Re: [PATCH -tip v8 09/13] kprobes: Enable stacktrace from pt_regs in kretprobe handler

Josh Poimboeuf
2021-06-18T14:04:32Z

On Fri, Jun 18, 2021 at 04:06:46PM +0900, Masami Hiramatsu wrote:
> Since the ORC unwinder from pt_regs requires setting up regs->ip
> correctly, set the correct return address to the regs->ip before
> calling user kretprobe handler.
>
> This allows the kretrprobe handler to trace stack from the
> kretprobe's pt_regs by stack_trace_save_regs() (eBPF will do
> this), instead of stack tracing from the handler context by
> stack_trace_save() (ftrace will do this).
>
> Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> Tested-by: Andrii Nakryik <andrii@kernel.org>

Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>

> ---
> Changes in v8:
> - Update comment to clarify why this is needed.
> 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 650cbe738666..ba729ed05cb3 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1896,6 +1896,9 @@ 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) {
>

--
Josh