Re: [PATCH] fprobe: Ensure running fprobe_exit_handler() finished before calling rethook_free()
From: Steven Rostedt <rostedt@goodmis.org>
Date: 2023-07-06 01:27:06
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On Wed, 28 Jun 2023 23:44:02 +0900 "Masami Hiramatsu (Google)" [off-list ref] wrote:
From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Ensure running fprobe_exit_handler() has finished before
calling rethook_free() in the unregister_fprobe() so that caller can free
the fprobe right after unregister_fprobe().
unregister_fprobe() ensured that all running fprobe_entry/exit_handler()
have finished by calling unregister_ftrace_function() which synchronizes
RCU. But commit 5f81018753df ("fprobe: Release rethook after the
ftrace_ops is unregistered") changed to call rethook_free() after
unregister_ftrace_function(). So call rethook_stop() to make rethook
disabled before unregister_ftrace_function() and ensure it again.I'm confused. I still don't understand why it is bad to call unregister_ftrace_function() *before* rethook_free(). Can you show the race condition you are trying to avoid? -- Steve
quoted hunk ↗ jump to hunk
Fixes: 5f81018753df ("fprobe: Release rethook after the ftrace_ops is unregistered") Cc: stable@vger.kernel.org Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org> --- include/linux/rethook.h | 1 + kernel/trace/fprobe.c | 3 +++ kernel/trace/rethook.c | 13 +++++++++++++ 3 files changed, 17 insertions(+)diff --git a/include/linux/rethook.h b/include/linux/rethook.h index c8ac1e5afcd1..bdbe6717f45a 100644 --- a/include/linux/rethook.h +++ b/include/linux/rethook.h@@ -59,6 +59,7 @@ struct rethook_node { }; struct rethook *rethook_alloc(void *data, rethook_handler_t handler); +void rethook_stop(struct rethook *rh); void rethook_free(struct rethook *rh); void rethook_add_node(struct rethook *rh, struct rethook_node *node); struct rethook_node *rethook_try_get(struct rethook *rh);diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c index 0121e8c0d54e..75517667b54f 100644 --- a/kernel/trace/fprobe.c +++ b/kernel/trace/fprobe.c@@ -364,6 +364,9 @@ int unregister_fprobe(struct fprobe *fp) fp->ops.saved_func != fprobe_kprobe_handler)) return -EINVAL; + if (fp->rethook) + rethook_stop(fp->rethook); + ret = unregister_ftrace_function(&fp->ops); if (ret < 0) return ret;diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c index 60f6cb2b486b..468006cce7ca 100644 --- a/kernel/trace/rethook.c +++ b/kernel/trace/rethook.c@@ -53,6 +53,19 @@ static void rethook_free_rcu(struct rcu_head *head) kfree(rh); } +/** + * rethook_stop() - Stop using a rethook. + * @rh: the struct rethook to stop. + * + * Stop using a rethook to prepare for freeing it. If you want to waitfor + * all running rethook handler before calling rethook_free(), you need to + * call this first and wait RCU, and call rethook_free(). + */ +void rethook_stop(struct rethook *rh) +{ + WRITE_ONCE(rh->handler, NULL); +} + /** * rethook_free() - Free struct rethook. * @rh: the struct rethook to be freed.