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

[Jiri Olsa]

On Wed, May 17, 2023 at 08:42:36PM +0900, Masami Hiramatsu wrote:
> On Wed, 17 May 2023 12:47:42 +0200
> Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> 
> > 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
> 
> Oops, I missed to push my fix.
> 
> https://lore.kernel.org/all/168100731160.79534.374827110083836722.stgit@devnote2/
> 
> > 
> > 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 ;-)
> 
> Jiri, I think the above will fix the issue, right?

yes, it's the same fix, great, thanks

jirka

> 
> > 
> > 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))
> > 
> > 
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>