[PATCH v5 1/2] kprobes: textmem API

Jarkko Sakkinen
2024-03-25T21:55:08Z

Tracing with kprobes while running a monolithic kernel is currently
impossible because CONFIG_KPROBES depends on CONFIG_MODULES because it uses
the kernel module allocator.

Introduce alloc_textmem() and free_textmem() for allocating executable
memory. If an arch implements these functions, it can mark this up with
the HAVE_ALLOC_EXECMEM kconfig flag.

At first this feature will be used for enabling kprobes without
modules support for arch/riscv.

Link: https://lore.kernel.org/all/20240325115632.04e37297491cadfbbf382767@kernel.org/
Suggested-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
v5:
- alloc_execmem() was missing GFP_KERNEL parameter. The patch set did
compile because 2/2 had the fixup (leaked there when rebasing the
patch set).
v4:
- Squashed a couple of unrequired CONFIG_MODULES checks.
- See https://lore.kernel.org/all/D034M18D63EC.2Y11D954YSZYK@kernel.org/
v3:
- A new patch added.
- For IS_DEFINED() I need advice as I could not really find that many
locations where it would be applicable.
---
arch/Kconfig | 16 +++++++++++++++-
include/linux/execmem.h | 13 +++++++++++++
kernel/kprobes.c | 17 ++++++++++++++---
kernel/trace/trace_kprobe.c | 8 ++++++++
4 files changed, 50 insertions(+), 4 deletions(-)
create mode 100644 include/linux/execmem.h

diff --git a/arch/Kconfig b/arch/Kconfig
index a5af0edd3eb8..33ba68b7168f 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -52,8 +52,8 @@ config GENERIC_ENTRY

config KPROBES
bool "Kprobes"
- depends on MODULES
depends on HAVE_KPROBES
+ select ALLOC_EXECMEM
select KALLSYMS
select TASKS_RCU if PREEMPTION
help
@@ -215,6 +215,20 @@ config HAVE_OPTPROBES
config HAVE_KPROBES_ON_FTRACE
bool

+config HAVE_ALLOC_EXECMEM
+ bool
+ help
+ Architectures that select this option are capable of allocating executable
+ memory, which can be used by subsystems but is not dependent of any of its
+ clients.
+
+config ALLOC_EXECMEM
+ bool "Executable (trampoline) memory allocation"
+ depends on MODULES || HAVE_ALLOC_EXECMEM
+ help
+ Select this for executable (trampoline) memory. Can be enabled when either
+ module allocator or arch-specific allocator is available.
+
config ARCH_CORRECT_STACKTRACE_ON_KRETPROBE
bool
help
diff --git a/include/linux/execmem.h b/include/linux/execmem.h
new file mode 100644
index 000000000000..ae2ff151523a
--- /dev/null
+++ b/include/linux/execmem.h
@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_EXECMEM_H
+#define _LINUX_EXECMEM_H
+
+#ifdef CONFIG_HAVE_ALLOC_EXECMEM
+void *alloc_execmem(unsigned long size, gfp_t gfp);
+void free_execmem(void *region);
+#else
+#define alloc_execmem(size, gfp) module_alloc(size)
+#define free_execmem(region) module_memfree(region)
+#endif
+
+#endif /* _LINUX_EXECMEM_H */
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 9d9095e81792..87fd8c14a938 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -44,6 +44,7 @@
#include <asm/cacheflush.h>
#include <asm/errno.h>
#include <linux/uaccess.h>
+#include <linux/execmem.h>

#define KPROBE_HASH_BITS 6
#define KPROBE_TABLE_SIZE (1 << KPROBE_HASH_BITS)
@@ -113,17 +114,17 @@ enum kprobe_slot_state {
void __weak *alloc_insn_page(void)
{
/*
- * Use module_alloc() so this page is within +/- 2GB of where the
+ * Use alloc_execmem() so this page is within +/- 2GB of where the
* kernel image and loaded module images reside. This is required
* for most of the architectures.
* (e.g. x86-64 needs this to handle the %rip-relative fixups.)
*/
- return module_alloc(PAGE_SIZE);
+ return alloc_execmem(PAGE_SIZE, GFP_KERNEL);
}

static void free_insn_page(void *page)
{
- module_memfree(page);
+ free_execmem(page);
}

struct kprobe_insn_cache kprobe_insn_slots = {
@@ -1580,6 +1581,7 @@ static int check_kprobe_address_safe(struct kprobe *p,
goto out;
}

