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

[Josh Poimboeuf]

On Fri, Jun 18, 2021 at 09:33:13AM +0900, Masami Hiramatsu wrote:
> On Thu, 17 Jun 2021 12:46:19 -0700
> Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
> 
> > On Thu, Jun 17, 2021 at 12:26 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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@xxxxxxxxxx> 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.

Agreed!  I get it now.  Thanks :-)

-- 
Josh