[RFC 15/19] kernel/api: add debugfs interface for kernel API specifications

Sasha Levin
2025-06-14T13:49:14Z

Add a debugfs interface to expose kernel API specifications at runtime.
This allows tools and users to query the complete API specifications
through the debugfs filesystem.

The interface provides:
- /sys/kernel/debug/kapi/list - lists all available API specifications
- /sys/kernel/debug/kapi/specs/<name> - detailed info for each API

Each specification file includes:
- Function name, version, and descriptions
- Execution context requirements and flags
- Parameter details with types, flags, and constraints
- Return value specifications and success conditions
- Error codes with descriptions and conditions
- Locking requirements and constraints
- Signal handling specifications
- Examples, notes, and deprecation status

This enables runtime introspection of kernel APIs for documentation
tools, static analyzers, and debugging purposes.

Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/api/Kconfig | 20 +++
kernel/api/Makefile | 5 +-
kernel/api/kapi_debugfs.c | 340 ++++++++++++++++++++++++++++++++++++++
3 files changed, 364 insertions(+), 1 deletion(-)
create mode 100644 kernel/api/kapi_debugfs.c

diff --git a/kernel/api/Kconfig b/kernel/api/Kconfig
index fde25ec70e134..d2754b21acc43 100644
--- a/kernel/api/Kconfig
+++ b/kernel/api/Kconfig
@@ -33,3 +33,23 @@ config KAPI_RUNTIME_CHECKS
development. The checks use WARN_ONCE to report violations.

If unsure, say N.
+
+config KAPI_SPEC_DEBUGFS
+ bool "Export kernel API specifications via debugfs"
+ depends on KAPI_SPEC
+ depends on DEBUG_FS
+ help
+ This option enables exporting kernel API specifications through
+ the debugfs filesystem. When enabled, specifications can be
+ accessed at /sys/kernel/debug/kapi/.
+
+ The debugfs interface provides:
+ - A list of all available API specifications
+ - Detailed information for each API including parameters,
+ return values, errors, locking requirements, and constraints
+ - Complete machine-readable representation of the specs
+
+ This is useful for documentation tools, static analyzers, and
+ runtime introspection of kernel APIs.
+
+ If unsure, say N.
diff --git a/kernel/api/Makefile b/kernel/api/Makefile
index 4120ded7e5cf1..07b8c007ec156 100644
--- a/kernel/api/Makefile
+++ b/kernel/api/Makefile
@@ -4,4 +4,7 @@
#

