[PATCH v2] arm64/simd: Avoid pointless clearing of FP/SIMD buffer

[Eric Biggers]

On Tue, Dec 09, 2025 at 06:48:49AM +0100, Ard Biesheuvel wrote:
> The buffer provided to kernel_neon_begin() is only used if the task is
> scheduled out while the FP/SIMD is in use by the kernel, or when such a
> section is interrupted by a softirq that also uses the FP/SIMD.
> 
> IOW, this happens rarely, and even if it happened often, there is still
> no reason for this buffer to be cleared beforehand, which happens
> unconditionally, due to the use of a compound literal expression.
> 
> So define that buffer variable explicitly, and mark it as
> __uninitialized so that it will not get cleared, even when
> -ftrivial-auto-var-init is in effect.
> 
> This requires some preprocessor gymnastics, due to the fact that the
> variable must be defined throughout the entire guarded scope, and the
> expression
> 
>   ({ struct user_fpsimd_state __uninitialized st; &st; })
> 
> is problematic in that regard, even though the compilers seem to
> permit it. So instead, repeat for 'for ()' trick that is also used in
> the implementation of the guarded scope helpers.
> 
> Cc: Will Deacon <will at kernel.org>,
> Cc: Catalin Marinas <catalin.marinas at arm.com>,
> Cc: Kees Cook <keescook at chromium.org>
> Cc: Eric Biggers <ebiggers at kernel.org>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  arch/arm64/include/asm/simd.h | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
> index 0941f6f58a14..69ecbd69ca8c 100644
> --- a/arch/arm64/include/asm/simd.h
> +++ b/arch/arm64/include/asm/simd.h
> @@ -48,6 +48,13 @@ DEFINE_LOCK_GUARD_1(ksimd,
>  		    kernel_neon_begin(_T->lock),
>  		    kernel_neon_end(_T->lock))
>  
> -#define scoped_ksimd()	scoped_guard(ksimd, &(struct user_fpsimd_state){})
> +#define __scoped_ksimd(_label)					\
> +	for (struct user_fpsimd_state __uninitialized __st;	\
> +	     true; ({ goto _label; }))				\
> +		if (0) {					\
> +_label:			break;					\
> +		} else scoped_guard(ksimd, &__st)
> +
> +#define scoped_ksimd()	__scoped_ksimd(__UNIQUE_ID(label))
>  
>  #endif
> 

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/log/?h=libcrypto-fixes

I added:

    Fixes: 4fa617cc6851 ("arm64/fpsimd: Allocate kernel mode FP/SIMD buffers on the stack")

- Eric