Re: [PATCH] kcov: don't instrument lib/find_bit.c

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, 23 Jul 2024 at 00:37, <andrey.konovalov@linux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@gmail.com>
>
> This file produces large amounts of flaky coverage not useful for the
> KCOV's intended use case (guiding the fuzzing process).
>
> Signed-off-by: Andrey Konovalov <andreyknvl@gmail.com>

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

> ---
>
> I noticed this while running one of the syzkaller's programs.
>
> In one run of the program, the number of KCOV entries amounts to ~300k,
> with the top ones:
>
>  117285 /home/user/src/lib/find_bit.c:137 (discriminator 10)
>  116752 /home/user/src/lib/find_bit.c:137 (discriminator 3)
>    2455 /home/user/src/lib/vsprintf.c:2559
>    2033 /home/user/src/fs/kernfs/dir.c:317
>    1662 /home/user/src/fs/kernfs/kernfs-internal.h:72
>    ...
>
> In another run (that triggers exactly the same behavior in the kernel),
> the amount of entries drops to ~110k:
>
>    7141 /home/user/src/lib/find_bit.c:137 (discriminator 10)
>    7110 /home/user/src/lib/find_bit.c:137 (discriminator 3)
>    2455 /home/user/src/lib/vsprintf.c:2559
>    2033 /home/user/src/fs/kernfs/dir.c:317
>    1662 /home/user/src/fs/kernfs/kernfs-internal.h:72
>     ...
>
> With this patch applied, the amount of KCOV entries for the same program
> remains somewhat stable at ~100k.
> ---
>  lib/Makefile | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/lib/Makefile b/lib/Makefile
> index 322bb127b4dc..0fde1c360f32 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -14,6 +14,7 @@ KCOV_INSTRUMENT_list_debug.o := n
>  KCOV_INSTRUMENT_debugobjects.o := n
>  KCOV_INSTRUMENT_dynamic_debug.o := n
>  KCOV_INSTRUMENT_fault-inject.o := n
> +KCOV_INSTRUMENT_find_bit.o := n
>
>  # string.o implements standard library functions like memset/memcpy etc.
>  # Use -ffreestanding to ensure that the compiler does not try to "optimize"
> --
> 2.25.1
>
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