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

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Message-Id: <20200826225144.6df009f67b3516a3d0d58e6e@kernel.org>
Date:   Wed, 26 Aug 2020 22:51:44 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     "Eddy_Wu@...ndmicro.com" <Eddy_Wu@...ndmicro.com>
Cc:     "peterz@...radead.org" <peterz@...radead.org>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: x86/kprobes: kretprobe fails to triggered if kprobe at function
 entry is not optimized (trigger by int3 breakpoint)

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

> 
> 
> > -----Original Message-----
> > From: peterz@...radead.org <peterz@...radead.org>
> > Sent: Wednesday, August 26, 2020 6:26 PM
> > To: Masami Hiramatsu <mhiramat@...nel.org>
> > Cc: Eddy Wu (RD-TW) <Eddy_Wu@...ndmicro.com>; linux-kernel@...r.kernel.org; x86@...nel.org
> > 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@...nel.org>