On Fri, 22 Nov 2019 at 12:26, <glider@xxxxxxxxxx> wrote:
>
> Clang may replace stackdepot_memcmp() with a call to instrumented bcmp(),
> which is exactly what we wanted to avoid creating stackdepot_memcmp().
> Building the file with -fno-builtin prevents such optimizations.
>
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> To: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> ---
> This patch was previously called "stackdepot: prevent Clang from optimizing
> away stackdepot_memcmp()".
>
> v3:
> - use -fno-builtin instead of a barrier
>
> Change-Id: I4495b617b15c0ab003a61c1f0d54d0026fa8b144
> ---
> lib/Makefile | 4 ++++
> lib/stackdepot.c | 2 +-
> 2 files changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/lib/Makefile b/lib/Makefile
> index c5892807e06f..58a3e1b1a868 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -215,6 +215,10 @@ obj-$(CONFIG_SG_POOL) += sg_pool.o
> obj-$(CONFIG_STMP_DEVICE) += stmp_device.o
> obj-$(CONFIG_IRQ_POLL) += irq_poll.o
>
> +# stackdepot.c should not be instrumented or call instrumented functions.
> +# Prevent the compiler from calling builtins like memcmp() or bcmp() from this
> +# file.
> +CFLAGS_stackdepot.o += -fno-builtin
> obj-$(CONFIG_STACKDEPOT) += stackdepot.o
> KASAN_SANITIZE_stackdepot.o := n
> KCOV_INSTRUMENT_stackdepot.o := n
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 0bc6182bc7a6..6d1123123e56 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -163,7 +163,7 @@ int stackdepot_memcmp(const unsigned long *u1, const unsigned long *u2,
> unsigned int n)
> {
> for ( ; n-- ; u1++, u2++) {
> - if (*u1 != *u2)
> + if ((*u1) != (*u2))
Why this change? Can stackdepot.c be reverted?
> return 1;
> }
> return 0;
> --
> 2.24.0.432.g9d3f5f5b63-goog
>
