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

Josh Poimboeuf
2021-06-17T04:39:09Z

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.

(I apologize for the churn, and my lack of responsiveness in general.
I've been working on moving to California, which is no small task with a
family, and in this housing market...)

--
Josh