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

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Date:   Thu, 17 Jun 2021 10:45:41 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.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 Thu, Jun 17, 2021 at 8:02 AM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> On Thu, 17 Jun 2021 23:40:01 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> > On Wed, 16 Jun 2021 23:40:32 -0500
> > Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >
> > > On Wed, Jun 16, 2021 at 11:39:11PM -0500, Josh Poimboeuf wrote:
> > > > On Thu, May 27, 2021 at 03:40:29PM +0900, Masami Hiramatsu wrote:
> > > > > To simplify the stacktrace with pt_regs from kretprobe handler,
> > > > > set the correct return address to the instruction pointer in
> > > > > the pt_regs before calling kretprobe handlers.
> > > > >
> > > > > Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > > > > Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> > > > > Tested-by: Andrii Nakryik <andrii@...nel.org>
> > > > > ---
> > > > >  Changes in v3:
> > > > >   - Cast the correct_ret_addr to unsigned long.
> > > > > ---
> > > > >  kernel/kprobes.c |    3 +++
> > > > >  1 file changed, 3 insertions(+)
> > > > >
> > > > > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > > > > index 54e5b89aad67..1598aca375c9 100644
> > > > > --- a/kernel/kprobes.c
> > > > > +++ b/kernel/kprobes.c
> > > > > @@ -1914,6 +1914,9 @@ unsigned long __kretprobe_trampoline_handler(struct pt_regs *regs,
> > > > >                 BUG_ON(1);
> > > > >         }
> > > > >
> > > > > +       /* Set the instruction pointer to the correct address */
> > > > > +       instruction_pointer_set(regs, (unsigned long)correct_ret_addr);
> > > > > +
> > > > >         /* Run them. */
> > > > >         first = current->kretprobe_instances.first;
> > > > >         while (first) {
> > > > >
> > > >
> > > > Hi Masami,
> > > >
> > > > 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?

> This changes the kretprobe to pass the return address via regs->ip to handler.
> Dynamic-event doesn't use it, but I'm not sure bcc is using it or not.
>
> Thank you,
>
> >
> > Thank you,
> >
> > >
> > > --
> > > Josh
> > >
> >
> >
> > --
> > Masami Hiramatsu <mhiramat@...nel.org>
>
>
> --
> Masami Hiramatsu <mhiramat@...nel.org>