# Core API specification framework
-obj-$(CONFIG_KAPI_SPEC) += kernel_api_spec.o
\ No newline at end of file
+obj-$(CONFIG_KAPI_SPEC) += kernel_api_spec.o
+
+# Debugfs interface for kernel API specs
+obj-$(CONFIG_KAPI_SPEC_DEBUGFS) += kapi_debugfs.o
\ No newline at end of file
diff --git a/kernel/api/kapi_debugfs.c b/kernel/api/kapi_debugfs.c
new file mode 100644
index 0000000000000..bf65ea6a49205
--- /dev/null
+++ b/kernel/api/kapi_debugfs.c
@@ -0,0 +1,340 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Kernel API specification debugfs interface
+ *
+ * This provides a debugfs interface to expose kernel API specifications
+ * at runtime, allowing tools and users to query the complete API specs.
+ */
+
+#include <linux/debugfs.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/seq_file.h>
+#include <linux/kernel_api_spec.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+
+/* External symbols for kernel API spec section */
+extern struct kernel_api_spec __start_kapi_specs[];
+extern struct kernel_api_spec __stop_kapi_specs[];
+
+static struct dentry *kapi_debugfs_root;
+
+/* Helper function to print parameter type as string */
+static const char *param_type_str(enum kapi_param_type type)
+{
+ switch (type) {
+ case KAPI_TYPE_INT: return "int";
+ case KAPI_TYPE_UINT: return "uint";
+ case KAPI_TYPE_PTR: return "ptr";
+ case KAPI_TYPE_STRUCT: return "struct";
+ case KAPI_TYPE_UNION: return "union";
+ case KAPI_TYPE_ARRAY: return "array";
+ case KAPI_TYPE_FD: return "fd";
+ case KAPI_TYPE_ENUM: return "enum";
+ case KAPI_TYPE_USER_PTR: return "user_ptr";
+ case KAPI_TYPE_PATH: return "path";
+ case KAPI_TYPE_FUNC_PTR: return "func_ptr";
+ case KAPI_TYPE_CUSTOM: return "custom";
+ default: return "unknown";
+ }
+}
+
+/* Helper to print parameter flags */
+static void print_param_flags(struct seq_file *m, u32 flags)
+{
+ seq_printf(m, " flags: ");
+ if (flags & KAPI_PARAM_IN) seq_printf(m, "IN ");
+ if (flags & KAPI_PARAM_OUT) seq_printf(m, "OUT ");
+ if (flags & KAPI_PARAM_INOUT) seq_printf(m, "INOUT ");
+ if (flags & KAPI_PARAM_OPTIONAL) seq_printf(m, "OPTIONAL ");
+ if (flags & KAPI_PARAM_CONST) seq_printf(m, "CONST ");
+ if (flags & KAPI_PARAM_USER) seq_printf(m, "USER ");
+ if (flags & KAPI_PARAM_VOLATILE) seq_printf(m, "VOLATILE ");
+ if (flags & KAPI_PARAM_DMA) seq_printf(m, "DMA ");
+ if (flags & KAPI_PARAM_ALIGNED) seq_printf(m, "ALIGNED ");
+ seq_printf(m, "\n");
+}
+
+/* Helper to print context flags */
+static void print_context_flags(struct seq_file *m, u32 flags)
+{
+ seq_printf(m, "Context flags: ");
+ if (flags & KAPI_CTX_PROCESS) seq_printf(m, "PROCESS ");
+ if (flags & KAPI_CTX_HARDIRQ) seq_printf(m, "HARDIRQ ");
+ if (flags & KAPI_CTX_SOFTIRQ) seq_printf(m, "SOFTIRQ ");
+ if (flags & KAPI_CTX_NMI) seq_printf(m, "NMI ");
+ if (flags & KAPI_CTX_SLEEPABLE) seq_printf(m, "SLEEPABLE ");
+ if (flags & KAPI_CTX_ATOMIC) seq_printf(m, "ATOMIC ");
+ if (flags & KAPI_CTX_PREEMPT_DISABLED) seq_printf(m, "PREEMPT_DISABLED ");
+ if (flags & KAPI_CTX_IRQ_DISABLED) seq_printf(m, "IRQ_DISABLED ");
+ seq_printf(m, "\n");
+}
+
+/* Show function for individual API spec */
+static int kapi_spec_show(struct seq_file *m, void *v)
+{
+ struct kernel_api_spec *spec = m->private;
+ int i;
+
+ seq_printf(m, "Kernel API Specification\n");
+ seq_printf(m, "========================\n\n");
+
+ /* Basic info */
+ seq_printf(m, "Name: %s\n", spec->name);
+ seq_printf(m, "Version: %u\n", spec->version);
+ seq_printf(m, "Description: %s\n", spec->description);
+ if (strlen(spec->long_description) > 0)
+ seq_printf(m, "Long description: %s\n", spec->long_description);
+
+ /* Context */
+ print_context_flags(m, spec->context_flags);
+ seq_printf(m, "\n");
+
+ /* Parameters */
+ if (spec->param_count > 0) {
+ seq_printf(m, "Parameters (%u):\n", spec->param_count);
