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SubjectRe: [PATCH -tip v7 09/13] kprobes: Setup instruction pointer in __kretprobe_trampoline_handler
On Thu, 17 Jun 2021 12:46:19 -0700
Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:

> 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.

So, there are 2 cases of the stacktrace from inside the kretprobe handler.

1) Call stack_trace_save() in the handler. This will unwind stack from the
handler's context. This is the case of the ftrace dynamic events.

2) Call stack_trace_save_regs(regs) in the handler with the pt_regs passed
by the kretprobe. This is the case of ebpf.

For the case 1, these patches can be dropped because ORC can unwind the
stack with UNWIND_HINT_FUNC. For the case 2, regs->ip must be set to the
correct (return) address so that ORC can find the correct entry from that
ip.

Thank you,

--
Masami Hiramatsu <mhiramat@kernel.org>

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