok;
+ if (!consume(p, TOKEN_KEYWORD_PTR, "expected 'ptr'"))
+ return -EINVAL;
+ if (!consume(p, TOKEN_LBRACKET, "expected '['"))
+ return -EINVAL;
+
+ tok = consume(p, TOKEN_IDENTIFIER, "expected identifier");
+ if (!tok)
+ return -EINVAL;
+
+ if (!consume(p, TOKEN_RBRACKET, "expected ']'"))
+ return -EINVAL;
+
+ ret = malloc(sizeof(*ret));
+ ret->type = NODE_POINTER;
+
+ points_to = strndup(tok->data.identifier.start, tok->data.identifier.length);
+ if (!points_to) {
+ free(ret);
+ return -EINVAL;
+ }
+
+ ret->data.pointer.points_to = points_to;
+ *node_ret = ret;
+ return 0;
+}
+
+static int parse_arr(struct parser *p, struct ast_node **node_ret)
+{
+ struct token *type, *num_elems;
+ struct ast_node *ret;
+
+ if (!consume(p, TOKEN_KEYWORD_ARR, "expected 'arr'") || !consume(p, TOKEN_LBRACKET, "expected '['"))
+ return -EINVAL;
+
+ type = advance(p);
+ if (!is_primitive(type))
+ return -EINVAL;
+
+ if (!consume(p, TOKEN_COMMA, "expected ','"))
+ return -EINVAL;
+
+ num_elems = consume(p, TOKEN_INTEGER, "expected integer");
+ if (!num_elems)
+ return -EINVAL;
+
+ if (!consume(p, TOKEN_RBRACKET, "expected ']'"))
+ return -EINVAL;
+
+ ret = malloc(sizeof(*ret));
+ if (!ret)
+ return -ENOMEM;
+
+ ret->type = NODE_ARRAY;
+ ret->data.array.num_elems = num_elems->data.integer;
+ ret->data.array.elem_size = primitive_byte_width(type->type);
+ ret->data.array.null_terminated = false;
+ *node_ret = ret;
+ return 0;
+}
+
+static int parse_str(struct parser *p, struct ast_node **node_ret)
+{
+ struct ast_node *ret;
+ struct token *len;
+
+ if (!consume(p, TOKEN_KEYWORD_STR, "expected 'str'") || !consume(p, TOKEN_LBRACKET, "expected '['"))
+ return -EINVAL;
+
+ len = consume(p, TOKEN_INTEGER, "expected integer");
+ if (!len)
+ return -EINVAL;
+
+ if (!consume(p, TOKEN_RBRACKET, "expected ']'"))
+ return -EINVAL;
+
+ ret = malloc(sizeof(*ret));
+ if (!ret)
+ return -ENOMEM;
+
+ /* A string is the susbet of byte arrays that are null-terminated. */
+ ret->type = NODE_ARRAY;
+ ret->data.array.num_elems = len->data.integer;
+ ret->data.array.elem_size = sizeof(char);
+ ret->data.array.null_terminated = true;
+ *node_ret = ret;
+ return 0;
+}
+
+static int parse_len(struct parser *p, struct ast_node **node_ret)
+{
+ struct token *type, *len;
+ struct ast_node *ret;
+
+ if (!consume(p, TOKEN_KEYWORD_LEN, "expected 'len'") || !consume(p, TOKEN_LBRACKET, "expected '['"))
+ return -EINVAL;
+
+ len = advance(p);
+ if (len->type != TOKEN_IDENTIFIER)
+ return -EINVAL;
+
+ if (!consume(p, TOKEN_COMMA, "expected ','"))
+ return -EINVAL;
+
+ type = advance(p);
+ if (!is_primitive(type))
+ return -EINVAL;
+
+ if (!consume(p, TOKEN_RBRACKET, "expected ']'"))
+ return -EINVAL;
+
+ ret = malloc(sizeof(*ret));
+ if (!ret)
+ return -ENOMEM;
+ ret->type = NODE_LENGTH;
+ ret->data.length.length_of = strndup(len->data.identifier.start, len->data.identifier.length);
+ ret->data.length.byte_width = primitive_byte_width(type->type);
+
+ *node_ret = ret;
+ return 0;
+}
+
+static int parse_type(struct parser *p, struct ast_node **node_ret)
+{
+ if (is_primitive(peek(p)))
+ return parse_primitive(p, node_ret);
+
+ if (peek(p)->type == TOKEN_KEYWORD_PTR)
+ return parse_ptr(p, node_ret);
+
+ if (peek(p)->type == TOKEN_KEYWORD_ARR)
+ return parse_arr(p, node_ret);
+
+ if (peek(p)->type == TOKEN_KEYWORD_STR)
+ return parse_str(p, node_ret);
+
+ if (peek(p)->type == TOKEN_KEYWORD_LEN)
+ return parse_len(p, node_ret);
+
+ return -EINVAL;
+}
+
+static int parse_region(struct parser *p, struct ast_node **node_ret)
+{
+ struct token *tok, *identifier;
+ struct ast_region *region;
+ struct ast_node *node;
+ struct ast_node *ret;
+ size_t i;
+ int err;
+
+ identifier = consume(p, TOKEN_IDENTIFIER, "expected identifier");
+ if (!identifier)
+ return -EINVAL;
+
+ ret = malloc(sizeof(*ret));
+ if (!ret)
+ return -ENOMEM;
+
+ tok = consume(p, TOKEN_LBRACE, "expected '{'");
+ if (!tok) {
+ err = -EINVAL;
+ goto fail_early;
+ }
+
+ region = &ret->data.region;
+ region->name = strndup(identifier->data.identifier.start, identifier->data.identifier.length);
+ if (!region->name) {
+ err = -ENOMEM;
+ goto fail_early;
+ }
+
+ region->num_members = 0;
+ while (!match(p, TOKEN_RBRACE)) {
+ err = parse_type(p, &node);
+ if (err)
+ goto fail;
+ region->members = realloc(region->members, ++region->num_members * sizeof(struct ast_node *));
+ region->members[region->num_members - 1] = node;
+ }
+
+ if (!consume(p, TOKEN_RBRACE, "expected '}'") || !consume(p, TOKEN_SEMICOLON, "expected ';'")) {
+ err = -EINVAL;
+ goto fail;
+ }
+
+ ret->type = NODE_REGION;
+ *node_ret = ret;
+ return 0;
+
+fail:
+ for (i = 0; i < region->num_members; i++)
+ free(region->members[i]);
+ free((void *)region->name);
+ free(region->members);
+fail_early:
+ free(ret);
+ return err;
+}
+
+static int parse_program(struct parser *p, struct ast_node **node_ret)
+{
+ struct ast_program *prog;
+ struct ast_node *reg;
+ struct ast_node *ret;
+ void *new_ptr;
+ size_t i;
+ int err;
+
+ ret = malloc(sizeof(*ret));
+ if (!ret)
+ return -ENOMEM;
+ ret->type = NODE_PROGRAM;
+
+ prog = &ret->data.program;
+ prog->num_members = 0;
+ prog->members = NULL;
+ while (!match(p, TOKEN_EOF)) {
+ err = parse_region(p, ®);
+ if (err)
+ goto fail;
+
+ new_ptr = realloc(prog->members, ++prog->num_members * sizeof(struct ast_node *));
+ if (!new_ptr) {
+ err = -ENOMEM;
+ goto fail;
+ }
+ prog->members = new_ptr;
+ prog->members[prog->num_members - 1] = reg;
+ }
+
+ *node_ret = ret;
+ return 0;
+
+fail:
+ for (i = 0; i < prog->num_members; i++)
+ free(prog->members[i]);
+ free(prog->members);
+ free(ret);
+ return err;
+}
+
+size_t node_alignment(struct ast_node *node)
+{
+ size_t max_alignment = 1;
+ size_t i;
+
+ switch (node->type) {
+ case NODE_PROGRAM:
+ for (i = 0; i < node->data.program.num_members; i++)
+ max_alignment = MAX(max_alignment, node_alignment(node->data.program.members[i]));
+ return max_alignment;
+ case NODE_REGION:
+ for (i = 0; i < node->data.region.num_members; i++)
+ max_alignment = MAX(max_alignment, node_alignment(node->data.region.members[i]));
+ return max_alignment;
+ case NODE_ARRAY:
+ return node->data.array.elem_size;
+ case NODE_LENGTH:
+ return node->data.length.byte_width;
+ case NODE_PRIMITIVE:
+ /* Primitives are aligned to their size. */
+ return node->data.primitive.byte_width;
+ case NODE_POINTER:
+ return sizeof(uintptr_t);
+ }
+
+ /* Anything should be at least 1-byte-aligned. */
+ return 1;
+}
+
+size_t node_size(struct ast_node *node)
+{
+ size_t total = 0;
+ size_t i;
+
+ switch (node->type) {
+ case NODE_PROGRAM:
+ for (i = 0; i < node->data.program.num_members; i++)
+ total += node_size(node->data.program.members[i]);
+ return total;
+ case NODE_REGION:
+ for (i = 0; i < node->data.region.num_members; i++) {
+ /* Account for padding within region. */
+ total = ROUND_UP_TO_MULTIPLE(total, node_alignment(node->data.region.members[i]));
+ total += node_size(node->data.region.members[i]);
+ }
+ return total;
+ case NODE_ARRAY:
+ return node->data.array.elem_size * node->data.array.num_elems +
+ (node->data.array.null_terminated ? 1 : 0);
+ case NODE_LENGTH:
+ return node->data.length.byte_width;
+ case NODE_PRIMITIVE:
+ return node->data.primitive.byte_width;
+ case NODE_POINTER:
+ return sizeof(uintptr_t);
+ }
+ return 0;
+}
+
+int parse(struct token **tokens, size_t token_count, struct ast_node **node_ret)
+{
+ struct parser p = { .tokens = tokens, .token_count = token_count, .curr_token = 0 };
+ return parse_program(&p, node_ret);
+}
diff --git a/tools/kfuzztest-bridge/input_parser.h b/tools/kfuzztest-bridge/input_parser.h
new file mode 100644
index 000000000000..c51241617389
--- /dev/null
+++ b/tools/kfuzztest-bridge/input_parser.h
@@ -0,0 +1,79 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Parser for KFuzzTest textual input format
+ *
+ * Copyright 2025 Google LLC
+ */
+#ifndef KFUZZTEST_BRIDGE_INPUT_PARSER_H
+#define KFUZZTEST_BRIDGE_INPUT_PARSER_H
+
+#include <stdlib.h>
+
+/* Rounds x up to the nearest multiple of n. */
+#define ROUND_UP_TO_MULTIPLE(x, n) (((n) == 0) ? (0) : (((x) + (n) - 1) / (n)) * (n))
+
+enum ast_node_type {
+ NODE_PROGRAM,
+ NODE_REGION,
+ NODE_ARRAY,
+ NODE_LENGTH,
+ NODE_PRIMITIVE,
+ NODE_POINTER,
+};
+
+struct ast_node; /* Forward declaration. */
+
+struct ast_program {
+ struct ast_node **members;
+ size_t num_members;
+};
+
+struct ast_region {
+ const char *name;
+ struct ast_node **members;
+ size_t num_members;
+};
+
+struct ast_array {
+ int elem_size;
+ int null_terminated; /* True iff the array should always end with 0. */
+ size_t num_elems;
+};
+
+struct ast_length {
+ size_t byte_width;
+ const char *length_of;
+};
+
+struct ast_primitive {
+ size_t byte_width;
+};
+
+struct ast_pointer {
+ const char *points_to;
+};
+
+struct ast_node {
+ enum ast_node_type type;
+ union {
+ struct ast_program program;
+ struct ast_region region;
+ struct ast_array array;
+ struct ast_length length;
+ struct ast_primitive primitive;
+ struct ast_pointer pointer;
+ } data;
+};
+
+struct parser {
+ struct token **tokens;
+ size_t token_count;
+ size_t curr_token;
+};
+
+int parse(struct token **tokens, size_t token_count, struct ast_node **node_ret);
+
+size_t node_size(struct ast_node *node);
+size_t node_alignment(struct ast_node *node);
+
+#endif /* KFUZZTEST_BRIDGE_INPUT_PARSER_H */
diff --git a/tools/kfuzztest-bridge/rand_stream.c b/tools/kfuzztest-bridge/rand_stream.c
new file mode 100644
index 000000000000..0b2965fa2c73
--- /dev/null
+++ b/tools/kfuzztest-bridge/rand_stream.c
@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Implements a cached file-reader for iterating over a byte stream of
+ * pseudo-random data
+ *
+ * Copyright 2025 Google LLC
+ */
+#include "rand_stream.h"
+
+static int refill(struct rand_stream *rs)
+{
+ size_t ret = fread(rs->buffer, sizeof(char), rs->buffer_size, rs->source);
+ rs->buffer_pos = 0;
+ if (ret != rs->buffer_size)
+ return -1;
+ return 0;
+}
+
+struct rand_stream *new_rand_stream(const char *path_to_file, size_t cache_size)
+{
+ struct rand_stream *rs;
+
+ rs = malloc(sizeof(*rs));
+ if (!rs)
+ return NULL;
+
+ rs->source = fopen(path_to_file, "rb");
+ if (!rs->source) {
+ free(rs);
+ return NULL;
+ }
+
+ rs->buffer = malloc(cache_size);
+ if (!rs->buffer) {
+ fclose(rs->source);
+ free(rs);
+ return NULL;
+ }
+
+ rs->buffer_size = cache_size;
+ if (refill(rs)) {
+ free(rs->buffer);
+ fclose(rs->source);
+ free(rs);
+ return NULL;
+ }
+
+ return rs;
+}
+
+int next_byte(struct rand_stream *rs, char *ret)
+{
+ int res;
+ if (rs->buffer_pos == rs->buffer_size) {
+ res = refill(rs);
+ if (res)
+ return res;
+ }
+ *ret = rs->buffer[rs->buffer_pos++];
+ return 0;
+}
diff --git a/tools/kfuzztest-bridge/rand_stream.h b/tools/kfuzztest-bridge/rand_stream.h
new file mode 100644
index 000000000000..3367376758a5
--- /dev/null
+++ b/tools/kfuzztest-bridge/rand_stream.h
@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Implements a cached file-reader for iterating over a byte stream of
+ * pseudo-random data
+ *
+ * Copyright 2025 Google LLC
+ */
+#ifndef KFUZZTEST_BRIDGE_RAND_STREAM_H
+#define KFUZZTEST_BRIDGE_RAND_STREAM_H
+
+#include <stdlib.h>
+#include <stdio.h>
+
+/**
+ * struct rand_stream - a cached bytestream reader
+ *
+ * Reads and returns bytes from a file, using cached pre-fetching to amortize
+ * the cost of reads.
