Re: [PATCH 1/3] kcov: remove #ifdef CONFIG_RANDOMIZE_BASE

Dmitry Vyukov
2017-09-19T13:03:52Z

On Tue, Sep 19, 2017 at 2:46 PM, Andrey Ryabinin
<aryabinin@virtuozzo.com> wrote:
> There is no need to surround kaslr_offset() with CONFIG_RANDOMIZE_BASE ifdef.
> kaslr_offset() will just return 0 if CONFIG_RANDOMIZE_BASE isn't set.
>
> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>

As you said this is a very hot function, but this changes code from:

2e: 48 8b 75 08 mov 0x8(%rbp),%rsi

to:

2b: 48 c7 c2 00 00 00 81 mov $0xffffffff81000000,%rdx
32: 48 81 ea 00 00 00 00 sub $0x0,%rdx
3c: 48 03 55 08 add 0x8(%rbp),%rdx

This also changes semantics, as returned PCs won't match nm/objdump output.

> ---
> kernel/kcov.c | 2 --
> 1 file changed, 2 deletions(-)
>
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 3f693a0f6f3e..2f0e7a7c7afc 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -69,9 +69,7 @@ void notrace __sanitizer_cov_trace_pc(void)
> unsigned long pos;
> unsigned long ip = _RET_IP_;
>
> -#ifdef CONFIG_RANDOMIZE_BASE
> ip -= kaslr_offset();
> -#endif
>
> /*
> * There is some code that runs in interrupts but for which
> --
> 2.13.5
>