Commit ab51e15d authored by Masami Hiramatsu's avatar Masami Hiramatsu Committed by Alexei Starovoitov
Browse files

fprobe: Introduce FPROBE_FL_KPROBE_SHARED flag for fprobe 



Introduce FPROBE_FL_KPROBE_SHARED flag for sharing fprobe callback with
kprobes safely from the viewpoint of recursion.

Since the recursion safety of the fprobe (and ftrace) is a bit different
from the kprobes, this may cause an issue if user wants to run the same
code from the fprobe and the kprobes.

The kprobes has per-cpu 'current_kprobe' variable which protects the
kprobe handler from recursion in any case. On the other hand, the fprobe
uses only ftrace_test_recursion_trylock(), which will allow interrupt
context calls another (or same) fprobe during the fprobe user handler is
running.

This is not a matter in cases if the common callback shared among the
kprobes and the fprobe has its own recursion detection, or it can handle
the recursion in the different contexts (normal/interrupt/NMI.)
But if it relies on the 'current_kprobe' recursion lock, it has to check
kprobe_running() and use kprobe_busy_*() APIs.

Fprobe has FPROBE_FL_KPROBE_SHARED flag to do this. If your common callback
code will be shared with kprobes, please set FPROBE_FL_KPROBE_SHARED
*before* registering the fprobe, like;

 fprobe.flags = FPROBE_FL_KPROBE_SHARED;

 register_fprobe(&fprobe, "func*", NULL);

This will protect your common callback from the nested call.

Signed-off-by: default avatarMasami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
Tested-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/164735293127.1084943.15687374237275817599.stgit@devnote2
parent 6ee64cc3

include/linux/fprobe.h

+12 −0
Original line number Diff line number Diff line
@@ -34,13 +34,25 @@ struct fprobe { 
	void (*exit_handler)(struct fprobe *fp, unsigned long entry_ip, struct pt_regs *regs);

};



/* This fprobe is soft-disabled. */

#define FPROBE_FL_DISABLED	1



/*

 * This fprobe handler will be shared with kprobes.

 * This flag must be set before registering.

 */

#define FPROBE_FL_KPROBE_SHARED	2



static inline bool fprobe_disabled(struct fprobe *fp)

{

	return (fp) ? fp->flags & FPROBE_FL_DISABLED : false;

}



static inline bool fprobe_shared_with_kprobes(struct fprobe *fp)

{

	return (fp) ? fp->flags & FPROBE_FL_KPROBE_SHARED : false;

}



#ifdef CONFIG_FPROBE

int register_fprobe(struct fprobe *fp, const char *filter, const char *notfilter);

int register_fprobe_ips(struct fprobe *fp, unsigned long *addrs, int num);

include/linux/kprobes.h

+3 −0
Original line number Diff line number Diff line
@@ -427,6 +427,9 @@ static inline struct kprobe *kprobe_running(void) 
{

	return NULL;

}

#define kprobe_busy_begin()	do {} while (0)

#define kprobe_busy_end()	do {} while (0)



static inline int register_kprobe(struct kprobe *p)

{

	return -EOPNOTSUPP;

kernel/trace/fprobe.c

+18 −1
Original line number Diff line number Diff line
@@ -56,6 +56,20 @@ static void fprobe_handler(unsigned long ip, unsigned long parent_ip, 
}

NOKPROBE_SYMBOL(fprobe_handler);



static void fprobe_kprobe_handler(unsigned long ip, unsigned long parent_ip,

				  struct ftrace_ops *ops, struct ftrace_regs *fregs)

{

	struct fprobe *fp = container_of(ops, struct fprobe, ops);



	if (unlikely(kprobe_running())) {

		fp->nmissed++;

		return;

	}

	kprobe_busy_begin();

	fprobe_handler(ip, parent_ip, ops, fregs);

	kprobe_busy_end();

}



static void fprobe_exit_handler(struct rethook_node *rh, void *data,

				struct pt_regs *regs)

{
@@ -110,6 +124,9 @@ static unsigned long *get_ftrace_locations(const char **syms, int num) 
static void fprobe_init(struct fprobe *fp)

{

	fp->nmissed = 0;

	if (fprobe_shared_with_kprobes(fp))

		fp->ops.func = fprobe_kprobe_handler;

	else

		fp->ops.func = fprobe_handler;

	fp->ops.flags |= FTRACE_OPS_FL_SAVE_REGS;

}