[PATCH] ARM: kprobes: move __kretprobe_trampoline to out of line assembler

[Nick Desaulniers]

On Mon, Sep 26, 2022 at 11:37 AM Nick Desaulniers
<ndesaulniers at google.com> wrote:
>
> diff --git a/arch/arm/probes/kprobes/core.c b/arch/arm/probes/kprobes/core.c
> index 1435b508aa36..17d7e0259e63 100644
> --- a/arch/arm/probes/kprobes/core.c
> +++ b/arch/arm/probes/kprobes/core.c
> @@ -375,58 +375,10 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
>         return NOTIFY_DONE;
>  }
>
> -/*
> - * 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, 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__ (
> -               "ldr    lr, =__kretprobe_trampoline     \n\t"
> -#ifdef CONFIG_FRAME_POINTER
> -       /* __kretprobe_trampoline makes a framepointer on pt_regs. */
> -#ifdef CONFIG_CC_IS_CLANG
> -               "stmdb  sp, {sp, lr, pc}        \n\t"
> -               "sub    sp, sp, #12             \n\t"
> -               /* In clang case, pt_regs->ip = lr. */
> -               "stmdb  sp!, {r0 - r11, lr}     \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, sp, lr, pc}    \n\t"
> -               "sub    sp, sp, #16             \n\t"
> -               "stmdb  sp!, {r0 - r11}         \n\t"
> -               /* fp points regs->r15 (pc) */
> -               "add    fp, sp, #60             \n\t"
> -#endif /* CONFIG_CC_IS_CLANG */
> -#else /* !CONFIG_FRAME_POINTER */
> -               /* store SP, LR on stack and add EABI unwind hint */
> -               "stmdb  sp, {sp, lr, pc}        \n\t"
> -               ".save  {sp, lr, pc}    \n\t"
> -               "sub    sp, sp, #16             \n\t"
> -               "stmdb  sp!, {r0 - r11}         \n\t"
> -               ".pad   #52                             \n\t"
> -#endif /* CONFIG_FRAME_POINTER */
> -               "mov    r0, sp                  \n\t"
> -               "bl     trampoline_handler      \n\t"
> -               "mov    lr, r0                  \n\t"
> -               "ldmia  sp!, {r0 - r11}         \n\t"
> -               "add    sp, sp, #16             \n\t"
> -#ifdef CONFIG_THUMB2_KERNEL
> -               "bx     lr                      \n\t"
> -#else
> -               "mov    pc, lr                  \n\t"
> -#endif
> -               : : : "memory");
> -}
> +/*void __kretprobe_trampoline(void);*/

^ d'oh, that commented out declaration should have been removed. Will
wait for feedback wrt. usage of UNWIND and Fixes tag to see if a v2 is
necessary, vs what can be modified if/when applied by maintainers.
-- 
Thanks,
~Nick Desaulniers