linux-kernel - Re: [PATCH -tip v7 09/13] kprobes: Setup instruction pointer in __kretprobe_trampoline

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Message-Id: <20210618085811.19f0a7b8c1e91d54483ba9f8@kernel.org>
Date:   Fri, 18 Jun 2021 08:58:11 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        Daniel Xu <dxu@...uu.xyz>,
        open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Jakub Kicinski <kuba@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kernel Team <kernel-team@...com>, Yonghong Song <yhs@...com>,
        linux-ia64@...r.kernel.org,
        Abhishek Sagar <sagar.abhishek@...il.com>
Subject: Re: [PATCH -tip v7 09/13] kprobes: Setup instruction pointer in
 __kretprobe_trampoline_handler

On Thu, 17 Jun 2021 13:21:59 -0500
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> On Thu, Jun 17, 2021 at 10:45:41AM -0700, Andrii Nakryiko wrote:
> > > > > > I know I suggested this patch, but I believe it would only be useful in
> > > > > > combination with the use of UNWIND_HINT_REGS in SAVE_REGS_STRING.  But I
> > > > > > think that would be tricky to pull off correctly.  Instead, we have
> > > > > > UNWIND_HINT_FUNC, which is working fine.
> > > > > >
> > > > > > So I'd suggest dropping this patch, as the unwinder isn't actually
> > > > > > reading regs->ip after all.
> > > > >
> > > > > ... and I guess this means patches 6-8 are no longer necessary.
> > > >
> > > > OK, I also confirmed that dropping those patche does not make any change
> > > > on the stacktrace.
> > > > Let me update the series without those.
> > >
> > > Oops, Andrii, can you also test the kernel without this patch?
> > > (you don't need to drop patch 6-8)
> > 
> > Hi Masami,
> > 
> > Dropping this patch and leaving all the other in place breaks stack
> > traces from kretprobes for BPF. I double checked with and without this
> > patch. Without this patch we are back to having broken stack traces. I
> > see either
> > 
> >   kretprobe_trampoline+0x0
> > 
> > or
> > 
> >   ftrace_trampoline+0xc8
> >   kretprobe_trampoline+0x0

Thanks for confirmation.

> > 
> > Is there any problem if you leave this patch as is?
> 
> Hm, I must be missing something then.  The patch is probably fine to
> keep, we just may need to improve the commit log so that it makes sense
> to me.

Yeah, I need to update the commit message so that this will help
the stacktrace from kretprobe's pt_regs, which will be used in bpf. 

Thank you!

> 
> Which unwinder are you using (CONFIG_UNWINDER_*)?
> 
> -- 
> Josh
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>