+ */
+struct rand_stream {
+ FILE *source;
+ char *buffer;
+ size_t buffer_size;
+ size_t buffer_pos;
+};
+
+/**
+ * new_rand_stream - return a new struct rand_stream
+ *
+ * @path_to_file: source of the output byte stream.
+ * @cache_size: size of the read-ahead cache in bytes.
+ */
+struct rand_stream *new_rand_stream(const char *path_to_file, size_t cache_size);
+
+/**
+ * next_byte - return the next byte from a struct rand_stream
+ *
+ * @rs: an initialized struct rand_stream.
+ * @ret: return pointer.
+ *
+ * @return 0 on success or a negative value on failure.
+ *
+ */
+int next_byte(struct rand_stream *rs, char *ret);
+
+#endif /* KFUZZTEST_BRIDGE_RAND_STREAM_H */
--
2.51.0.318.gd7df087d1a-goog
^ permalink raw reply related [flat|nested] 15+ messages in thread
* [PATCH v2 RFC 5/7] kfuzztest: add ReST documentation
2025-09-01 16:42 [PATCH v2 RFC 0/7] KFuzzTest: a new kernel fuzzing framework Ethan Graham
` (3 preceding siblings ...)
2025-09-01 16:42 ` [PATCH v2 RFC 4/7] tools: add kfuzztest-bridge utility Ethan Graham
@ 2025-09-01 16:42 ` Ethan Graham
2025-09-01 16:42 ` [PATCH v2 RFC 6/7] kfuzztest: add KFuzzTest sample fuzz targets Ethan Graham
2025-09-01 16:42 ` [PATCH v2 RFC 7/7] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing Ethan Graham
6 siblings, 0 replies; 15+ messages in thread
From: Ethan Graham @ 2025-09-01 16:42 UTC (permalink / raw)
To: ethangraham, glider
Cc: andreyknvl, brendan.higgins, davidgow, dvyukov, jannh, elver,
rmoar, shuah, tarasmadan, kasan-dev, kunit-dev, linux-kernel,
linux-mm, dhowells, lukas, ignat, herbert, davem, linux-crypto
From: Ethan Graham <ethangraham@google.com>
Add Documentation/dev-tools/kfuzztest.rst and reference it in the
dev-tools index.
Signed-off-by: Ethan Graham <ethangraham@google.com>
---
v2:
- Add documentation for kfuzztest-bridge tool introduced in patch 4.
---
---
Documentation/dev-tools/index.rst | 1 +
Documentation/dev-tools/kfuzztest.rst | 371 ++++++++++++++++++++++++++
2 files changed, 372 insertions(+)
create mode 100644 Documentation/dev-tools/kfuzztest.rst
diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index 65c54b27a60b..00ccc4da003b 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -32,6 +32,7 @@ Documentation/process/debugging/index.rst
kfence
kselftest
kunit/index
+ kfuzztest
ktap
checkuapi
gpio-sloppy-logic-analyzer
diff --git a/Documentation/dev-tools/kfuzztest.rst b/Documentation/dev-tools/kfuzztest.rst
new file mode 100644
index 000000000000..aeaf433a320e
--- /dev/null
+++ b/Documentation/dev-tools/kfuzztest.rst
@@ -0,0 +1,371 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. Copyright 2025 Google LLC
+
+=========================================
+Kernel Fuzz Testing Framework (KFuzzTest)
+=========================================
+
+Overview
+========
+
+The Kernel Fuzz Testing Framework (KFuzzTest) is a framework designed to expose
+internal kernel functions to a userspace fuzzing engine.
+
+It is intended for testing stateless or low-state functions that are difficult
+to reach from the system call interface, such as routines involved in file
+format parsing or complex data transformations. This provides a method for
+in-situ fuzzing of kernel code without requiring that it be built as a separate
+userspace library or that its dependencies be stubbed out.
+
+The framework consists of four main components:
+
+1. An API, based on the ``FUZZ_TEST`` macro, for defining test targets
+ directly in the kernel tree.
+2. A binary serialization format for passing complex, pointer-rich data
+ structures from userspace to the kernel.
+3. A ``debugfs`` interface through which a userspace fuzzer submits
+ serialized test inputs.
+4. Metadata embedded in dedicated ELF sections of the ``vmlinux`` binary to
+ allow for the discovery of available fuzz targets by external tooling.
+
+.. warning::
+ KFuzzTest is a debugging and testing tool. It exposes internal kernel
+ functions to userspace with minimal sanitization and is designed for
+ use in controlled test environments only. It must **NEVER** be enabled
+ in production kernels.
+
+Supported Architectures
+=======================
+
+KFuzzTest is currently only supported for x86_64.
+
+Usage
+=====
+
+To enable KFuzzTest, configure the kernel with::
+
+ CONFIG_KFUZZTEST=y
+
+which depends on ``CONFIG_DEBUGFS`` for receiving userspace inputs, and
+``CONFIG_DEBUG_KERNEL`` as an additional guardrail for preventing KFuzzTest
+from finding its way into a production build accidentally.
+
+The KFuzzTest sample fuzz targets can be built in with
+``CONFIG_SAMPLE_KFUZZTEST``.
+
+KFuzzTest currently only supports code that is built into the kernel, as the
+core module's startup process discovers fuzz targets, constraints, and
+annotations from a dedicated ELF section during startup.
+
+Declaring a KFuzzTest target
+----------------------------
+
+A fuzz target is defined directly in a .c file, typically alongside the function
+being tested. This process involves three main parts: defining an input
+structure, writing the test body using the ``FUZZ_TEST`` macro, and optionally
+adding metadata for the fuzzer.
+
+The following example illustrates how to create a fuzz target for a function
+``int process_data(const char *data, size_t len)``.
+
+.. code-block:: c
+
+ /*
+ * 1. Define a struct to model the inputs for the function under test.
+ * Each field corresponds to an argument needed by the function.
+ */
+ struct process_data_inputs {
+ const char *data;
+ size_t len;
+ };
+
+ /*
+ * 2. Define the fuzz target using the FUZZ_TEST macro.
+ * The first parameter is a unique name for the target.
+ * The second parameter is the input struct defined above.
+ */
+ FUZZ_TEST(test_process_data, struct process_data_inputs)
+ {
+ /*
+ * Within this body, the 'arg' variable is a pointer to a
+ * fully initialized 'struct process_data_inputs'.
+ */
+
+ /*
+ * 3. (Optional) Add constraints to define preconditions.
+ * This check ensures 'arg->data' is not NULL. If the condition
+ * is not met, the test exits early. This also creates metadata
+ * to inform the fuzzer.
+ */
+ KFUZZTEST_EXPECT_NOT_NULL(process_data_inputs, data);
+
+ /*
+ * 4. (Optional) Add annotations to provide semantic hints.
+ * This annotation informs the fuzzer that the 'len' field
+ * is the length of the buffer pointed to by 'data'.
+ * Annotations do not add any runtime checks.
+ */
+ KFUZZTEST_ANNOTATE_LEN(process_data_inputs, len, data);
+
+ /*
+ * 5. Call the kernel function with the provided inputs.
+ * Memory errors like out-of-bounds accesses on 'arg->data' will
+ * be detected by KASAN or other memory error detection tools.
+ */
+ process_data(arg->data, arg->len);
+ }
+
+KFuzzTest provides two families of macros to improve the quality of fuzzing:
+
+- ``KFUZZTEST_EXPECT_*``: These macros define constraints, which are
+ preconditions that must be true for the test to proceed. They are enforced
+ with a runtime check in the kernel. If a check fails, the current test run is
+ aborted. This metadata helps the userspace fuzzer avoid generating invalid
+ inputs.
+
+- ``KFUZZTEST_ANNOTATE_*``: These macros define annotations, which are purely
+ semantic hints for the fuzzer. They do not add any runtime checks and exist
+ only to help the fuzzer generate more intelligent and structurally correct
+ inputs. For example, KFUZZTEST_ANNOTATE_LEN links a size field to a pointer
+ field, which is a common pattern in C APIs.
+
+Metadata
+--------
+
+Macros ``FUZZ_TEST``, `KFUZZTEST_EXPECT_*`` and ``KFUZZTEST_ANNOTATE_*`` embed
+metadata into several sections within the main ``.data`` section of the final
+``vmlinux`` binary; ``.kfuzztest_target``, ``.kfuzztest_constraint`` and
+``.kfuzztest_annotation`` respectively.
+
+This serves two purposes:
+
+1. The core module uses the ``.kfuzztest_target`` section at boot to discover
+ every ``FUZZ_TEST`` instance and create its ``debugfs`` directory and
+ ``input`` file.
+2. Userspace fuzzers can read this metadata from the ``vmlinux`` binary to
+ discover targets and learn about their rules and structure in order to
+ generate correct and effective inputs.
+
+The metadata in the ``.kfuzztest_*`` sections consists of arrays of fixed-size C
+structs (e.g., ``struct kfuzztest_target``). Fields within these structs that
+are pointers, such as ``name`` or ``arg_type_name``, contain addresses that
+point to other locations in the ``vmlinux`` binary. A userspace tool that
+parsing the ELF file must resolve these pointers to read the data that they
+reference. For example, to get a target's name, a tool must:
+
+1. Read the ``struct kfuzztest_target`` from the ``.kfuzztest_target`` section.
+2. Read the address in the ``.name`` field.
+3. Use that address to locate and read null-terminated string from its position
+ elsewhere in the binary (e.g., ``.rodata``).
+
+Tooling Dependencies
+--------------------
+
+For userspace tools to parse the ``vmlinux`` binary and make use of emitted
+KFuzzTest metadata, the kernel must be compiled with DWARF debug information.
+This is required for tools to understand the layout of C structs, resolve type
+information, and correctly interpret constraints and annotations.
+
+When using KFuzzTest with automated fuzzing tools, either
+``CONFIG_DEBUG_INFO_DWARF4`` or ``CONFIG_DEBUG_INFO_DWARF5`` should be enabled.