+ for (i = 0; i < spec->param_count; i++) {
+ struct kapi_param_spec *param = &spec->params[i];
+ seq_printf(m, " [%d] %s:\n", i, param->name);
+ seq_printf(m, " type: %s (%s)\n",
+ param_type_str(param->type), param->type_name);
+ print_param_flags(m, param->flags);
+ if (strlen(param->description) > 0)
+ seq_printf(m, " description: %s\n", param->description);
+ if (param->size > 0)
+ seq_printf(m, " size: %zu\n", param->size);
+ if (param->alignment > 0)
+ seq_printf(m, " alignment: %zu\n", param->alignment);
+
+ /* Print constraints if any */
+ if (param->constraint_type != KAPI_CONSTRAINT_NONE) {
+ seq_printf(m, " constraints:\n");
+ switch (param->constraint_type) {
+ case KAPI_CONSTRAINT_RANGE:
+ seq_printf(m, " type: range\n");
+ seq_printf(m, " min: %lld\n", param->min_value);
+ seq_printf(m, " max: %lld\n", param->max_value);
+ break;
+ case KAPI_CONSTRAINT_MASK:
+ seq_printf(m, " type: mask\n");
+ seq_printf(m, " valid_bits: 0x%llx\n", param->valid_mask);
+ break;
+ case KAPI_CONSTRAINT_ENUM:
+ seq_printf(m, " type: enum\n");
+ seq_printf(m, " count: %u\n", param->enum_count);
+ break;
+ case KAPI_CONSTRAINT_CUSTOM:
+ seq_printf(m, " type: custom\n");
+ if (strlen(param->constraints) > 0)
+ seq_printf(m, " description: %s\n",
+ param->constraints);
+ break;
+ default:
+ break;
+ }
+ }
+ seq_printf(m, "\n");
+ }
+ }
+
+ /* Return value */
+ seq_printf(m, "Return value:\n");
+ seq_printf(m, " type: %s\n", spec->return_spec.type_name);
+ if (strlen(spec->return_spec.description) > 0)
+ seq_printf(m, " description: %s\n", spec->return_spec.description);
+
+ switch (spec->return_spec.check_type) {
+ case KAPI_RETURN_EXACT:
+ seq_printf(m, " success: == %lld\n", spec->return_spec.success_value);
+ break;
+ case KAPI_RETURN_RANGE:
+ seq_printf(m, " success: [%lld, %lld]\n",
+ spec->return_spec.success_min,
+ spec->return_spec.success_max);
+ break;
+ case KAPI_RETURN_FD:
+ seq_printf(m, " success: valid file descriptor (>= 0)\n");
+ break;
+ case KAPI_RETURN_ERROR_CHECK:
+ seq_printf(m, " success: error check\n");
+ break;
+ case KAPI_RETURN_CUSTOM:
+ seq_printf(m, " success: custom check\n");
+ break;
+ default:
+ break;
+ }
+ seq_printf(m, "\n");
+
+ /* Errors */
+ if (spec->error_count > 0) {
+ seq_printf(m, "Errors (%u):\n", spec->error_count);
+ for (i = 0; i < spec->error_count; i++) {
+ struct kapi_error_spec *err = &spec->errors[i];
+ seq_printf(m, " %s (%d): %s\n",
+ err->name, err->error_code, err->description);
+ if (strlen(err->condition) > 0)
+ seq_printf(m, " condition: %s\n", err->condition);
+ }
+ seq_printf(m, "\n");
+ }
+
+ /* Locks */
+ if (spec->lock_count > 0) {
+ seq_printf(m, "Locks (%u):\n", spec->lock_count);
+ for (i = 0; i < spec->lock_count; i++) {
+ struct kapi_lock_spec *lock = &spec->locks[i];
+ const char *type_str;
+ switch (lock->lock_type) {
+ case KAPI_LOCK_MUTEX: type_str = "mutex"; break;
+ case KAPI_LOCK_SPINLOCK: type_str = "spinlock"; break;
+ case KAPI_LOCK_RWLOCK: type_str = "rwlock"; break;
+ case KAPI_LOCK_SEMAPHORE: type_str = "semaphore"; break;
+ case KAPI_LOCK_RCU: type_str = "rcu"; break;
+ case KAPI_LOCK_SEQLOCK: type_str = "seqlock"; break;
+ default: type_str = "unknown"; break;
+ }
+ seq_printf(m, " %s (%s): %s\n",
+ lock->lock_name, type_str, lock->description);
+ if (lock->acquired)
+ seq_printf(m, " acquired by function\n");
+ if (lock->released)
+ seq_printf(m, " released by function\n");
+ }
+ seq_printf(m, "\n");
+ }
+
+ /* Constraints */
+ if (spec->constraint_count > 0) {
+ seq_printf(m, "Additional constraints (%u):\n", spec->constraint_count);
+ for (i = 0; i < spec->constraint_count; i++) {
+ seq_printf(m, " - %s\n", spec->constraints[i].description);
+ }
+ seq_printf(m, "\n");
+ }
+
+ /* Signals */
+ if (spec->signal_count > 0) {
+ seq_printf(m, "Signal handling (%u):\n", spec->signal_count);
