Re: [PATCH] crypto: lib/curve25519-hacl64 - Disable KASAN with clang-17 and older

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Mon, Jun 09, 2025 at 03:45:20PM -0700, Nathan Chancellor wrote:
> After commit 6f110a5e4f99 ("Disable SLUB_TINY for build testing"), which
> causes CONFIG_KASAN to be enabled in allmodconfig again, arm64
> allmodconfig builds with clang-17 and older show an instance of
> -Wframe-larger-than (which breaks the build with CONFIG_WERROR=y):
> 
>   lib/crypto/curve25519-hacl64.c:757:6: error: stack frame size (2336) exceeds limit (2048) in 'curve25519_generic' [-Werror,-Wframe-larger-than]
>     757 | void curve25519_generic(u8 mypublic[CURVE25519_KEY_SIZE],
>         |      ^
> 
> When KASAN is disabled, the stack usage is roughly quartered:
> 
>   lib/crypto/curve25519-hacl64.c:757:6: error: stack frame size (608) exceeds limit (128) in 'curve25519_generic' [-Werror,-Wframe-larger-than]
>     757 | void curve25519_generic(u8 mypublic[CURVE25519_KEY_SIZE],
>         |      ^
> 
> Using '-Rpass-analysis=stack-frame-layout' shows the following variables
> and many, many 8-byte spills when KASAN is enabled:
> 
>   Offset: [SP-144], Type: Variable, Align: 8, Size: 40
>   Offset: [SP-464], Type: Variable, Align: 8, Size: 320
>   Offset: [SP-784], Type: Variable, Align: 8, Size: 320
>   Offset: [SP-864], Type: Variable, Align: 32, Size: 80
>   Offset: [SP-896], Type: Variable, Align: 32, Size: 32
>   Offset: [SP-1016], Type: Variable, Align: 8, Size: 120
> 
> When KASAN is disabled, there are still spills but not at many and the
> variables list is smaller:
> 
>   Offset: [SP-192], Type: Variable, Align: 32, Size: 80
>   Offset: [SP-224], Type: Variable, Align: 32, Size: 32
>   Offset: [SP-344], Type: Variable, Align: 8, Size: 120
> 
> Disable KASAN for this file when using clang-17 or older to avoid
> blowing out the stack, clearing up the warning.
> 
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  lib/crypto/Makefile | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/lib/crypto/Makefile b/lib/crypto/Makefile
> index 3e79283b617d..18664127ecd6 100644
> --- a/lib/crypto/Makefile
> +++ b/lib/crypto/Makefile
> @@ -35,6 +35,10 @@ obj-$(CONFIG_CRYPTO_LIB_CURVE25519_GENERIC)	+= libcurve25519-generic.o
>  libcurve25519-generic-y				:= curve25519-fiat32.o
>  libcurve25519-generic-$(CONFIG_ARCH_SUPPORTS_INT128)	:= curve25519-hacl64.o
>  libcurve25519-generic-y				+= curve25519-generic.o
> +# clang versions prior to 18 may blow out the stack with KASAN
> +ifeq ($(call clang-min-version, 180000),)
> +KASAN_SANITIZE_curve25519-hacl64.o := n
> +endif

Seems reasonable. We've had a hassle with this for a while.

    Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>