+
+Input Format
+============
+
+KFuzzTest targets receive their inputs from userspace via a write to a dedicated
+debugfs ``/sys/kernel/debug/kfuzztest/<test-name>/input``.
+
+The data written to this file must be a single binary blob that follows a
+specific serialization format. This format is designed to allow complex,
+pointer-rich C structures to be represented in a flat buffer, requiring only a
+single kernel allocation and copy from userspace.
+
+An input is first prefixed by an 8-byte header containing a magic value in the
+first four bytes, defined as ``KFUZZTEST_HEADER_MAGIC`` in
+`<include/linux/kfuzztest.h>``, and a version number in the subsequent four
+bytes.
+
+Version 0
+---------
+
+In version 0 (i.e., when the version number in the 8-byte header is equal to 0),
+the input format consists of three main parts laid out sequentially: a region
+array, a relocation table, and the payload.::
+
+ +----------------+---------------------+-----------+----------------+
+ | region array | relocation table | padding | payload |
+ +----------------+---------------------+-----------+----------------+
+
+Region Array
+^^^^^^^^^^^^
+
+This component is a header that describes how the raw data in the Payload is
+partitioned into logical memory regions. It consists of a count of regions
+followed by an array of ``struct reloc_region``, where each entry defines a
+single region with its size and offset from the start of the payload.
+
+.. code-block:: c
+
+ struct reloc_region {
+ uint32_t offset;
+ uint32_t size;
+ };
+
+ struct reloc_region_array {
+ uint32_t num_regions;
+ struct reloc_region regions[];
+ };
+
+By convention, region 0 represents the top-level input struct that is passed
+as the arg variable to the FUZZ_TEST body. Subsequent regions typically
+represent data buffers pointed to by fields within that struct. Region array
+entries must be ordered by offset ascending, and must not overlap with one
+another.
+
+To satisfy C language alignment requirements and prevent potential hardware
+faults, the memory address of each region's data must be correctly aligned for
+the type it represents. The framework allocates a base buffer that is suitably
+aligned for any C type. Therefore, the userspace tool that generates the input
+is responsible for calculating each region's offset within the payload to ensure
+this alignment is maintained.
+
+Relocation Table
+^^^^^^^^^^^^^^^^
+
+The relocation table provides the instructions for the kernel to "hydrate" the
+payload by patching pointer fields. It contains an array of
+``struct reloc_entry`` items. Each entry acts as a linking instruction,
+specifying:
+
+- The location of a pointer that needs to be patched (identified by a region
+ ID and an offset within that region).
+
+- The target region that the pointer should point to (identified by the
+ target's region ID) or ``KFUZZTEST_REGIONID_NULL`` if the pointer is ``NULL``.
+
+This table also specifies the amount of padding between its end and the start
+of the payload, which should be at least 8 bytes.
+
+.. code-block:: c
+
+ struct reloc_entry {
+ uint32_t region_id;
+ uint32_t region_offset;
+ uint32_t value;
+ };
+
+ struct reloc_table {
+ uint32_t num_entries;
+ uint32_t padding_size;
+ struct reloc_entry entries[];
+ };
+
+Payload
+^^^^^^^
+
+The payload contains the raw binary data for all regions, concatenated together
+according to their specified offsets.
+
+- Alignment: The start of the payload must be aligned to the most restrictive
+ alignment requirement of all its constituent regions. The framework ensures
+ that each region within the payload is then placed at an offset that respects
+ its own type's alignment.
+
+- Padding and Poisoning: The space between the end of one region's data and the
+ beginning of the next must be sufficient for padding. In KASAN builds,
+ KFuzzTest poisons this unused padding, allowing for precise detection of
+ out-of-bounds memory accesses between adjacent buffers. This padding should
+ be at least ``KFUZZTEST_POISON_SIZE`` bytes as defined in
+ `include/linux/kfuzztest.h``.
+
+KFuzzTest Bridge Tool
+=====================
+
+The kfuzztest-bridge program is a userspace utility that encodes a random byte
+stream into the structured binary format expected by a KFuzzTest harness. It
+allows users to describe the target's input structure textually, making it easy
+to perform smoke tests or connect harnesses to blob-based fuzzing engines.
+
+This tool is intended to be simple, both in usage and implementation. Its
+structure and DSL are sufficient for simpler use-cases. For more advanced
+coverage-guided fuzzing it is recommended to use syzkaller which implements
+deeper support for KFuzzTest targets.
+
+Usage
+-----
+
+The tool can be built with ``make tools/kfuzztest-bridge``. In the case of libc
+incompatibilities, the tool may have to be built on the target system.
+
+Example:
+
+.. code-block:: sh
+
+ ./kfuzztest-bridge \
+ "foo { u32 ptr[bar] }; bar { ptr[data] len[data, u64]}; data { arr[u8, 42] };" \
+ "my-fuzz-target" /dev/urandom
+
+The command takes three arguments
+
+1. A string describing the input structure (see `Textual Format`_ sub-section).
+2. The name of the target test, which corresponds to its directory in
+ ``/sys/kernel/debug/kfuzztest/``.
+3. A path to a file providing a stream of random data, such as
+ ``/dev/urandom``.
+
+The structure string in the example corresponds to the following C data
+structures:
+
+.. code-block:: c
+
+ struct foo {
+ u32 a;
+ struct bar *b;
+ };
+
+ struct bar {
+ struct data *d;
+ u64 data_len; /* Equals 42. */
+ };
+
+ struct data {
+ char arr[42];
+ };
+
+Textual Format
+--------------
+
+The textual format is a human-readable representation of the region-based binary
+format used by KFuzzTest. It is described by the following grammar:
+
+.. code-block:: text
+
+ schema ::= region ( ";" region )* [";"]
+ region ::= identifier "{" type+ "}"
+ type ::= primitive | pointer | array | length | string
+ primitive ::= "u8" | "u16" | "u32" | "u64"
+ pointer ::= "ptr" "[" identifier "]"
+ array ::= "arr" "[" primitive "," integer "]"
+ length ::= "len" "[" identifier "," primitive "]"
+ string ::= "str" "[" integer "]"
+ identifier ::= [a-zA-Z_][a-zA-Z1-9_]*
+ integer ::= [0-9]+
+
+Pointers must reference a named region. To fuzz a raw buffer, the buffer must be
+defined in its own region, as shown below:
+
+.. code-block:: c
+
+ struct my_struct {
+ char *buf;
+ size_t buflen;
+ };
+
+This would correspond to the following textual description:
+
+.. code-block:: text
+
+ my_struct { ptr[buf] len[buf, u64] }; buf { arr[u8, n] };
+
+Where ``n`` is some integer value defining the size of the byte array inside of
+the ``buf`` region.
--
2.51.0.318.gd7df087d1a-goog
^ permalink raw reply related [flat|nested] 15+ messages in thread
* [PATCH v2 RFC 6/7] kfuzztest: add KFuzzTest sample fuzz targets
2025-09-01 16:42 [PATCH v2 RFC 0/7] KFuzzTest: a new kernel fuzzing framework Ethan Graham
` (4 preceding siblings ...)
2025-09-01 16:42 ` [PATCH v2 RFC 5/7] kfuzztest: add ReST documentation Ethan Graham
@ 2025-09-01 16:42 ` Ethan Graham
2025-09-01 16:42 ` [PATCH v2 RFC 7/7] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing Ethan Graham
6 siblings, 0 replies; 15+ messages in thread
From: Ethan Graham @ 2025-09-01 16:42 UTC (permalink / raw)
To: ethangraham, glider
Cc: andreyknvl, brendan.higgins, davidgow, dvyukov, jannh, elver,
rmoar, shuah, tarasmadan, kasan-dev, kunit-dev, linux-kernel,
linux-mm, dhowells, lukas, ignat, herbert, davem, linux-crypto
From: Ethan Graham <ethangraham@google.com>
Add two simple fuzz target samples to demonstrate the KFuzzTest API and
provide basic self-tests for the framework.
These examples showcase how a developer can define a fuzz target using
the FUZZ_TEST(), constraint, and annotation macros, and serve as runtime
sanity checks for the core logic. For example, they test that out-of-bounds
memory accesses into poisoned padding regions are correctly detected in a
KASAN build.
These have been tested by writing syzkaller-generated inputs into their
debugfs 'input' files and verifying that the correct KASAN reports were
triggered.
Signed-off-by: Ethan Graham <ethangraham@google.com>
---
samples/Kconfig | 7 +++
samples/Makefile | 1 +
samples/kfuzztest/Makefile | 3 ++
samples/kfuzztest/overflow_on_nested_buffer.c | 52 +++++++++++++++++++
samples/kfuzztest/underflow_on_buffer.c | 41 +++++++++++++++
5 files changed, 104 insertions(+)
create mode 100644 samples/kfuzztest/Makefile
create mode 100644 samples/kfuzztest/overflow_on_nested_buffer.c
create mode 100644 samples/kfuzztest/underflow_on_buffer.c
diff --git a/samples/Kconfig b/samples/Kconfig
index ffef99950206..4be51a21d010 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -321,6 +321,13 @@ config SAMPLE_HUNG_TASK
if 2 or more processes read the same file concurrently, it will
be detected by the hung_task watchdog.
+config SAMPLE_KFUZZTEST
+ bool "Build KFuzzTest sample targets"
+ depends on KFUZZTEST
+ help
+ Build KFuzzTest sample targets that serve as selftests for input
+ deserialization and inter-region redzone poisoning logic.
+
source "samples/rust/Kconfig"
source "samples/damon/Kconfig"
diff --git a/samples/Makefile b/samples/Makefile
index 07641e177bd8..3a0e7f744f44 100644
--- a/samples/Makefile
+++ b/samples/Makefile
@@ -44,4 +44,5 @@ obj-$(CONFIG_SAMPLE_DAMON_WSSE) += damon/
obj-$(CONFIG_SAMPLE_DAMON_PRCL) += damon/
obj-$(CONFIG_SAMPLE_DAMON_MTIER) += damon/
obj-$(CONFIG_SAMPLE_HUNG_TASK) += hung_task/
+obj-$(CONFIG_SAMPLE_KFUZZTEST) += kfuzztest/
obj-$(CONFIG_SAMPLE_TSM_MR) += tsm-mr/
diff --git a/samples/kfuzztest/Makefile b/samples/kfuzztest/Makefile
new file mode 100644
index 000000000000..4f8709876c9e
--- /dev/null
+++ b/samples/kfuzztest/Makefile
@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+obj-$(CONFIG_SAMPLE_KFUZZTEST) += overflow_on_nested_buffer.o underflow_on_buffer.o
diff --git a/samples/kfuzztest/overflow_on_nested_buffer.c b/samples/kfuzztest/overflow_on_nested_buffer.c
new file mode 100644
index 000000000000..8b4bab1d6d4a
--- /dev/null
+++ b/samples/kfuzztest/overflow_on_nested_buffer.c
@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This file contains a KFuzzTest example target that ensures that a buffer
+ * overflow on a nested region triggers a KASAN OOB access report.
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <linux/kfuzztest.h>
+
+static void overflow_on_nested_buffer(const char *a, size_t a_len, const char *b, size_t b_len)
+{
+ size_t i;
+ pr_info("a = [%px, %px)", a, a + a_len);
+ pr_info("b = [%px, %px)", b, b + b_len);
+
+ /* Ensure that all bytes in arg->b are accessible. */
+ for (i = 0; i < b_len; i++)
+ READ_ONCE(b[i]);
+ /*
+ * Check that all bytes in arg->a are accessible, and provoke an OOB on
+ * the first byte to the right of the buffer which will trigger a KASAN
+ * report.
+ */
+ for (i = 0; i <= a_len; i++)
+ READ_ONCE(a[i]);
+}
+
+struct nested_buffers {
+ const char *a;
+ size_t a_len;
+ const char *b;
+ size_t b_len;
+};
+
+/**
+ * The KFuzzTest input format specifies that struct nested buffers should
+ * be expanded as:
+ *
+ * | a | b | pad[8] | *a | pad[8] | *b |
+ *
+ * where the padded regions are poisoned. We expect to trigger a KASAN report by
+ * overflowing one byte into the `a` buffer.
