Re: [PATCH v5 1/2] kprobes: textmem API

Masami Hiramatsu
2024-03-26T00:58:41Z

On Mon, 25 Mar 2024 23:55:01 +0200
Jarkko Sakkinen <jarkko@kernel.org> wrote:

> 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

You don't need this block, because these APIs have dummy functions.

> /* Check if 'p' is probing a module. */
> *probed_mod = __module_text_address((unsigned long) p->addr);
> if (*probed_mod) {

So this block never be true if !CONFIG_MODULES automatically, and it should be
optimized out by compiler.

> @@ -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 */

I think this block should be moved right before remove_module_kprobe_blacklist().
Then we can gather the code depending on modules in one place.

>
> 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 */

So, keep the kprobe_module_nb outside of this #ifdef as I sed.

>
> 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

Then we don't need this #ifdef.

>
> 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 */

As I similar to the above, let's move trace_kprobe_module_nb outside
of #ifdef.

>
> 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 */

And remove this #ifdef.

Thank you!

>
> return 0;
> }
> --
> 2.44.0
>

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>