Re: [PATCH 7/8] ARM: kprobes: Make a frame pointer on __kretprobe_trampoline

Nick Desaulniers
2021-10-11T19:06:50Z

On Fri, Oct 8, 2021 at 5:29 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> Currently kretprobe on ARM just fills r0-r11 of pt_regs, but
> that is not enough for the stacktrace. Moreover, from the user
> kretprobe handler, stacktrace needs a frame pointer on the
> __kretprobe_trampoline.
>
> This adds a frame pointer on __kretprobe_trampoline for both gcc
> and clang case. Those have different frame pointer so we need
> different but similar stack on pt_regs.
>
> Gcc makes the frame pointer (fp) to point the 'pc' address of
> the {fp, ip (=sp), lr, pc}, this means {r11, r13, r14, r15}.
> Thus if we save the r11 (fp) on pt_regs->r12, we can make this
> set on the end of pt_regs.
>
> On the other hand, Clang makes the frame pointer to point the
> 'fp' address of {fp, lr} on stack. Since the next to the
> pt_regs->lr is pt_regs->sp, I reused the pair of pt_regs->fp
> and pt_regs->ip.
> So this stores the 'lr' on pt_regs->ip and make the fp to point
> pt_regs->fp.
>
> For both cases, saves __kretprobe_trampoline address to
> pt_regs->lr, so that the stack tracer can identify this frame
> pointer has been made by the __kretprobe_trampoline.
>
> Note that if the CONFIG_FRAME_POINTER is not set, this keeps
> fp as is.
>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
> arch/arm/probes/kprobes/core.c | 29 ++++++++++++++++++++++++-----
> 1 file changed, 24 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm/probes/kprobes/core.c b/arch/arm/probes/kprobes/core.c
> index 95f23b47ba27..7cbd65a22769 100644
> --- a/arch/arm/probes/kprobes/core.c
> +++ b/arch/arm/probes/kprobes/core.c
> @@ -368,16 +368,35 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
> /*
> * When a retprobed function returns, trampoline_handler() is called,
> * calling the kretprobe's handler. We construct a struct pt_regs to
> - * give a view of registers r0-r11 to the user return-handler. This is
> - * not a complete pt_regs structure, but that should be plenty sufficient
> - * for kretprobe handlers which should normally be interested in r0 only
> - * anyway.
> + * give a view of registers r0-r11, sp, lr, and pc to the user
> + * return-handler. This is not a complete pt_regs structure, but that
> + * should be enough for stacktrace from the return handler with or
> + * without pt_regs.
> */
> void __naked __kprobes __kretprobe_trampoline(void)
> {
> __asm__ __volatile__ (
> - "sub sp, sp, #16 \n\t"
> + "ldr lr, =__kretprobe_trampoline \n\t"
> + "stmdb sp!, {sp, lr, pc} \n\t"
> +#ifdef CONFIG_FRAME_POINTER
> + /* __kretprobe_trampoline makes a framepointer on pt_regs. */
> +#ifdef CONFIG_CC_IS_CLANG
> + /* In clang case, pt_regs->ip = lr. */
> + "stmdb sp!, {lr} \n\t"
> "stmdb sp!, {r0 - r11} \n\t"
> + /* fp points regs->r11 (fp) */
> + "add fp, sp, #44 \n\t"
> +#else /* !CONFIG_CC_IS_CLANG */
> + /* In gcc case, pt_regs->ip = fp. */
> + "stmdb sp!, {fp} \n\t"
> + "stmdb sp!, {r0 - r11} \n\t"
> + /* fp points regs->r15 (pc) */
> + "add fp, sp, #60 \n\t"

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> +#endif /* CONFIG_CC_IS_CLANG */
> +#else /* !CONFIG_FRAME_POINTER */
> + "sub sp, sp, #4 \n\t"
> + "stmdb sp!, {r0 - r11} \n\t"
> +#endif /* CONFIG_FRAME_POINTER */
> "mov r0, sp \n\t"
> "bl trampoline_handler \n\t"
> "mov lr, r0 \n\t"
>

--
Thanks,
~Nick Desaulniers