+ */
+FUZZ_TEST(test_overflow_on_nested_buffer, struct nested_buffers)
+{
+ KFUZZTEST_EXPECT_NOT_NULL(nested_buffers, a);
+ KFUZZTEST_EXPECT_NOT_NULL(nested_buffers, b);
+ KFUZZTEST_ANNOTATE_LEN(nested_buffers, a_len, a);
+ KFUZZTEST_ANNOTATE_LEN(nested_buffers, b_len, b);
+
+ overflow_on_nested_buffer(arg->a, arg->a_len, arg->b, arg->b_len);
+}
diff --git a/samples/kfuzztest/underflow_on_buffer.c b/samples/kfuzztest/underflow_on_buffer.c
new file mode 100644
index 000000000000..fbe214274037
--- /dev/null
+++ b/samples/kfuzztest/underflow_on_buffer.c
@@ -0,0 +1,41 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This file contains a KFuzzTest example target that ensures that a buffer
+ * underflow on a region triggers a KASAN OOB access report.
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <linux/kfuzztest.h>
+
+static void underflow_on_buffer(char *buf, size_t buflen)
+{
+ size_t i;
+
+ pr_info("buf = [%px, %px)", buf, buf + buflen);
+
+ /* First ensure that all bytes in arg->b are accessible. */
+ for (i = 0; i < buflen; i++)
+ READ_ONCE(buf[i]);
+ /*
+ * Provoke a buffer overflow on the first byte preceding b, triggering
+ * a KASAN report.
+ */
+ READ_ONCE(*((char *)buf - 1));
+}
+
+struct some_buffer {
+ char *buf;
+ size_t buflen;
+};
+
+/**
+ * Tests that the region between struct some_buffer and the expanded *buf field
+ * is correctly poisoned by accessing the first byte before *buf.
+ */
+FUZZ_TEST(test_underflow_on_buffer, struct some_buffer)
+{
+ KFUZZTEST_EXPECT_NOT_NULL(some_buffer, buf);
+ KFUZZTEST_ANNOTATE_LEN(some_buffer, buflen, buf);
+
+ underflow_on_buffer(arg->buf, arg->buflen);
+}
--
2.51.0.318.gd7df087d1a-goog
^ permalink raw reply related [flat|nested] 15+ messages in thread
* [PATCH v2 RFC 7/7] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing
2025-09-01 16:42 [PATCH v2 RFC 0/7] KFuzzTest: a new kernel fuzzing framework Ethan Graham
` (5 preceding siblings ...)
2025-09-01 16:42 ` [PATCH v2 RFC 6/7] kfuzztest: add KFuzzTest sample fuzz targets Ethan Graham
@ 2025-09-01 16:42 ` Ethan Graham
2025-09-03 8:58 ` Ignat Korchagin
6 siblings, 1 reply; 15+ messages in thread
From: Ethan Graham @ 2025-09-01 16:42 UTC (permalink / raw)
To: ethangraham, glider
Cc: andreyknvl, brendan.higgins, davidgow, dvyukov, jannh, elver,
rmoar, shuah, tarasmadan, kasan-dev, kunit-dev, linux-kernel,
linux-mm, dhowells, lukas, ignat, herbert, davem, linux-crypto
From: Ethan Graham <ethangraham@google.com>
Add KFuzzTest targets for pkcs7_parse_message, rsa_parse_pub_key, and
rsa_parse_priv_key to serve as real-world examples of how the framework is used.
These functions are ideal candidates for KFuzzTest as they perform complex
parsing of user-controlled data but are not directly exposed at the syscall
boundary. This makes them difficult to exercise with traditional fuzzing tools
and showcases the primary strength of the KFuzzTest framework: providing an
interface to fuzz internal functions.
The targets are defined within /lib/tests, alongside existing KUnit
tests.
Signed-off-by: Ethan Graham <ethangraham@google.com>
---
v2:
- Move KFuzzTest targets outside of the source files into dedicated
_kfuzz.c files under /crypto/asymmetric_keys/tests/ as suggested by
Ignat Korchagin and Eric Biggers.
---
---
crypto/asymmetric_keys/Kconfig | 15 ++++++++
crypto/asymmetric_keys/Makefile | 2 +
crypto/asymmetric_keys/tests/Makefile | 2 +
crypto/asymmetric_keys/tests/pkcs7_kfuzz.c | 22 +++++++++++
.../asymmetric_keys/tests/rsa_helper_kfuzz.c | 38 +++++++++++++++++++
5 files changed, 79 insertions(+)
create mode 100644 crypto/asymmetric_keys/tests/Makefile
create mode 100644 crypto/asymmetric_keys/tests/pkcs7_kfuzz.c
create mode 100644 crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c
diff --git a/crypto/asymmetric_keys/Kconfig b/crypto/asymmetric_keys/Kconfig
index e1345b8f39f1..7a4c5eb18624 100644
--- a/crypto/asymmetric_keys/Kconfig
+++ b/crypto/asymmetric_keys/Kconfig
@@ -104,3 +104,18 @@ config FIPS_SIGNATURE_SELFTEST_ECDSA
depends on CRYPTO_ECDSA=y || CRYPTO_ECDSA=FIPS_SIGNATURE_SELFTEST
endif # ASYMMETRIC_KEY_TYPE
+
+config PKCS7_MESSAGE_PARSER_KFUZZ
+ bool "Build fuzz target for PKCS#7 parser"
+ depends on KFUZZTEST
+ depends on PKCS7_MESSAGE_PARSER
+ default y
+ help
+ Builds the KFuzzTest targets for PKCS#7.
+
+config RSA_HELPER_KFUZZ
+ bool "Build fuzz targets for RSA helpers"
+ depends on KFUZZTEST
+ default y
+ help
+ Builds the KFuzzTest targets for RSA helper functions.
diff --git a/crypto/asymmetric_keys/Makefile b/crypto/asymmetric_keys/Makefile
index bc65d3b98dcb..77b825aee6b2 100644
--- a/crypto/asymmetric_keys/Makefile
+++ b/crypto/asymmetric_keys/Makefile
@@ -67,6 +67,8 @@ obj-$(CONFIG_PKCS7_TEST_KEY) += pkcs7_test_key.o
pkcs7_test_key-y := \
pkcs7_key_type.o
+obj-y += tests/
+
#
# Signed PE binary-wrapped key handling
#
diff --git a/crypto/asymmetric_keys/tests/Makefile b/crypto/asymmetric_keys/tests/Makefile
new file mode 100644
index 000000000000..42a779c9042a
--- /dev/null
+++ b/crypto/asymmetric_keys/tests/Makefile
@@ -0,0 +1,2 @@
+obj-$(CONFIG_PKCS7_MESSAGE_PARSER_KFUZZ) += pkcs7_kfuzz.o
+obj-$(CONFIG_RSA_HELPER_KFUZZ) += rsa_helper_kfuzz.o
diff --git a/crypto/asymmetric_keys/tests/pkcs7_kfuzz.c b/crypto/asymmetric_keys/tests/pkcs7_kfuzz.c
new file mode 100644
index 000000000000..84d0b0d8d0eb
--- /dev/null
+++ b/crypto/asymmetric_keys/tests/pkcs7_kfuzz.c
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * PKCS#7 parser KFuzzTest target
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <crypto/pkcs7.h>
+#include <linux/kfuzztest.h>
+
+struct pkcs7_parse_message_arg {
+ const void *data;
+ size_t datalen;
+};
+
+FUZZ_TEST(test_pkcs7_parse_message, struct pkcs7_parse_message_arg)
+{
+ KFUZZTEST_EXPECT_NOT_NULL(pkcs7_parse_message_arg, data);
+ KFUZZTEST_ANNOTATE_LEN(pkcs7_parse_message_arg, datalen, data);
+ KFUZZTEST_EXPECT_LE(pkcs7_parse_message_arg, datalen, 16 * PAGE_SIZE);
+
+ pkcs7_parse_message(arg->data, arg->datalen);
+}
diff --git a/crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c b/crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c
new file mode 100644
index 000000000000..5877e54cb75a
--- /dev/null
+++ b/crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c
@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * RSA key extract helper KFuzzTest targets
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <linux/kfuzztest.h>
+#include <crypto/internal/rsa.h>
+
+struct rsa_parse_pub_key_arg {
+ const void *key;
+ size_t key_len;
+};
+
+FUZZ_TEST(test_rsa_parse_pub_key, struct rsa_parse_pub_key_arg)
+{
+ KFUZZTEST_EXPECT_NOT_NULL(rsa_parse_pub_key_arg, key);
+ KFUZZTEST_ANNOTATE_LEN(rsa_parse_pub_key_arg, key_len, key);
+ KFUZZTEST_EXPECT_LE(rsa_parse_pub_key_arg, key_len, 16 * PAGE_SIZE);
+
+ struct rsa_key out;
+ rsa_parse_pub_key(&out, arg->key, arg->key_len);
+}
+
+struct rsa_parse_priv_key_arg {
+ const void *key;
+ size_t key_len;
+};
+
+FUZZ_TEST(test_rsa_parse_priv_key, struct rsa_parse_priv_key_arg)
+{
+ KFUZZTEST_EXPECT_NOT_NULL(rsa_parse_priv_key_arg, key);
+ KFUZZTEST_ANNOTATE_LEN(rsa_parse_priv_key_arg, key_len, key);
+ KFUZZTEST_EXPECT_LE(rsa_parse_priv_key_arg, key_len, 16 * PAGE_SIZE);
+
+ struct rsa_key out;
+ rsa_parse_priv_key(&out, arg->key, arg->key_len);
+}
--
2.51.0.318.gd7df087d1a-goog
^ permalink raw reply related [flat|nested] 15+ messages in thread
* Re: [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures
2025-09-01 16:42 ` [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures Ethan Graham
@ 2025-09-02 10:37 ` Marco Elver
2025-09-03 8:40 ` Alexander Potapenko
` (2 subsequent siblings)
3 siblings, 0 replies; 15+ messages in thread
From: Marco Elver @ 2025-09-02 10:37 UTC (permalink / raw)
To: Ethan Graham
Cc: ethangraham, glider, andreyknvl, brendan.higgins, davidgow,
dvyukov, jannh, rmoar, shuah, tarasmadan, kasan-dev, kunit-dev,
linux-kernel, linux-mm, dhowells, lukas, ignat, herbert, davem,
linux-crypto
On Mon, 1 Sept 2025 at 18:43, Ethan Graham <ethan.w.s.graham@gmail.com> wrote:
>
> From: Ethan Graham <ethangraham@google.com>
>
> Add the foundational user-facing components for the KFuzzTest framework.
> This includes the main API header <linux/kfuzztest.h>, the Kconfig
> option to enable the feature, and the required linker script changes
> which introduce three new ELF sections in vmlinux.
>
> Note that KFuzzTest is intended strictly for debug builds only, and
> should never be enabled in a production build. The fact that it exposes
> internal kernel functions and state directly to userspace may constitute
> a serious security vulnerability if used for any reason other than
> testing.
>
> The header defines:
> - The FUZZ_TEST() macro for creating test targets.
> - The data structures required for the binary serialization format,
> which allows passing complex inputs from userspace.
> - The metadata structures for test targets, constraints and annotations,
> which are placed in dedicated ELF sections (.kfuzztest_*) for discovery.
>
> This patch only adds the public interface and build integration; no
> runtime logic is included.
