Re: [PATCH v2 3/8] riscv: stacktrace: disable KASAN instrumentation for stacktrace.o

[Shuai Xue <xueshuai@linux.alibaba.com>]

On 5/28/26 4:23 PM, Wang Han wrote:
> KASAN records stack traces for every alloc/free, which means it walks
> the unwinder very frequently. Instrumenting the stack trace collection
> code itself adds substantial overhead and makes the traces themselves
> noisier.
> 
> Mark stacktrace.o as not KASAN-instrumented, matching the arm, arm64
> and x86 treatment of their stack unwinding code. This is a prerequisite
> preference for the upcoming reliable unwinder, but the change is valid
> on its own.
> 
> Signed-off-by: Wang Han <wanghan@linux.alibaba.com>
> ---
>   arch/riscv/kernel/Makefile | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> index cabb99cadfb6..1cb6c9ab2981 100644
> --- a/arch/riscv/kernel/Makefile
> +++ b/arch/riscv/kernel/Makefile
> @@ -44,6 +44,11 @@ CFLAGS_REMOVE_return_address.o	= $(CC_FLAGS_FTRACE)
>   CFLAGS_REMOVE_sbi_ecall.o = $(CC_FLAGS_FTRACE)
>   endif
>   
> +# When KASAN is enabled, a stack trace is recorded for every alloc/free, which
> +# can significantly impact performance. Avoid instrumenting the stack trace
> +# collection code to minimize this impact.
> +KASAN_SANITIZE_stacktrace.o := n
> +

I checked the three referenced arches:
   - arm    (arch/arm/kernel/Makefile):    KASAN only
   - arm64  (arch/arm64/kernel/Makefile):  KASAN only
   - x86    (arch/x86/kernel/Makefile):    KASAN *and* KCOV
            (KCOV_INSTRUMENT_stacktrace.o := n, plus dumpstack and
             the unwind_*.o TUs)

So as written, this patch matches arm/arm64 but NOT x86. KCOV
instruments every basic-block edge; the unwinder is a hot path
(doubly so under KASAN, where it runs on every alloc/free), so the
same rationale that justifies disabling KASAN applies to KCOV. I'd
suggest making the claim true by adding:

  KCOV_INSTRUMENT_stacktrace.o := n

(RISC-V keeps its entire unwinder in stacktrace.o, so unlike x86
there's no dumpstack/unwind_*.o to also annotate — the single TU
covers the equivalent scope.)

Alternatively, if you'd rather keep it minimal, just drop "and x86"
from the changelog so the claim matches the code.


Thanks.
Shuai

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