[PATCH] ARM: kretprobes: fix out-of-range literal issue in naked asm routine

[Masami Hiramatsu]

On Fri, 25 Mar 2022 09:03:02 +0100
Ard Biesheuvel <ardb at kernel.org> wrote:

> __kretprobe_trampoline is a naked C function implemented in asm, and the
> first thing it does is take its own address and record it in the frame
> record, so that a backtrace will properly traverse its stackframe.
> 
> As it turns out, using 'LDR <reg>, =<symbol>' notation is problematic
> here, as it relies on a literal pool to be emitted sufficiently close to
> the LDR instruction, and since the compiler is not aware of this need,
> this results in assembler errors in some cases.
> 
> Since the address taken is guaranteed to be the address of the
> instruction itself, just use ADR instead.

Ah, indeed :)

This looks good to me.

Reviewed-by: Masami Hiramatsu <mhiramat at kernel.org>
Tested-by: Masami Hiramatsu <mhiramat at kernel.org>

Thank you very much!

> 
> Cc: Masami Hiramatsu <mhiramat at kernel.org>
> Cc: Steven Rostedt <rostedt at goodmis.org>
> Cc: Nick Desaulniers <ndesaulniers at google.com>
> Reported-by: kernel test robot <lkp at intel.com>
> Fixes: 7e9bf33b8124 ("ARM: kprobes: Make a frame pointer on __kretprobe_trampoline")
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  arch/arm/probes/kprobes/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/probes/kprobes/core.c b/arch/arm/probes/kprobes/core.c
> index 9090c3a74dcc..068639a777c8 100644
> --- a/arch/arm/probes/kprobes/core.c
> +++ b/arch/arm/probes/kprobes/core.c
> @@ -377,7 +377,7 @@ void __naked __kprobes __kretprobe_trampoline(void)
>  {
>  	__asm__ __volatile__ (
>  #ifdef CONFIG_FRAME_POINTER
> -		"ldr	lr, =__kretprobe_trampoline	\n\t"
> +		"adr	lr, .			\n\t"
>  	/* __kretprobe_trampoline makes a framepointer on pt_regs. */
>  #ifdef CONFIG_CC_IS_CLANG
>  		"stmdb	sp, {sp, lr, pc}	\n\t"
> -- 
> 2.30.2
> 


-- 
Masami Hiramatsu <mhiramat at kernel.org>