>
> Signed-off-by: Ethan Graham <ethangraham@google.com>
> ---
> arch/x86/kernel/vmlinux.lds.S | 22 ++
> include/linux/kfuzztest.h | 508 ++++++++++++++++++++++++++++++++++
> lib/Kconfig.debug | 1 +
> lib/kfuzztest/Kconfig | 20 ++
> 4 files changed, 551 insertions(+)
> create mode 100644 include/linux/kfuzztest.h
> create mode 100644 lib/kfuzztest/Kconfig
>
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 4fa0be732af1..484e3e1ffb9f 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -112,6 +112,26 @@ ASSERT(__relocate_kernel_end - __relocate_kernel_start <= KEXEC_CONTROL_CODE_MAX
> #else
> #define KEXEC_RELOCATE_KERNEL
> #endif
> +
> +#ifdef CONFIG_KFUZZTEST
> +#define KFUZZTEST_TABLE \
> + . = ALIGN(PAGE_SIZE); \
> + __kfuzztest_targets_start = .; \
> + KEEP(*(.kfuzztest_target)); \
> + __kfuzztest_targets_end = .; \
> + . = ALIGN(PAGE_SIZE); \
> + __kfuzztest_constraints_start = .; \
> + KEEP(*(.kfuzztest_constraint)); \
> + __kfuzztest_constraints_end = .; \
> + . = ALIGN(PAGE_SIZE); \
> + __kfuzztest_annotations_start = .; \
> + KEEP(*(.kfuzztest_annotation)); \
> + __kfuzztest_annotations_end = .;
> +
> +#else /* CONFIG_KFUZZTEST */
> +#define KFUZZTEST_TABLE
> +#endif /* CONFIG_KFUZZTEST */
> +
> PHDRS {
> text PT_LOAD FLAGS(5); /* R_E */
> data PT_LOAD FLAGS(6); /* RW_ */
> @@ -199,6 +219,8 @@ SECTIONS
> CONSTRUCTORS
> KEXEC_RELOCATE_KERNEL
>
> + KFUZZTEST_TABLE
> +
> /* rarely changed data like cpu maps */
> READ_MOSTLY_DATA(INTERNODE_CACHE_BYTES)
>
> diff --git a/include/linux/kfuzztest.h b/include/linux/kfuzztest.h
> new file mode 100644
> index 000000000000..11a647c1d925
> --- /dev/null
> +++ b/include/linux/kfuzztest.h
> @@ -0,0 +1,508 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * The Kernel Fuzz Testing Framework (KFuzzTest) API for defining fuzz targets
> + * for internal kernel functions.
> + *
> + * For more information please see Documentation/dev-tools/kfuzztest.rst.
> + *
> + * Copyright 2025 Google LLC
> + */
> +#ifndef KFUZZTEST_H
> +#define KFUZZTEST_H
> +
> +#include <linux/fs.h>
> +#include <linux/printk.h>
> +#include <linux/types.h>
> +
> +#define KFUZZTEST_HEADER_MAGIC (0xBFACE)
> +#define KFUZZTEST_V0 (0)
> +
> +/**
> + * @brief The KFuzzTest Input Serialization Format
> + *
> + * KFuzzTest receives its input from userspace as a single binary blob. This
> + * format allows for the serialization of complex, pointer-rich C structures
> + * into a flat buffer that can be safely passed into the kernel. This format
> + * requires only a single copy from userspace into a kenrel buffer, and no
> + * further kernel allocations. Pointers are patched internally using a "region"
> + * system where each region corresponds to some pointed-to data.
> + *
> + * Regions should be padded to respect alignment constraints of their underlying
> + * types, and should be followed by at least 8 bytes of padding. These padded
> + * regions are poisoned by KFuzzTest to ensure that KASAN catches OOB accesses.
> + *
> + * The format consists of a prefix and three main components:
> + * 1. An 8-byte header: Contains KFUZZTEST_MAGIC in the first 4 bytes, and the
> + * version number in the subsequent 4 bytes. This ensures backwards
> + * compatibility in the event of future format changes.
> + * 2. A reloc_region_array: Defines the memory layout of the target structure
> + * by partitioning the payload into logical regions. Each logical region
> + * should contain the byte representation of the type that it represents,
> + * including any necessary padding. The region descriptors should be
> + * ordered by offset ascending.
> + * 3. A reloc_table: Provides "linking" instructions that tell the kernel how
> + * to patch pointer fields to point to the correct regions. By design,
> + * the first region (index 0) is passed as input into a FUZZ_TEST.
> + * 4. A Payload: The raw binary data for the structure and its associated
> + * buffers. This should be aligned to the maximum alignment of all
> + * regions to satisfy alignment requirements of the input types, but this
> + * isn't checked by the parser.
> + *
> + * For a detailed specification of the binary layout see the full documentation
> + * at: Documentation/dev-tools/kfuzztest.rst
> + */
> +
> +/**
> + * struct reloc_region - single contiguous memory region in the payload
> + *
> + * @offset: The byte offset of this region from the start of the payload, which
> + * should be aligned to the alignment requirements of the region's
> + * underlying type.
> + * @size: The size of this region in bytes.
> + */
> +struct reloc_region {
> + uint32_t offset;
> + uint32_t size;
> +};
> +
> +/**
> + * struct reloc_region_array - array of regions in an input
> + * @num_regions: The total number of regions defined.
> + * @regions: A flexible array of `num_regions` region descriptors.
> + */
> +struct reloc_region_array {
> + uint32_t num_regions;
> + struct reloc_region regions[];
> +};
> +
> +/**
> + * struct reloc_entry - a single pointer to be patched in an input
> + *
> + * @region_id: The index of the region in the `reloc_region_array` that
> + * contains the pointer.
> + * @region_offset: The start offset of the pointer inside of the region.
> + * @value: contains the index of the pointee region, or KFUZZTEST_REGIONID_NULL
> + * if the pointer is NULL.
> + */
> +struct reloc_entry {
> + uint32_t region_id;
> + uint32_t region_offset;
> + uint32_t value;
> +};
> +
> +/**
> + * struct reloc_entry - array of relocations required by an input
> + *
> + * @num_entries: the number of pointer relocations.
> + * @padding_size: the number of padded bytes between the last relocation in
> + * entries, and the start of the payload data. This should be at least
> + * 8 bytes, as it is used for poisoning.
> + * @entries: array of relocations.
> + */
> +struct reloc_table {
> + uint32_t num_entries;
> + uint32_t padding_size;
> + struct reloc_entry entries[];
> +};
> +
> +/**
> + * kfuzztest_parse_and_relocate - validate and relocate a KFuzzTest input
> + *
> + * @input: A buffer containing the serialized input for a fuzz target.
> + * @input_size: the size in bytes of the @input buffer.
> + * @arg_ret: return pointer for the test case's input structure.
> + */
> +int kfuzztest_parse_and_relocate(void *input, size_t input_size, void **arg_ret);
> +
> +/*
> + * Dump some information on the parsed headers and payload. Can be useful for
> + * debugging inputs when writing an encoder for the KFuzzTest input format.
> + */
> +__attribute__((unused)) static inline void kfuzztest_debug_header(struct reloc_region_array *regions,
> + struct reloc_table *rt, void *payload_start,
> + void *payload_end)
> +{
> + uint32_t i;
> +
> + pr_info("regions: { num_regions = %u } @ %px", regions->num_regions, regions);
> + for (i = 0; i < regions->num_regions; i++) {
> + pr_info(" region_%u: { start: 0x%x, size: 0x%x }", i, regions->regions[i].offset,
> + regions->regions[i].size);
> + }
> +
> + pr_info("reloc_table: { num_entries = %u, padding = %u } @ offset 0x%lx", rt->num_entries, rt->padding_size,
> + (char *)rt - (char *)regions);
> + for (i = 0; i < rt->num_entries; i++) {
> + pr_info(" reloc_%u: { src: %u, offset: 0x%x, dst: %u }", i, rt->entries[i].region_id,
> + rt->entries[i].region_offset, rt->entries[i].value);
> + }
> +
> + pr_info("payload: [0x%lx, 0x%lx)", (char *)payload_start - (char *)regions,
> + (char *)payload_end - (char *)regions);
> +}
> +
> +struct kfuzztest_target {
> + const char *name;
> + const char *arg_type_name;
> + ssize_t (*write_input_cb)(struct file *filp, const char __user *buf, size_t len, loff_t *off);
> +} __aligned(32);
> +
> +/**
> + * FUZZ_TEST - defines a KFuzzTest target
> + *
> + * @test_name: The unique identifier for the fuzz test, which is used to name
> + * the debugfs entry, e.g., /sys/kernel/debug/kfuzztest/@test_name.
> + * @test_arg_type: The struct type that defines the inputs for the test. This
> + * must be the full struct type (e.g., "struct my_inputs"), not a typedef.
> + *
> + * Context:
> + * This macro is the primary entry point for the KFuzzTest framework. It
> + * generates all the necessary boilerplate for a fuzz test, including:
> + * - A static `struct kfuzztest_target` instance that is placed in a
> + * dedicated ELF section for discovery by userspace tools.
> + * - A `debugfs` write callback that handles receiving serialized data from
> + * a fuzzer, parsing it, and "hydrating" it into a valid C struct.
> + * - A function stub where the developer places the test logic.
> + *
> + * User-Provided Logic:
> + * The developer must provide the body of the fuzz test logic within the curly
> + * braces following the macro invocation. Within this scope, the framework
> + * provides the following variables:
> + *
> + * - `arg`: A pointer of type `@test_arg_type *` to the fully hydrated input
> + * structure. All pointer fields within this struct have been relocated
> + * and are valid kernel pointers. This is the primary variable to use
> + * for accessing fuzzing inputs.
> + *
> + * - `regions`: A pointer of type `struct reloc_region_array *`. This is an
> + * advanced feature that allows access to the raw region metadata, which
> + * can be useful for checking the actual allocated size of a buffer via
> + * `KFUZZTEST_REGION_SIZE(n)`.
I don't see `regions` being passed. I only see:
+ static void kfuzztest_logic_##test_name(test_arg_type *arg)
Am I looking at the wrong one?
> + * Example Usage:
> + *
> + * // 1. The kernel function we want to fuzz.
> + * int process_data(const char *data, size_t len);
> + *
> + * // 2. Define a struct to hold all inputs for the function.
> + * struct process_data_inputs {
> + * const char *data;
> + * size_t len;
> + * };
> + *
> + * // 3. Define the fuzz test using the FUZZ_TEST macro.
> + * FUZZ_TEST(process_data_fuzzer, struct process_data_inputs)
> + * {
> + * int ret;
> + * // Use KFUZZTEST_EXPECT_* to enforce preconditions.
> + * // The test will exit early if data is NULL.
> + * KFUZZTEST_EXPECT_NOT_NULL(process_data_inputs, data);
> + *
> + * // Use KFUZZTEST_ANNOTATE_* to provide hints to the fuzzer.
> + * // This links the 'len' field to the 'data' buffer.
> + * KFUZZTEST_ANNOTATE_LEN(process_data_inputs, len, data);
> + *
> + * // Call the function under test using the 'arg' variable. OOB memory
> + * // accesses will be caught by KASAN, but the user can also choose to
> + * // validate the return value and log any failures.
> + * ret = process_data(arg->data, arg->len);
> + * }
> + */
> +#define FUZZ_TEST(test_name, test_arg_type) \
> + static ssize_t kfuzztest_write_cb_##test_name(struct file *filp, const char __user *buf, size_t len, \
> + loff_t *off); \
> + static void kfuzztest_logic_##test_name(test_arg_type *arg); \
> + const struct kfuzztest_target __fuzz_test__##test_name __section(".kfuzztest_target") __used = { \
> + .name = #test_name, \
> + .arg_type_name = #test_arg_type, \
> + .write_input_cb = kfuzztest_write_cb_##test_name, \
> + }; \
> + static ssize_t kfuzztest_write_cb_##test_name(struct file *filp, const char __user *buf, size_t len, \
> + loff_t *off) \
> + { \
> + test_arg_type *arg; \
> + void *buffer; \
> + int ret; \
> + \
> + buffer = kmalloc(len, GFP_KERNEL); \
Should there be some kind of cap on the max allocation size, because
the user space tool controls this. If something went wrong on the user
space side, it can DoS the kernel and make for a poor debugging
experience.
Perhaps print a message like pr_warn(#test_name ": oversized input of
size %sz") and return -EINVAL.
> + if (!buffer) \
> + return -ENOMEM; \
> + ret = simple_write_to_buffer(buffer, len, off, buf, len); \
> + if (ret < 0) \
> + goto out; \
> + ret = kfuzztest_parse_and_relocate(buffer, len, (void **)&arg); \
> + if (ret < 0) \
> + goto out; \
> + kfuzztest_logic_##test_name(arg); \
> + ret = len; \
> +out: \
> + kfree(buffer); \
> + return ret; \
> + } \
> + static void kfuzztest_logic_##test_name(test_arg_type *arg)
> +
> +enum kfuzztest_constraint_type {
> + EXPECT_EQ,
> + EXPECT_NE,
> + EXPECT_LT,
> + EXPECT_LE,
> + EXPECT_GT,
> + EXPECT_GE,
> + EXPECT_IN_RANGE,
> +};
> +
> +/**
> + * struct kfuzztest_constraint - a metadata record for a domain constraint
> + *
> + * Domain constraints are rules about the input data that must be satisfied for
> + * a fuzz test to proceed. While they are enforced in the kernel with a runtime
> + * check, they are primarily intended as a discoverable contract for userspace
> + * fuzzers.
