Re: [PATCH 1/2] arm64: kasan: do not instrument stacktrace.c

[Mark Rutland <mark.rutland@arm.com>]

On Sun, May 22, 2022 at 01:50:58AM +0200, andrey.konovalov@linux.dev wrote:
> From: Andrey Konovalov <andreyknvl@google.com>
> 
> Disable KASAN instrumentation of arch/arm64/kernel/stacktrace.c.
> 
> This speeds up Generic KASAN by 5-20%.
> 
> As a side-effect, KASAN is now unable to detect bugs in the stack trace
> collection code. This is taken as an acceptable downside.
> 
> Also replace READ_ONCE_NOCHECK() with READ_ONCE() in stacktrace.c.
> As the file is now not instrumented, there is no need to use the
> NOCHECK version of READ_ONCE().
> 
> Suggested-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  arch/arm64/kernel/Makefile     | 3 +++
>  arch/arm64/kernel/stacktrace.c | 4 ++--
>  2 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index fa7981d0d917..da8cf6905c76 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -14,6 +14,9 @@ CFLAGS_REMOVE_return_address.o = $(CC_FLAGS_FTRACE)
>  CFLAGS_REMOVE_syscall.o	 = -fstack-protector -fstack-protector-strong
>  CFLAGS_syscall.o	+= -fno-stack-protector
>  
> +# Do not instrument to improve performance.
> +KASAN_SANITIZE_stacktrace.o := n

Can we make that a little more descriptive? e.g.

# When KASAN is enabled, a stacktrace is recorded for every alloc/free, which
# can significantly impact performance. Avoid instrumenting the stacktrace code
# to minimize this impact.
KASAN_SANITIZE_stacktrace.o := n

With that:

  Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> +
>  # It's not safe to invoke KCOV when portions of the kernel environment aren't
>  # available or are out-of-sync with HW state. Since `noinstr` doesn't always
>  # inhibit KCOV instrumentation, disable it for the entire compilation unit.
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index e4103e085681..33e96ae4b15f 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -110,8 +110,8 @@ static int notrace unwind_frame(struct task_struct *tsk,
>  	 * Record this frame record's values and location. The prev_fp and
>  	 * prev_type are only meaningful to the next unwind_frame() invocation.
>  	 */
> -	frame->fp = READ_ONCE_NOCHECK(*(unsigned long *)(fp));
> -	frame->pc = READ_ONCE_NOCHECK(*(unsigned long *)(fp + 8));
> +	frame->fp = READ_ONCE(*(unsigned long *)(fp));
> +	frame->pc = READ_ONCE(*(unsigned long *)(fp + 8));
>  	frame->prev_fp = fp;
>  	frame->prev_type = info.type;
>  
> -- 
> 2.25.1
>