+ for (i = 0; i < spec->signal_count; i++) {
+ struct kapi_signal_spec *sig = &spec->signals[i];
+ seq_printf(m, " %s (%d):\n", sig->signal_name, sig->signal_num);
+ seq_printf(m, " direction: ");
+ if (sig->direction & KAPI_SIGNAL_SEND) seq_printf(m, "send ");
+ if (sig->direction & KAPI_SIGNAL_RECEIVE) seq_printf(m, "receive ");
+ if (sig->direction & KAPI_SIGNAL_HANDLE) seq_printf(m, "handle ");
+ if (sig->direction & KAPI_SIGNAL_BLOCK) seq_printf(m, "block ");
+ if (sig->direction & KAPI_SIGNAL_IGNORE) seq_printf(m, "ignore ");
+ seq_printf(m, "\n");
+ seq_printf(m, " action: ");
+ switch (sig->action) {
+ case KAPI_SIGNAL_ACTION_DEFAULT: seq_printf(m, "default"); break;
+ case KAPI_SIGNAL_ACTION_TERMINATE: seq_printf(m, "terminate"); break;
+ case KAPI_SIGNAL_ACTION_COREDUMP: seq_printf(m, "coredump"); break;
+ case KAPI_SIGNAL_ACTION_STOP: seq_printf(m, "stop"); break;
+ case KAPI_SIGNAL_ACTION_CONTINUE: seq_printf(m, "continue"); break;
+ case KAPI_SIGNAL_ACTION_CUSTOM: seq_printf(m, "custom"); break;
+ case KAPI_SIGNAL_ACTION_RETURN: seq_printf(m, "return"); break;
+ case KAPI_SIGNAL_ACTION_RESTART: seq_printf(m, "restart"); break;
+ default: seq_printf(m, "unknown"); break;
+ }
+ seq_printf(m, "\n");
+ if (strlen(sig->description) > 0)
+ seq_printf(m, " description: %s\n", sig->description);
+ }
+ seq_printf(m, "\n");
+ }
+
+ /* Additional info */
+ if (strlen(spec->examples) > 0) {
+ seq_printf(m, "Examples:\n%s\n\n", spec->examples);
+ }
+ if (strlen(spec->notes) > 0) {
+ seq_printf(m, "Notes:\n%s\n\n", spec->notes);
+ }
+ if (strlen(spec->since_version) > 0) {
+ seq_printf(m, "Since: %s\n", spec->since_version);
+ }
+ if (spec->deprecated) {
+ seq_printf(m, "DEPRECATED");
+ if (strlen(spec->replacement) > 0)
+ seq_printf(m, " - use %s instead", spec->replacement);
+ seq_printf(m, "\n");
+ }
+
+ return 0;
+}
+
+static int kapi_spec_open(struct inode *inode, struct file *file)
+{
+ return single_open(file, kapi_spec_show, inode->i_private);
+}
+
+static const struct file_operations kapi_spec_fops = {
+ .open = kapi_spec_open,
+ .read = seq_read,
+ .llseek = seq_lseek,
+ .release = single_release,
+};
+
+/* Show all available API specs */
+static int kapi_list_show(struct seq_file *m, void *v)
+{
+ struct kernel_api_spec *spec;
+ int count = 0;
+
+ seq_printf(m, "Available Kernel API Specifications\n");
+ seq_printf(m, "===================================\n\n");
+
+ for (spec = __start_kapi_specs; spec < __stop_kapi_specs; spec++) {
+ seq_printf(m, "%s - %s\n", spec->name, spec->description);
+ count++;
+ }
+
+ seq_printf(m, "\nTotal: %d specifications\n", count);
+ return 0;
+}
+
+static int kapi_list_open(struct inode *inode, struct file *file)
+{
+ return single_open(file, kapi_list_show, NULL);
+}
+
+static const struct file_operations kapi_list_fops = {
+ .open = kapi_list_open,
+ .read = seq_read,
+ .llseek = seq_lseek,
+ .release = single_release,
+};
+
+static int __init kapi_debugfs_init(void)
+{
+ struct kernel_api_spec *spec;
+ struct dentry *spec_dir;
+
+ /* Create main directory */
+ kapi_debugfs_root = debugfs_create_dir("kapi", NULL);
+
+ /* Create list file */
+ debugfs_create_file("list", 0444, kapi_debugfs_root, NULL, &kapi_list_fops);
+
+ /* Create specs subdirectory */
+ spec_dir = debugfs_create_dir("specs", kapi_debugfs_root);
+
+ /* Create a file for each API spec */
+ for (spec = __start_kapi_specs; spec < __stop_kapi_specs; spec++) {
+ debugfs_create_file(spec->name, 0444, spec_dir, spec, &kapi_spec_fops);
+ }
+
+ pr_info("Kernel API debugfs interface initialized\n");
+ return 0;
+}
+
+static void __exit kapi_debugfs_exit(void)
+{
+ debugfs_remove_recursive(kapi_debugfs_root);
+}
+
+/* Initialize as part of kernel, not as a module */
+fs_initcall(kapi_debugfs_init);
\ No newline at end of file
--
2.39.5