> + *
> + * Instances of this struct are generated by the KFUZZTEST_EXPECT_* macros
> + * and placed into the read-only ".kfuzztest_constraint" ELF section of the
> + * vmlinux binary. A fuzzer can parse this section to learn about the
> + * constraints and generate valid inputs more intelligently.
> + *
> + * For an example of how these constraints are used within a fuzz test, see the
> + * documentation for the FUZZ_TEST() macro.
> + *
> + * @input_type: The name of the input struct type, without the leading
> + * "struct ".
> + * @field_name: The name of the field within the struct that this constraint
> + * applies to.
> + * @value1: The primary value used in the comparison (e.g., the upper
> + * bound for EXPECT_LE).
> + * @value2: The secondary value, used only for multi-value comparisons
> + * (e.g., the upper bound for EXPECT_IN_RANGE).
> + * @type: The type of the constraint.
> + */
> +struct kfuzztest_constraint {
> + const char *input_type;
> + const char *field_name;
> + uintptr_t value1;
> + uintptr_t value2;
> + enum kfuzztest_constraint_type type;
> +} __aligned(64);
> +
> +#define __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val1, val2, tpe) \
> + static struct kfuzztest_constraint __constraint_##arg_type##_##field __section(".kfuzztest_constraint") \
> + __used = { \
> + .input_type = "struct " #arg_type, \
> + .field_name = #field, \
> + .value1 = (uintptr_t)val1, \
> + .value2 = (uintptr_t)val2, \
> + .type = tpe, \
> + }
> +
> +/**
> + * KFUZZTEST_EXPECT_EQ - constrain a field to be equal to a value
> + *
> + * @arg_type: name of the input structure, without the leading "struct ".
> + * @field: some field that is comparable
> + * @val: a value of the same type as @arg_type.@field
> + */
> +#define KFUZZTEST_EXPECT_EQ(arg_type, field, val) \
> + do { \
> + if (arg->field != val) \
> + return; \
> + __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_EQ); \
> + } while (0)
> +
> +/**
> + * KFUZZTEST_EXPECT_NE - constrain a field to be not equal to a value
> + *
> + * @arg_type: name of the input structure, without the leading "struct ".
> + * @field: some field that is comparable.
> + * @val: a value of the same type as @arg_type.@field.
> + */
> +#define KFUZZTEST_EXPECT_NE(arg_type, field, val) \
> + do { \
> + if (arg->field == val) \
> + return; \
> + __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_NE); \
> + } while (0)
> +
> +/**
> + * KFUZZTEST_EXPECT_LT - constrain a field to be less than a value
> + *
> + * @arg_type: name of the input structure, without the leading "struct ".
> + * @field: some field that is comparable.
> + * @val: a value of the same type as @arg_type.@field.
> + */
> +#define KFUZZTEST_EXPECT_LT(arg_type, field, val) \
> + do { \
> + if (arg->field >= val) \
> + return; \
> + __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_LT); \
> + } while (0)
> +
> +/**
> + * KFUZZTEST_EXPECT_LE - constrain a field to be less than or equal to a value
> + *
> + * @arg_type: name of the input structure, without the leading "struct ".
> + * @field: some field that is comparable.
> + * @val: a value of the same type as @arg_type.@field.
> + */
> +#define KFUZZTEST_EXPECT_LE(arg_type, field, val) \
> + do { \
> + if (arg->field > val) \
> + return; \
> + __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_LE); \
> + } while (0)
> +
> +/**
> + * KFUZZTEST_EXPECT_GT - constrain a field to be greater than a value
> + *
> + * @arg_type: name of the input structure, without the leading "struct ".
> + * @field: some field that is comparable.
> + * @val: a value of the same type as @arg_type.@field.
> + */
> +#define KFUZZTEST_EXPECT_GT(arg_type, field, val) \
> + do { \
> + if (arg->field <= val) \
> + return; \
> + __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_GT) \
> + } while (0)
> +
> +/**
> + * KFUZZTEST_EXPECT_GE - constrain a field to be greater than or equal to a value
> + *
> + * @arg_type: name of the input structure, without the leading "struct ".
> + * @field: some field that is comparable.
> + * @val: a value of the same type as @arg_type.@field.
> + */
> +#define KFUZZTEST_EXPECT_GE(arg_type, field, val) \
> + do { \
> + if (arg->field < val) \
> + return; \
> + __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_GE)` \
Stray `
> + } while (0)
> +
> +/**
> + * KFUZZTEST_EXPECT_GE - constrain a pointer field to be non-NULL
> + *
> + * @arg_type: name of the input structure, without the leading "struct ".
> + * @field: some field that is comparable.
> + * @val: a value of the same type as @arg_type.@field.
> + */
> +#define KFUZZTEST_EXPECT_NOT_NULL(arg_type, field) KFUZZTEST_EXPECT_NE(arg_type, field, NULL)
> +
> +/**
> + * KFUZZTEST_EXPECT_IN_RANGE - constrain a field to be within a range
> + *
> + * @arg_type: name of the input structure, without the leading "struct ".
> + * @field: some field that is comparable.
> + * @lower_bound: a lower bound of the same type as @arg_type.@field.
> + * @upper_bound: an upper bound of the same type as @arg_type.@field.
> + */
> +#define KFUZZTEST_EXPECT_IN_RANGE(arg_type, field, lower_bound, upper_bound) \
> + do { \
> + if (arg->field < lower_bound || arg->field > upper_bound) \
> + return; \
> + __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, lower_bound, upper_bound, EXPECT_IN_RANGE) \
> + } while (0)
> +
> +/**
> + * Annotations express attributes about structure fields that can't be easily
> + * or safely verified at runtime. They are intended as hints to the fuzzing
> + * engine to help it generate more semantically correct and effective inputs.
> + * Unlike constraints, annotations do not add any runtime checks and do not
> + * cause a test to exit early.
> + *
> + * For example, a `char *` field could be a raw byte buffer or a C-style
> + * null-terminated string. A fuzzer that is aware of this distinction can avoid
> + * creating inputs that would cause trivial, uninteresting crashes from reading
> + * past the end of a non-null-terminated buffer.
> + */
> +enum kfuzztest_annotation_attribute : uint8_t {
> + ATTRIBUTE_LEN,
> + ATTRIBUTE_STRING,
> + ATTRIBUTE_ARRAY,
> +};
> +
> +/**
> + * struct kfuzztest_annotation - a metadata record for a fuzzer hint
> + *
> + * This struct captures a single hint about a field in the input structure.
> + * Instances are generated by the KFUZZTEST_ANNOTATE_* macros and are placed
> + * into the read-only ".kfuzztest_annotation" ELF section of the vmlinux binary.
> + *
> + * A userspace fuzzer can parse this section to understand the semantic
> + * relationships between fields (e.g., which field is a length for which
> + * buffer) and the expected format of the data (e.g., a null-terminated
> + * string). This allows the fuzzer to be much more intelligent during input
> + * generation and mutation.
> + *
> + * For an example of how annotations are used within a fuzz test, see the
> + * documentation for the FUZZ_TEST() macro.
> + *
> + * @input_type: The name of the input struct type.
> + * @field_name: The name of the field being annotated (e.g., the data
> + * buffer field).
> + * @linked_field_name: For annotations that link two fields (like
> + * ATTRIBUTE_LEN), this is the name of the related field (e.g., the
> + * length field). For others, this may be unused.
> + * @attrib: The type of the annotation hint.
> + */
> +struct kfuzztest_annotation {
> + const char *input_type;
> + const char *field_name;
> + const char *linked_field_name;
> + enum kfuzztest_annotation_attribute attrib;
> +} __aligned(32);
> +
> +#define __KFUZZTEST_ANNOTATE(arg_type, field, linked_field, attribute) \
> + static struct kfuzztest_annotation __annotation_##arg_type##_##field __section(".kfuzztest_annotation") \
> + __used = { \
> + .input_type = "struct " #arg_type, \
> + .field_name = #field, \
> + .linked_field_name = #linked_field, \
> + .attrib = attribute, \
> + }
> +
> +/**
> + * KFUZZTEST_ANNOTATE_STRING - annotate a char* field as a C string
> + *
> + * We define a C string as a sequence of non-zero characters followed by exactly
> + * one null terminator.
> + *
> + * @arg_type: name of the input structure, without the leading "struct ".
> + * @field: the name of the field to annotate.
> + */
> +#define KFUZZTEST_ANNOTATE_STRING(arg_type, field) __KFUZZTEST_ANNOTATE(arg_type, field, NULL, ATTRIBUTE_STRING)
> +
> +/**
> + * KFUZZTEST_ANNOTATE_ARRAY - annotate a pointer as an array
> + *
> + * We define an array as a contiguous memory region containing zero or more
> + * elements of the same type.
> + *
> + * @arg_type: name of the input structure, without the leading "struct ".
> + * @field: the name of the field to annotate.
> + */
> +#define KFUZZTEST_ANNOTATE_ARRAY(arg_type, field) __KFUZZTEST_ANNOTATE(arg_type, field, NULL, ATTRIBUTE_ARRAY)
> +
> +/**
> + * KFUZZTEST_ANNOTATE_LEN - annotate a field as the length of another
> + *
> + * This expresses the relationship `arg_type.field == len(linked_field)`, where
> + * `linked_field` is an array.
> + *
> + * @arg_type: name of the input structure, without the leading "struct ".
> + * @field: the name of the field to annotate.
> + * @linked_field: the name of an array field with length @field.
> + */
> +#define KFUZZTEST_ANNOTATE_LEN(arg_type, field, linked_field) \
> + __KFUZZTEST_ANNOTATE(arg_type, field, linked_field, ATTRIBUTE_LEN)
> +
> +#define KFUZZTEST_REGIONID_NULL U32_MAX
> +
> +/**
> + * The end of the input should be padded by at least this number of bytes as
> + * it is poisoned to detect out of bounds accesses at the end of the last
> + * region.
> + */
> +#define KFUZZTEST_POISON_SIZE 0x8
> +
> +#endif /* KFUZZTEST_H */
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index ebe33181b6e6..3542e94204c8 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1947,6 +1947,7 @@ endmenu
> menu "Kernel Testing and Coverage"
>
> source "lib/kunit/Kconfig"
> +source "lib/kfuzztest/Kconfig"
>
> config NOTIFIER_ERROR_INJECTION
> tristate "Notifier error injection"
> diff --git a/lib/kfuzztest/Kconfig b/lib/kfuzztest/Kconfig
> new file mode 100644
> index 000000000000..f9fb5abf8d27
> --- /dev/null
> +++ b/lib/kfuzztest/Kconfig
> @@ -0,0 +1,20 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +
> +config KFUZZTEST
> + bool "KFuzzTest - enable support for internal fuzz targets"
> + depends on DEBUG_FS && DEBUG_KERNEL
> + help
> + Enables support for the kernel fuzz testing framework (KFuzzTest), an
> + interface for exposing internal kernel functions to a userspace fuzzing
> + engine. KFuzzTest targets are exposed via a debugfs interface that
> + accepts serialized userspace inputs, and is designed to make it easier
> + to fuzz deeply nested kernel code that is hard to reach from the system
> + call boundary. Using a simple macro-based API, developers can add a new
> + fuzz target with minimal boilerplate code.
> +
> + It is strongly recommended to also enable CONFIG_KASAN for byte-accurate
> + out-of-bounds detection, as KFuzzTest was designed with this in mind. It
> + is also recommended to enable CONFIG_KCOV for coverage guided fuzzing.
> +
> + WARNING: This exposes internal kernel functions directly to userspace
> + and must NEVER be enabled in production builds.
