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

[Nick Desaulniers]

On Fri, Oct 8, 2021 at 5:29 AM Masami Hiramatsu <mhiramat at 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 at 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 at 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