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

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Date:   Thu, 17 Jun 2021 19:58:24 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        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 Fri, Jun 18, 2021 at 08:58:11AM +0900, Masami Hiramatsu wrote:
> 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. 

Yes, I presume it's because when bpf unwinds from the kretprobe regs,
the unwinder starts from regs->ip, which is otherwise undefined because
it's skipped by SAVE_REGS_STRING.

-- 
Josh