> --
> 2.51.0.318.gd7df087d1a-goog
>
^ permalink raw reply [flat|nested] 15+ messages in thread
* Re: [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures
2025-09-01 16:42 ` [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures Ethan Graham
2025-09-02 10:37 ` Marco Elver
@ 2025-09-03 8:40 ` Alexander Potapenko
2025-09-03 10:15 ` Alexander Potapenko
2025-09-03 11:35 ` Alexander Potapenko
3 siblings, 0 replies; 15+ messages in thread
From: Alexander Potapenko @ 2025-09-03 8:40 UTC (permalink / raw)
To: Ethan Graham
Cc: ethangraham, andreyknvl, brendan.higgins, davidgow, dvyukov,
jannh, elver, rmoar, shuah, tarasmadan, kasan-dev, kunit-dev,
linux-kernel, linux-mm, dhowells, lukas, ignat, herbert, davem,
linux-crypto
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -112,6 +112,26 @@ ASSERT(__relocate_kernel_end - __relocate_kernel_start <= KEXEC_CONTROL_CODE_MAX
> #else
> #define KEXEC_RELOCATE_KERNEL
> #endif
> +
> +#ifdef CONFIG_KFUZZTEST
> +#define KFUZZTEST_TABLE \
> + . = ALIGN(PAGE_SIZE); \
> + __kfuzztest_targets_start = .; \
> + KEEP(*(.kfuzztest_target)); \
> + __kfuzztest_targets_end = .; \
> + . = ALIGN(PAGE_SIZE); \
> + __kfuzztest_constraints_start = .; \
> + KEEP(*(.kfuzztest_constraint)); \
> + __kfuzztest_constraints_end = .; \
> + . = ALIGN(PAGE_SIZE); \
> + __kfuzztest_annotations_start = .; \
> + KEEP(*(.kfuzztest_annotation)); \
> + __kfuzztest_annotations_end = .;
> +
> +#else /* CONFIG_KFUZZTEST */
> +#define KFUZZTEST_TABLE
> +#endif /* CONFIG_KFUZZTEST */
I think the definition of KFUZZTEST_TABLE should better be in
include/asm-generic/vmlinux.lds.h, so that it can be used by other
architectures.
> + * KFuzzTest receives its input from userspace as a single binary blob. This
> + * format allows for the serialization of complex, pointer-rich C structures
> + * into a flat buffer that can be safely passed into the kernel. This format
> + * requires only a single copy from userspace into a kenrel buffer, and no
Nit: kernel
> + * further kernel allocations. Pointers are patched internally using a "region"
> + * system where each region corresponds to some pointed-to data.
> + *
> + * Regions should be padded to respect alignment constraints of their underlying
> + * types, and should be followed by at least 8 bytes of padding. These padded
> + * regions are poisoned by KFuzzTest to ensure that KASAN catches OOB accesses.
> + *
> + * The format consists of a prefix and three main components:
Nit: s/prefix/header?
> + * 1. An 8-byte header: Contains KFUZZTEST_MAGIC in the first 4 bytes, and the
> + * version number in the subsequent 4 bytes. This ensures backwards
> + * compatibility in the event of future format changes.
> + * 2. A reloc_region_array: Defines the memory layout of the target structure
> + * by partitioning the payload into logical regions. Each logical region
> + * should contain the byte representation of the type that it represents,
> + * including any necessary padding. The region descriptors should be
> + * ordered by offset ascending.
> + * 3. A reloc_table: Provides "linking" instructions that tell the kernel how
> + * to patch pointer fields to point to the correct regions. By design,
> + * the first region (index 0) is passed as input into a FUZZ_TEST.
> + * 4. A Payload: The raw binary data for the structure and its associated
> + * buffers. This should be aligned to the maximum alignment of all
> + * regions to satisfy alignment requirements of the input types, but this
> + * isn't checked by the parser.
Maybe also call it "target structure" here?
> + * For a detailed specification of the binary layout see the full documentation
> + * at: Documentation/dev-tools/kfuzztest.rst
> + */
> +
> +/**
> + * struct reloc_region - single contiguous memory region in the payload
> + *
> + * @offset: The byte offset of this region from the start of the payload, which
> + * should be aligned to the alignment requirements of the region's
> + * underlying type.
> + * @size: The size of this region in bytes.
> + */
> +struct reloc_region {
> + uint32_t offset;
> + uint32_t size;
> +};
> +
> +/**
> + * struct reloc_region_array - array of regions in an input
Nit: newline here for consistency.
> +#define __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val1, val2, tpe) \
> + static struct kfuzztest_constraint __constraint_##arg_type##_##field __section(".kfuzztest_constraint") \
> + __used = { \
> + .input_type = "struct " #arg_type, \
> + .field_name = #field, \
> + .value1 = (uintptr_t)val1, \
> + .value2 = (uintptr_t)val2, \
> + .type = tpe, \
> + }
> +
> +/**
> + * KFUZZTEST_EXPECT_EQ - constrain a field to be equal to a value
> + *
> + * @arg_type: name of the input structure, without the leading "struct ".
> + * @field: some field that is comparable
> + * @val: a value of the same type as @arg_type.@field
> + */
> +#define KFUZZTEST_EXPECT_EQ(arg_type, field, val) \
> + do { \
> + if (arg->field != val) \
> + return; \
> + __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_EQ); \
Doesn't the compiler complain about defining __used in the middle of the block?
Maybe move it before the if statement?
> + * KFUZZTEST_EXPECT_NE - constrain a field to be not equal to a value
Nit: you could probably save some space and extract the boilerplate
from KFUZZTEST_EXPECT_XX into a helper macro.
> +config KFUZZTEST
> + bool "KFuzzTest - enable support for internal fuzz targets"
> + depends on DEBUG_FS && DEBUG_KERNEL
Given that you only have the sections defined for x86, you should
probably put something like "depends on X86_64" here.
If you go for it, please mention somewhere that the framework is only
available for x86_64, and add "x86:" to the patch title.
An alternative would be to add KFUZZTEST_TABLE to vmlinux.lds.S for
every architecture.
^ permalink raw reply [flat|nested] 15+ messages in thread
* Re: [PATCH v2 RFC 7/7] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing
2025-09-01 16:42 ` [PATCH v2 RFC 7/7] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing Ethan Graham
@ 2025-09-03 8:58 ` Ignat Korchagin
0 siblings, 0 replies; 15+ messages in thread
From: Ignat Korchagin @ 2025-09-03 8:58 UTC (permalink / raw)
To: Ethan Graham
Cc: ethangraham, glider, andreyknvl, brendan.higgins, davidgow,
dvyukov, jannh, elver, rmoar, shuah, tarasmadan, kasan-dev,
kunit-dev, linux-kernel, linux-mm, dhowells, lukas, herbert,
davem, linux-crypto
On Mon, Sep 1, 2025 at 5:43 PM Ethan Graham <ethan.w.s.graham@gmail.com> wrote:
>
> From: Ethan Graham <ethangraham@google.com>
>
> Add KFuzzTest targets for pkcs7_parse_message, rsa_parse_pub_key, and
> rsa_parse_priv_key to serve as real-world examples of how the framework is used.
>
> These functions are ideal candidates for KFuzzTest as they perform complex
> parsing of user-controlled data but are not directly exposed at the syscall
> boundary. This makes them difficult to exercise with traditional fuzzing tools
> and showcases the primary strength of the KFuzzTest framework: providing an
> interface to fuzz internal functions.
nit: can I ask for another real example? AFAIK this subsystem is
rarely used (at least directly by users). However, one user-controlled
widely used parser terrifies me: load_script() function from
binfmt_script.c, which parses the shebang line for scripts. I would
really like to see what this framework can do to fuzz that.
> The targets are defined within /lib/tests, alongside existing KUnit
> tests.
>
> Signed-off-by: Ethan Graham <ethangraham@google.com>
>
> ---
> v2:
> - Move KFuzzTest targets outside of the source files into dedicated
> _kfuzz.c files under /crypto/asymmetric_keys/tests/ as suggested by
> Ignat Korchagin and Eric Biggers.
> ---
> ---
> crypto/asymmetric_keys/Kconfig | 15 ++++++++
> crypto/asymmetric_keys/Makefile | 2 +
> crypto/asymmetric_keys/tests/Makefile | 2 +
> crypto/asymmetric_keys/tests/pkcs7_kfuzz.c | 22 +++++++++++
> .../asymmetric_keys/tests/rsa_helper_kfuzz.c | 38 +++++++++++++++++++
> 5 files changed, 79 insertions(+)
> create mode 100644 crypto/asymmetric_keys/tests/Makefile
> create mode 100644 crypto/asymmetric_keys/tests/pkcs7_kfuzz.c
> create mode 100644 crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c
>
> diff --git a/crypto/asymmetric_keys/Kconfig b/crypto/asymmetric_keys/Kconfig
> index e1345b8f39f1..7a4c5eb18624 100644
> --- a/crypto/asymmetric_keys/Kconfig
> +++ b/crypto/asymmetric_keys/Kconfig
> @@ -104,3 +104,18 @@ config FIPS_SIGNATURE_SELFTEST_ECDSA
> depends on CRYPTO_ECDSA=y || CRYPTO_ECDSA=FIPS_SIGNATURE_SELFTEST
>
> endif # ASYMMETRIC_KEY_TYPE
> +
> +config PKCS7_MESSAGE_PARSER_KFUZZ
I'm a bit worried about the scalability of defining one (visible)
config option per fuzz file/module. Is there a use-case, where a user
would want to enable some targets, but not the others? Can it be
unconditionally enabled and compiled only if CONFIG_KFUZZTEST=y?
> + bool "Build fuzz target for PKCS#7 parser"
> + depends on KFUZZTEST
> + depends on PKCS7_MESSAGE_PARSER
> + default y
> + help
> + Builds the KFuzzTest targets for PKCS#7.
> +
> +config RSA_HELPER_KFUZZ
> + bool "Build fuzz targets for RSA helpers"
> + depends on KFUZZTEST
> + default y
> + help
> + Builds the KFuzzTest targets for RSA helper functions.
> diff --git a/crypto/asymmetric_keys/Makefile b/crypto/asymmetric_keys/Makefile
> index bc65d3b98dcb..77b825aee6b2 100644
> --- a/crypto/asymmetric_keys/Makefile
> +++ b/crypto/asymmetric_keys/Makefile
> @@ -67,6 +67,8 @@ obj-$(CONFIG_PKCS7_TEST_KEY) += pkcs7_test_key.o
> pkcs7_test_key-y := \
> pkcs7_key_type.o
>
> +obj-y += tests/
> +
> #
> # Signed PE binary-wrapped key handling
> #
> diff --git a/crypto/asymmetric_keys/tests/Makefile b/crypto/asymmetric_keys/tests/Makefile
> new file mode 100644
> index 000000000000..42a779c9042a
> --- /dev/null
> +++ b/crypto/asymmetric_keys/tests/Makefile
> @@ -0,0 +1,2 @@
> +obj-$(CONFIG_PKCS7_MESSAGE_PARSER_KFUZZ) += pkcs7_kfuzz.o
> +obj-$(CONFIG_RSA_HELPER_KFUZZ) += rsa_helper_kfuzz.o
> diff --git a/crypto/asymmetric_keys/tests/pkcs7_kfuzz.c b/crypto/asymmetric_keys/tests/pkcs7_kfuzz.c
> new file mode 100644
> index 000000000000..84d0b0d8d0eb
> --- /dev/null
> +++ b/crypto/asymmetric_keys/tests/pkcs7_kfuzz.c
> @@ -0,0 +1,22 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * PKCS#7 parser KFuzzTest target
> + *
> + * Copyright 2025 Google LLC
> + */
> +#include <crypto/pkcs7.h>
> +#include <linux/kfuzztest.h>
> +
> +struct pkcs7_parse_message_arg {
> + const void *data;
> + size_t datalen;
> +};
> +
> +FUZZ_TEST(test_pkcs7_parse_message, struct pkcs7_parse_message_arg)
> +{
> + KFUZZTEST_EXPECT_NOT_NULL(pkcs7_parse_message_arg, data);
> + KFUZZTEST_ANNOTATE_LEN(pkcs7_parse_message_arg, datalen, data);
> + KFUZZTEST_EXPECT_LE(pkcs7_parse_message_arg, datalen, 16 * PAGE_SIZE);
> +
> + pkcs7_parse_message(arg->data, arg->datalen);
> +}
> diff --git a/crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c b/crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c
> new file mode 100644
> index 000000000000..5877e54cb75a
> --- /dev/null
> +++ b/crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c
> @@ -0,0 +1,38 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * RSA key extract helper KFuzzTest targets
> + *
> + * Copyright 2025 Google LLC
> + */
> +#include <linux/kfuzztest.h>
> +#include <crypto/internal/rsa.h>
> +
> +struct rsa_parse_pub_key_arg {
> + const void *key;
> + size_t key_len;
> +};
> +
> +FUZZ_TEST(test_rsa_parse_pub_key, struct rsa_parse_pub_key_arg)
> +{
> + KFUZZTEST_EXPECT_NOT_NULL(rsa_parse_pub_key_arg, key);
> + KFUZZTEST_ANNOTATE_LEN(rsa_parse_pub_key_arg, key_len, key);
> + KFUZZTEST_EXPECT_LE(rsa_parse_pub_key_arg, key_len, 16 * PAGE_SIZE);
> +
> + struct rsa_key out;
> + rsa_parse_pub_key(&out, arg->key, arg->key_len);
> +}
> +
> +struct rsa_parse_priv_key_arg {
> + const void *key;
> + size_t key_len;
> +};
> +
> +FUZZ_TEST(test_rsa_parse_priv_key, struct rsa_parse_priv_key_arg)
> +{
> + KFUZZTEST_EXPECT_NOT_NULL(rsa_parse_priv_key_arg, key);
> + KFUZZTEST_ANNOTATE_LEN(rsa_parse_priv_key_arg, key_len, key);
> + KFUZZTEST_EXPECT_LE(rsa_parse_priv_key_arg, key_len, 16 * PAGE_SIZE);
> +
> + struct rsa_key out;
> + rsa_parse_priv_key(&out, arg->key, arg->key_len);
> +}
> --
> 2.51.0.318.gd7df087d1a-goog
>
Ignat
^ permalink raw reply [flat|nested] 15+ messages in thread
* Re: [PATCH v2 RFC 3/7] kfuzztest: implement core module and input processing
2025-09-01 16:42 ` [PATCH v2 RFC 3/7] kfuzztest: implement core module and input processing Ethan Graham
@ 2025-09-03 9:53 ` Alexander Potapenko
0 siblings, 0 replies; 15+ messages in thread
From: Alexander Potapenko @ 2025-09-03 9:53 UTC (permalink / raw)
To: Ethan Graham
Cc: ethangraham, andreyknvl, brendan.higgins, davidgow, dvyukov,
jannh, elver, rmoar, shuah, tarasmadan, kasan-dev, kunit-dev,
linux-kernel, linux-mm, dhowells, lukas, ignat, herbert, davem,
linux-crypto
> +/**
> + * struct kfuzztest_simple_fuzzer_state - Global state for the KFTF module.
