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

Masami Hiramatsu
2021-06-17T14:40:01Z

On Wed, 16 Jun 2021 23:40:32 -0500
Josh Poimboeuf <jpoimboe@redhat.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@redhat.com>
> > > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> > > Tested-by: Andrii Nakryik <andrii@kernel.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.

Thank you,

>
> --
> Josh
>

--
Masami Hiramatsu <mhiramat@kernel.org>