Re: x86/kprobes: kretprobe fails to triggered if kprobe at function entry is not optimized (trigger by int3 breakpoint)

[Masami Hiramatsu]

On Wed, 26 Aug 2020 13:36:15 +0000
"Eddy_Wu@xxxxxxxxxxxxxx" <Eddy_Wu@xxxxxxxxxxxxxx> wrote:

> 
> 
> > -----Original Message-----
> > From: peterz@xxxxxxxxxxxxx <peterz@xxxxxxxxxxxxx>
> > Sent: Wednesday, August 26, 2020 6:26 PM
> > To: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > Cc: Eddy Wu (RD-TW) <Eddy_Wu@xxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx
> > Subject: Re: x86/kprobes: kretprobe fails to triggered if kprobe at function entry is not optimized (trigger by int3 breakpoint)
> >
> > On Wed, Aug 26, 2020 at 07:00:41PM +0900, Masami Hiramatsu wrote:
> > > Of course, this doesn't solve the llist_del_first() contention in the
> > > pre_kretprobe_handler(). So anyway we need a lock for per-probe llist
> > > (if I understand llist.h comment correctly.)
> >
> > Bah, lemme think about that. Kprobes really shouldn't be using locks :/
> 
> Maybe we can have per-cpu free list for retprobe_instance?
> This ensure we only have one user requesting free instance at a time, given that pre_kretprobe_handler() wont recursive.

That will restrict kretprobe not to probe the recursive call loop... 
and if a thread is scheduled, another thread will not be probed too.

I think lockless kretprobe is finally implemented by merging it's
shadow-stack with func-graph tracer.

Thank you,

> 
> We may be wasting memory if target function perfer some cpu though.
> 
> 
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-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>