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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Thu, 17 Jun 2021 11:31:03 -0700
Message-ID: <CAEf4BzbQxxAWEvE7BfrBPCPzBjrAEVL9cg-duwbFNzEmbPPW2w@mail.gmail.com>
Subject: Re: [PATCH -tip v7 09/13] kprobes: Setup instruction pointer in __kretprobe_trampoline_handler
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>,
        Daniel Xu <dxu@dxuuu.xyz>,
        open list <linux-kernel@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, Jakub Kicinski <kuba@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Kernel Team <kernel-team@fb.com>, Yonghong Song <yhs@fb.com>,
        linux-ia64@vger.kernel.org,
        Abhishek Sagar <sagar.abhishek@gmail.com>
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On Thu, Jun 17, 2021 at 11:22 AM Josh Poimboeuf <jpoimboe@redhat.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
> >
> > 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.
>
> Which unwinder are you using (CONFIG_UNWINDER_*)?
>

$ rg UNWINDER ~/linux-build/default/.config
5585:CONFIG_UNWINDER_ORC=y
5586:# CONFIG_UNWINDER_FRAME_POINTER is not set
5587:# CONFIG_UNWINDER_GUESS is not set

> --
> Josh
>