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

Josh Poimboeuf
2021-06-17T04:40:32Z

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.

--
Josh