Re: [PATCH RFC] kprobes: Remove MODULES dependency

Masami Hiramatsu
2020-07-10T10:33:10Z

Hi Jarkko,

On Fri, 10 Jul 2020 02:45:19 +0300
Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> wrote:

> Remove MODULES dependency and migrate from module_alloc to vmalloc().
> According to Andi, the history with this dependency is that kprobes
> originally required custom LKM's, which does not hold today anymore.
>
> Right now one has to compile LKM support only to enable kprobes. With
> this change applied, it is somewhat easier to create custom test
> kernel's with a proper debugging capabilities, thus making Linux more
> developer friendly.

Agreed. Now we have several way to access kprobes via ftrace/perf/bpf
without modules. So it's a good time to remove CONFIG_MODULE dependency.

But I also would like to minimize the changes for code readability.
I made some comments below.

>
> Cc: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> ---
> arch/Kconfig | 1 -
> arch/x86/kernel/kprobes/core.c | 5 +++--
> kernel/kprobes.c | 22 ++++++++++++++++++++--
> kernel/trace/trace_kprobe.c | 12 ++++++++++++
> 4 files changed, 35 insertions(+), 5 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 8cc35dc556c7..bb59cdf335ab 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -61,7 +61,6 @@ config OPROFILE_NMI_TIMER
>
> config KPROBES
> bool "Kprobes"
> - depends on MODULES
> depends on HAVE_KPROBES
> select KALLSYMS
> help
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index ada39ddbc922..dc7b8d6fd20d 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -42,6 +42,7 @@
> #include <linux/moduleloader.h>
> #include <linux/vmalloc.h>
> #include <linux/pgtable.h>
> +#include <linux/vmalloc.h>
>
> #include <asm/text-patching.h>
> #include <asm/cacheflush.h>
> @@ -423,7 +424,7 @@ void *alloc_insn_page(void)
> {
> void *page;
>
> - page = module_alloc(PAGE_SIZE);
> + page = vmalloc(PAGE_SIZE);

No, you can not use vmalloc here. The reason why we use module_alloc()
is to allocate the executable memory for trampoline code.
So, you need to use vmalloc_exec() instead.

> if (!page)
> return NULL;
>
> @@ -446,7 +447,7 @@ void *alloc_insn_page(void)
> /* Recover page to RW mode before releasing it */
> void free_insn_page(void *page)
> {
> - module_memfree(page);
> + vfree(page);
> }
>
> static int arch_copy_kprobe(struct kprobe *p)
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 4a904cc56d68..02680642ea11 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -35,6 +35,7 @@
> #include <linux/ftrace.h>
> #include <linux/cpu.h>
> #include <linux/jump_label.h>
> +#include <linux/vmalloc.h>
>
> #include <asm/sections.h>
> #include <asm/cacheflush.h>
> @@ -111,12 +112,12 @@ enum kprobe_slot_state {
>
> void __weak *alloc_insn_page(void)
> {
> - return module_alloc(PAGE_SIZE);
> + return vmalloc(PAGE_SIZE);

Ditto.

> }
>
> void __weak free_insn_page(void *page)
> {
> - module_memfree(page);
> + vfree(page);
> }
>
> struct kprobe_insn_cache kprobe_insn_slots = {
> @@ -563,8 +564,11 @@ static void kprobe_optimizer(struct work_struct *work)
> mutex_lock(&kprobe_mutex);
> cpus_read_lock();
> mutex_lock(&text_mutex);
> +
> +#ifdef CONFIG_MODULES
> /* Lock modules while optimizing kprobes */
> mutex_lock(&module_mutex);
> +#endif

Hmm, can you reduce these "#ifdef CONFIG_MODULE"s ?

e.g.

#ifdef CONFIG_MODULES
static void lock_modules(void)
{
mutex_lock(&module_mutex);
}
...
#else
#define lock_modules() do { } while (0)
...
#endif

>
> /*
> * Step 1: Unoptimize kprobes and collect cleaned (unused and disarmed)
> @@ -589,7 +593,9 @@ static void kprobe_optimizer(struct work_struct *work)
> /* Step 4: Free cleaned kprobes after quiesence period */
> do_free_cleaned_kprobes();
>
> +#ifdef CONFIG_MODULES
> mutex_unlock(&module_mutex);
> +#endif
> mutex_unlock(&text_mutex);
> cpus_read_unlock();
>
> @@ -739,6 +745,7 @@ static int reuse_unused_kprobe(struct kprobe *ap)
> return 0;
> }
>

> +#ifdef CONFIG_MODULE
> /* Remove optimized instructions */
> static void kill_optimized_kprobe(struct kprobe *p)
> {
> @@ -764,6 +771,7 @@ static void kill_optimized_kprobe(struct kprobe *p)
> /* Don't touch the code, because it is already freed. */
> arch_remove_optimized_kprobe(op);
> }
> +#endif

This optprobe related one is OK to keep in this place.