+#ifdef CONFIG_MODULES
/* Check if 'p' is probing a module. */
*probed_mod = __module_text_address((unsigned long) p->addr);
if (*probed_mod) {
@@ -1603,6 +1605,8 @@ static int check_kprobe_address_safe(struct kprobe *p,
ret = -ENOENT;
}
}
+#endif
+
out:
preempt_enable();
jump_label_unlock();
@@ -2482,6 +2486,7 @@ int kprobe_add_area_blacklist(unsigned long start, unsigned long end)
return 0;
}

+#ifdef CONFIG_MODULES
/* Remove all symbols in given area from kprobe blacklist */
static void kprobe_remove_area_blacklist(unsigned long start, unsigned long end)
{
@@ -2499,6 +2504,7 @@ static void kprobe_remove_ksym_blacklist(unsigned long entry)
{
kprobe_remove_area_blacklist(entry, entry + 1);
}
+#endif /* CONFIG_MODULES */

int __weak arch_kprobe_get_kallsym(unsigned int *symnum, unsigned long *value,
char *type, char *sym)
@@ -2564,6 +2570,7 @@ static int __init populate_kprobe_blacklist(unsigned long *start,
return ret ? : arch_populate_kprobe_blacklist();
}

+#ifdef CONFIG_MODULES
static void add_module_kprobe_blacklist(struct module *mod)
{
unsigned long start, end;
@@ -2665,6 +2672,7 @@ static struct notifier_block kprobe_module_nb = {
.notifier_call = kprobes_module_callback,
.priority = 0
};
+#endif /* CONFIG_MODULES */

void kprobe_free_init_mem(void)
{
@@ -2724,8 +2732,11 @@ static int __init init_kprobes(void)
err = arch_init_kprobes();
if (!err)
err = register_die_notifier(&kprobe_exceptions_nb);
+
+#ifdef CONFIG_MODULES
if (!err)
err = register_module_notifier(&kprobe_module_nb);
+#endif

kprobes_initialized = (err == 0);
kprobe_sysctls_init();
diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index c4c6e0e0068b..af70bb1e9c3a 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -111,6 +111,7 @@ static nokprobe_inline bool trace_kprobe_within_module(struct trace_kprobe *tk,
return strncmp(module_name(mod), name, len) == 0 && name[len] == ':';
}

+#ifdef CONFIG_MODULES
static nokprobe_inline bool trace_kprobe_module_exist(struct trace_kprobe *tk)
{
char *p;
@@ -129,6 +130,9 @@ static nokprobe_inline bool trace_kprobe_module_exist(struct trace_kprobe *tk)

return ret;
}
+#else
+#define trace_kprobe_module_exist(tk) false /* aka a module never exists */
+#endif /* CONFIG_MODULES */

static bool trace_kprobe_is_busy(struct dyn_event *ev)
{
@@ -670,6 +674,7 @@ static int register_trace_kprobe(struct trace_kprobe *tk)
return ret;
}

+#ifdef CONFIG_MODULES
/* Module notifier call back, checking event on the module */
static int trace_kprobe_module_callback(struct notifier_block *nb,
unsigned long val, void *data)
@@ -704,6 +709,7 @@ static struct notifier_block trace_kprobe_module_nb = {
.notifier_call = trace_kprobe_module_callback,
.priority = 1 /* Invoked after kprobe module callback */
};
+#endif /* CONFIG_MODULES */

static int count_symbols(void *data, unsigned long unused)
{
@@ -1897,8 +1903,10 @@ static __init int init_kprobe_trace_early(void)
if (ret)
return ret;

+#ifdef CONFIG_MODULES
if (register_module_notifier(&trace_kprobe_module_nb))
return -EINVAL;
+#endif /* CONFIG_MODULES */

return 0;
}
--
2.44.0