[PATCH v6 1/2] kprobes: implemente trampoline memory allocator

[Jarkko Sakkinen]

Tracing with kprobes while running a monolithic kernel is currently
impossible because CONFIG_KPROBES depends on CONFIG_MODULES.

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

The second new kconfig flag is ALLOC_EXECMEM, which can be selected if
either MODULES is selected or HAVE_ALLOC_EXECMEM is support by the arch. If
HAVE_ALLOC_EXECMEM is not supported by an arch, module_alloc() and
module_memfree() are used as a fallback, thus retaining backwards
compatibility to earlier kernel versions.

This will allow architecture to enable kprobes traces without requiring
to enable module.

The support can be implemented with four easy steps:

1. Implement alloc_execmem().
2. Implement free_execmem().
3. Edit arch/<arch>/Makefile.
4. Set HAVE_ALLOC_EXECMEM in arch/<arch>/Kconfig.

Link: https://lore.kernel.org/all/20240325115632.04e37297491cadfbbf382767@xxxxxxxxxx/
Suggested-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
---
v6:
- Use null pointer for notifiers and register the module notifier only if
  IS_ENABLED(CONFIG_MODULES) is set.
- Fixed typo in the commit message and wrote more verbose description
  of the feature.
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@xxxxxxxxxx/
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            | 19 +++++++++++++++----
 kernel/trace/trace_kprobe.c | 15 +++++++++++++--
 4 files changed, 56 insertions(+), 7 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..9ed07a4bf9e3 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;
@@ -2660,6 +2667,9 @@ static int kprobes_module_callback(struct notifier_block *nb,
 	mutex_unlock(&kprobe_mutex);
 	return NOTIFY_DONE;
 }
+#else
+#define kprobes_module_callback	(NULL)
+#endif /* CONFIG_MODULES */
 
 static struct notifier_block kprobe_module_nb = {
 	.notifier_call = kprobes_module_callback,
@@ -2724,7 +2734,8 @@ static int __init init_kprobes(void)
 	err = arch_init_kprobes();
 	if (!err)
 		err = register_die_notifier(&kprobe_exceptions_nb);
-	if (!err)
+
+	if (!err && IS_ENABLED(CONFIG_MODULES))
 		err = register_module_notifier(&kprobe_module_nb);
 
 	kprobes_initialized = (err == 0);
diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index c4c6e0e0068b..13ea0d0994d5 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)
@@ -699,6 +704,9 @@ static int trace_kprobe_module_callback(struct notifier_block *nb,
 
 	return NOTIFY_DONE;
 }
+#else
+#define trace_kprobe_module_callback (NULL)
+#endif /* CONFIG_MODULES */
 
 static struct notifier_block trace_kprobe_module_nb = {
 	.notifier_call = trace_kprobe_module_callback,
@@ -1897,8 +1905,11 @@ static __init int init_kprobe_trace_early(void)
 	if (ret)
 		return ret;
 
-	if (register_module_notifier(&trace_kprobe_module_nb))
-		return -EINVAL;
+	if (IS_ENABLED(CONFIG_MODULES)) {
+		ret = register_module_notifier(&trace_kprobe_module_nb);
+		if (ret)
+			return -EINVAL;
+	}
 
 	return 0;
 }
-- 
2.44.0