>
> static inline
> void __prepare_optimized_kprobe(struct optimized_kprobe *op, struct kprobe *p)
> @@ -1608,6 +1616,7 @@ static int check_kprobe_address_safe(struct kprobe *p,
> goto out;
> }
>
> +#ifdef CONFIG_MODULE
> /*
> * If the module freed .init.text, we couldn't insert
> * kprobes in there.
> @@ -1618,6 +1627,7 @@ static int check_kprobe_address_safe(struct kprobe *p,
> *probed_mod = NULL;
> ret = -ENOENT;
> }
> +#endif
> }
> out:
> preempt_enable();
> @@ -2090,6 +2100,7 @@ NOKPROBE_SYMBOL(pre_handler_kretprobe);
>
> #endif /* CONFIG_KRETPROBES */
>
> +#ifdef CONFIG_MODULE
> /* Set the kprobe gone and remove its instruction buffer. */
> static void kill_kprobe(struct kprobe *p)
> {
> @@ -2114,6 +2125,7 @@ static void kill_kprobe(struct kprobe *p)
> */
> arch_remove_kprobe(p);
> }
> +#endif
>
> /* Disable one kprobe */
> int disable_kprobe(struct kprobe *kp)
> @@ -2214,6 +2226,7 @@ int kprobe_add_area_blacklist(unsigned long start, unsigned long end)
> return 0;
> }
>
> +#ifdef CONFIG_MODULE
> /* Remove all symbols in given area from kprobe blacklist */
> static void kprobe_remove_area_blacklist(unsigned long start, unsigned long end)
> {
> @@ -2231,6 +2244,7 @@ static void kprobe_remove_ksym_blacklist(unsigned long entry)
> {
> kprobe_remove_area_blacklist(entry, entry + 1);
> }
> +#endif
>
> int __init __weak arch_populate_kprobe_blacklist(void)
> {
> @@ -2274,6 +2288,7 @@ static int __init populate_kprobe_blacklist(unsigned long *start,
> return ret ? : arch_populate_kprobe_blacklist();
> }
>
> +#ifdef CONFIG_MODULE
> static void add_module_kprobe_blacklist(struct module *mod)
> {
> unsigned long start, end;
> @@ -2375,6 +2390,7 @@ static struct notifier_block kprobe_module_nb = {
> .notifier_call = kprobes_module_callback,
> .priority = 0
> };
> +#endif /* CONFIG_MODULE */
>
> /* Markers of _kprobe_blacklist section */
> extern unsigned long __start_kprobe_blacklist[];
> @@ -2425,8 +2441,10 @@ static int __init init_kprobes(void)
> err = arch_init_kprobes();
> if (!err)
> err = register_die_notifier(&kprobe_exceptions_nb);
> +#ifdef CONFIG_MODULE
> if (!err)
> err = register_module_notifier(&kprobe_module_nb);
> +#endif
>
> kprobes_initialized = (err == 0);
>
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index aefb6065b508..30969b38fce8 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -103,6 +103,7 @@ static nokprobe_inline bool trace_kprobe_has_gone(struct trace_kprobe *tk)
> return !!(kprobe_gone(&tk->rp.kp));
> }
>
> +#ifdef CONFIG_MODULE
> static nokprobe_inline bool trace_kprobe_within_module(struct trace_kprobe *tk,
> struct module *mod)
> {
> @@ -110,7 +111,9 @@ static nokprobe_inline bool trace_kprobe_within_module(struct trace_kprobe *tk,
> const char *name = trace_kprobe_symbol(tk);
> return strncmp(mod->name, name, len) == 0 && name[len] == ':';
> }
> +#endif
>
> +#ifdef CONFIG_MODULE
> static nokprobe_inline bool trace_kprobe_module_exist(struct trace_kprobe *tk)
> {
> char *p;
> @@ -129,6 +132,7 @@ static nokprobe_inline bool trace_kprobe_module_exist(struct trace_kprobe *tk)
>
> return ret;
> }
> +#endif

Here, too. If you make a dummy function in case of !CONFIG_MODULE,
we don't need to modify caller-side code.

>
> static bool trace_kprobe_is_busy(struct dyn_event *ev)
> {
> @@ -608,10 +612,12 @@ static int append_trace_kprobe(struct trace_kprobe *tk, struct trace_kprobe *to)
>
> /* Register k*probe */
> ret = __register_trace_kprobe(tk);
> +#ifdef CONFIG_MODULE
> if (ret == -ENOENT && !trace_kprobe_module_exist(tk)) {
> pr_warn("This probe might be able to register after target module is loaded. Continue.\n");
> ret = 0;
> }
> +#endif

So, please make a dummy trace_kprobe_module_exist() and keep this untouched.

>
> if (ret)
> trace_probe_unlink(&tk->tp);
> @@ -651,10 +657,12 @@ static int register_trace_kprobe(struct trace_kprobe *tk)
>
> /* Register k*probe */
> ret = __register_trace_kprobe(tk);
> +#ifdef CONFIG_MODULE
> if (ret == -ENOENT && !trace_kprobe_module_exist(tk)) {
> pr_warn("This probe might be able to register after target module is loaded. Continue.\n");
> ret = 0;
> }
> +#endif

Ditto.

>
> if (ret < 0)
> unregister_kprobe_event(tk);
> @@ -666,6 +674,7 @@ static int register_trace_kprobe(struct trace_kprobe *tk)
> return ret;
> }
>
> +#ifdef CONFIG_MODULE
> /* Module notifier call back, checking event on the module */
> static int trace_kprobe_module_callback(struct notifier_block *nb,
> unsigned long val, void *data)
> @@ -700,6 +709,7 @@ static struct notifier_block trace_kprobe_module_nb = {
> .notifier_call = trace_kprobe_module_callback,
> .priority = 1 /* Invoked after kprobe module callback */
> };
> +#endif

Unless it makes any build error, please keep it untouched.

>
> /* Convert certain expected symbols into '_' when generating event names */
> static inline void sanitize_event_name(char *name)
> @@ -1891,8 +1901,10 @@ static __init int init_kprobe_trace_early(void)
> if (ret)
> return ret;
>
> +#ifdef CONFIG_MODULE
> if (register_module_notifier(&trace_kprobe_module_nb))
> return -EINVAL;
> +#endif

Ditto.

Thank you,

>
> return 0;
> }
> --
> 2.25.1
>

--
Masami Hiramatsu <mhiramat@kernel.org>