Re: [PATCH v3 2/4] fprobe: make fprobe_kprobe_handler recursion free

[Jiri Olsa]

On Wed, May 17, 2023 at 11:45:07AM +0800, Ze Gao wrote:
> Current implementation calls kprobe related functions before doing
> ftrace recursion check in fprobe_kprobe_handler, which opens door
> to kernel crash due to stack recursion if preempt_count_{add, sub}
> is traceable in kprobe_busy_{begin, end}.
> 
> Things goes like this without this patch quoted from Steven:
> "
> fprobe_kprobe_handler() {
>    kprobe_busy_begin() {
>       preempt_disable() {
>          preempt_count_add() {  <-- trace
>             fprobe_kprobe_handler() {
> 		[ wash, rinse, repeat, CRASH!!! ]
> "
> 
> By refactoring the common part out of fprobe_kprobe_handler and
> fprobe_handler and call ftrace recursion detection at the very beginning,
> the whole fprobe_kprobe_handler is free from recursion.
> 
> Signed-off-by: Ze Gao <zegao@xxxxxxxxxxx>
> Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> Link: https://lore.kernel.org/linux-trace-kernel/20230516071830.8190-3-zegao@xxxxxxxxxxx
> ---
>  kernel/trace/fprobe.c | 59 ++++++++++++++++++++++++++++++++-----------
>  1 file changed, 44 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index 9abb3905bc8e..097c740799ba 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -20,30 +20,22 @@ struct fprobe_rethook_node {
>  	char data[];
>  };
>  
> -static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
> -			   struct ftrace_ops *ops, struct ftrace_regs *fregs)
> +static inline void __fprobe_handler(unsigned long ip, unsigned long
> +		parent_ip, struct ftrace_ops *ops, struct ftrace_regs *fregs)
>  {
>  	struct fprobe_rethook_node *fpr;
>  	struct rethook_node *rh = NULL;
>  	struct fprobe *fp;
>  	void *entry_data = NULL;
> -	int bit, ret;
> +	int ret;
>  

this change uncovered bug for me introduced by [1]

the bpf's kprobe multi uses either fprobe's entry_handler or exit_handler,
so the 'ret' value is undefined for return probe path and occasionally we
won't setup rethook and miss the return probe

we can either squash this change into your patch or I can make separate
patch for that.. but given that [1] is quite recent we could just silently
fix that ;-)

jirka


[1] 39d954200bf6 fprobe: Skip exit_handler if entry_handler returns !0

---
diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index 9abb3905bc8e..293184227394 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c
@@ -27,7 +27,7 @@ static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
 	struct rethook_node *rh = NULL;
 	struct fprobe *fp;
 	void *entry_data = NULL;
-	int bit, ret;
+	int bit, ret = 0;
 
 	fp = container_of(ops, struct fprobe, ops);
 	if (fprobe_disabled(fp))