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

Andrii Nakryiko
2021-06-17T19:46:19Z

On Thu, Jun 17, 2021 at 12:26 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Thu, Jun 17, 2021 at 11:31:03AM -0700, Andrii Nakryiko wrote:
> > 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
>
> Do the stack traces end there? Or do they continue normally after that?

That's the entire stack trace.

>
> --
> Josh
>