s/KFTF/KFuzzTest
> +
> + pr_info("KFuzzTest: registered target %s", targ->name);
> + }
> +
> + /* Taint the kernel after successfully creating the debugfs entries. */
> + add_taint(TAINT_TEST, LOCKDEP_STILL_OK);
Maybe elaborate a little that we taint the kernel because these entry
points allow calling (almost) arbitrary kernel code upon user request?
> + /* Patch pointers. */
> + for (i = 0; i < rt->num_entries; i++) {
> + re = rt->entries[i];
> + src = regions->regions[re.region_id];
> + ptr_location = (uintptr_t *)((char *)payload_start + src.offset + re.region_offset);
> + if (re.value == KFUZZTEST_REGIONID_NULL)
> + *ptr_location = (uintptr_t)NULL;
> + else if (re.value < regions->num_regions) {
> + dst = regions->regions[re.value];
> + *ptr_location = (uintptr_t)((char *)payload_start + dst.offset);
> + } else
> + return -EINVAL;
There should be braces around this return statement, see
https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces.
> +
> +static bool kfuzztest_input_is_valid(struct reloc_region_array *regions, struct reloc_table *rt, void *payload_start,
> + void *payload_end)
> +{
> + size_t payload_size = (char *)payload_end - (char *)payload_start;
You seem to be casting payload_start and payload_end to char* almost
everywhere, maybe declare them as [unsigned] char * instead?
^ permalink raw reply [flat|nested] 15+ messages in thread
* Re: [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures
2025-09-01 16:42 ` [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures Ethan Graham
2025-09-02 10:37 ` Marco Elver
2025-09-03 8:40 ` Alexander Potapenko
@ 2025-09-03 10:15 ` Alexander Potapenko
2025-09-03 11:35 ` Alexander Potapenko
3 siblings, 0 replies; 15+ messages in thread
From: Alexander Potapenko @ 2025-09-03 10:15 UTC (permalink / raw)
To: Ethan Graham
Cc: ethangraham, andreyknvl, brendan.higgins, davidgow, dvyukov,
jannh, elver, rmoar, shuah, tarasmadan, kasan-dev, kunit-dev,
linux-kernel, linux-mm, dhowells, lukas, ignat, herbert, davem,
linux-crypto
> + * kfuzztest_parse_and_relocate - validate and relocate a KFuzzTest input
> + *
> + * @input: A buffer containing the serialized input for a fuzz target.
> + * @input_size: the size in bytes of the @input buffer.
> + * @arg_ret: return pointer for the test case's input structure.
> + */
> +int kfuzztest_parse_and_relocate(void *input, size_t input_size, void **arg_ret);
Given that this function is declared in "kfuzztest: implement core
module and input processing", maybe swap the order of the two patches?
^ permalink raw reply [flat|nested] 15+ messages in thread
* Re: [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures
2025-09-01 16:42 ` [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures Ethan Graham
` (2 preceding siblings ...)
2025-09-03 10:15 ` Alexander Potapenko
@ 2025-09-03 11:35 ` Alexander Potapenko
3 siblings, 0 replies; 15+ messages in thread
From: Alexander Potapenko @ 2025-09-03 11:35 UTC (permalink / raw)
To: Ethan Graham
Cc: ethangraham, andreyknvl, brendan.higgins, davidgow, dvyukov,
jannh, elver, rmoar, shuah, tarasmadan, kasan-dev, kunit-dev,
linux-kernel, linux-mm, dhowells, lukas, ignat, herbert, davem,
linux-crypto
> + static ssize_t kfuzztest_write_cb_##test_name(struct file *filp, const char __user *buf, size_t len, \
> + loff_t *off) \
> + { \
> + test_arg_type *arg; \
> + void *buffer; \
> + int ret; \
> + \
> + buffer = kmalloc(len, GFP_KERNEL); \
> + if (!buffer) \
> + return -ENOMEM; \
> + ret = simple_write_to_buffer(buffer, len, off, buf, len); \
> + if (ret < 0) \
> + goto out; \
> + ret = kfuzztest_parse_and_relocate(buffer, len, (void **)&arg); \
> + if (ret < 0) \
> + goto out; \
> + kfuzztest_logic_##test_name(arg); \
> + ret = len; \
> +out: \
> + kfree(buffer); \
> + return ret; \
> + } \
> + static void kfuzztest_logic_##test_name(test_arg_type *arg)
simple_write_to_buffer() may write less than len bytes if it hits a
protected page.
You should check that `ret == len` and return -EFAULT if they differ.
^ permalink raw reply [flat|nested] 15+ messages in thread
* Re: [PATCH v2 RFC 4/7] tools: add kfuzztest-bridge utility
2025-09-01 16:42 ` [PATCH v2 RFC 4/7] tools: add kfuzztest-bridge utility Ethan Graham
@ 2025-09-03 14:07 ` Alexander Potapenko
0 siblings, 0 replies; 15+ messages in thread
From: Alexander Potapenko @ 2025-09-03 14:07 UTC (permalink / raw)
To: Ethan Graham
Cc: ethangraham, andreyknvl, brendan.higgins, davidgow, dvyukov,
jannh, elver, rmoar, shuah, tarasmadan, kasan-dev, kunit-dev,
linux-kernel, linux-mm, dhowells, lukas, ignat, herbert, davem,
linux-crypto
> + fd = openat(AT_FDCWD, buf, O_WRONLY, 0);
> + if (fd < 0)
> + return fd;
> +
> + bytes_written = write(fd, (void *)data, data_size);
We need a check for bytes_written == data_size here.
There's no way we can use a while-loop to ensure everything was
written (because the debugfs handler expects us to write the whole
packet at once), but at least a sanity check won't hurt.
> + err = tokenize(input_fmt, &tokens, &num_tokens);
> + if (err) {
> + printf("tokenization failed: %s\n", strerror(-err));
> + return err;
> + }
I would probably make tokenization part of parse(), but that's up to you.
> +
> + err = parse(tokens, num_tokens, &ast_prog);
> + if (err) {
> + printf("parsing failed: %s\n", strerror(-err));
> + return err;
> + }
> +
> + rs = new_rand_stream(input_filepath, 1024);
You probably need to destroy this stream after use, like you destroy the buffer.
Same for the tokens.
> +
> +int append_bytes(struct byte_buffer *buf, const char *bytes, size_t num_bytes)
> +{
> + size_t req_size;
> + size_t new_size;
> + char *new_ptr;
> +
> + req_size = buf->num_bytes + num_bytes;
> + new_size = buf->alloc_size;
> +
> + while (req_size > new_size)
> + new_size *= 2;
> + if (new_size != buf->alloc_size) {
> + new_ptr = realloc(buf->buffer, new_size);
> + if (!buf->buffer)
You should be checking for !new_ptr here.
> +
> +static bool is_alpha(char c)
> +{
> + return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z');
> +}
> +
> +static bool is_whitespace(char c)
> +{
> + switch (c) {
> + case ' ':
> + case '\r':
> + case '\t':
> + case '\n':
> + return true;
> + default:
> + return false;
> + }
> +}
> +
> +static void skip_whitespace(struct lexer *l)
> +{
> + for (;;) {
> + if (is_whitespace(peek(l))) {
> + advance(l);
> + } else {
> + return;
> + }
> + }
> +}
while (is_whitespace(peek(l))) {
advance(l);
}
> --- /dev/null
> +++ b/tools/kfuzztest-bridge/input_parser.c
> @@ -0,0 +1,373 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Parser for KFuzzTest textual input format
Some description of the format would be useful here.
> + *
> + * Copyright 2025 Google LLC
> + */
> +#include <asm-generic/errno-base.h>
> +#include <stdio.h>
> +#include <string.h>
> +
> +#include "input_lexer.h"
> +#include "input_parser.h"
> +
> +#define MAX(a, b) ((a) > (b) ? (a) : (b))
> +
> +static struct token *peek(struct parser *p)
> +{
> + return p->tokens[p->curr_token];
> +}
> +
> +static struct token *advance(struct parser *p)
> +{
> + struct token *tok = peek(p);
> + p->curr_token++;
> + return tok;
> +}
It would be nice to check for p->token_count here.
> + region->num_members = 0;
> + while (!match(p, TOKEN_RBRACE)) {
> + err = parse_type(p, &node);
> + if (err)
> + goto fail;
> + region->members = realloc(region->members, ++region->num_members * sizeof(struct ast_node *));
Missing a NULL check here.
^ permalink raw reply [flat|nested] 15+ messages in thread
end of thread, other threads:[~2025-09-03 14:07 UTC | newest]
Thread overview: 15+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2025-09-01 16:42 [PATCH v2 RFC 0/7] KFuzzTest: a new kernel fuzzing framework Ethan Graham
2025-09-01 16:42 ` [PATCH v2 RFC 1/7] mm/kasan: implement kasan_poison_range Ethan Graham
2025-09-01 16:42 ` [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures Ethan Graham
2025-09-02 10:37 ` Marco Elver
2025-09-03 8:40 ` Alexander Potapenko
2025-09-03 10:15 ` Alexander Potapenko
2025-09-03 11:35 ` Alexander Potapenko
2025-09-01 16:42 ` [PATCH v2 RFC 3/7] kfuzztest: implement core module and input processing Ethan Graham
2025-09-03 9:53 ` Alexander Potapenko
2025-09-01 16:42 ` [PATCH v2 RFC 4/7] tools: add kfuzztest-bridge utility Ethan Graham
2025-09-03 14:07 ` Alexander Potapenko
2025-09-01 16:42 ` [PATCH v2 RFC 5/7] kfuzztest: add ReST documentation Ethan Graham
2025-09-01 16:42 ` [PATCH v2 RFC 6/7] kfuzztest: add KFuzzTest sample fuzz targets Ethan Graham
2025-09-01 16:42 ` [PATCH v2 RFC 7/7] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing Ethan Graham
2025-09-03 8:58 ` Ignat Korchagin
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).