From: Ard Biesheuvel Date: Tue, 9 Dec 2025 05:48:49 +0000 (+0100) Subject: arm64/simd: Avoid pointless clearing of FP/SIMD buffer X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=c4b502d60a71cf0c0c938f133dc4c0e2adc17b44;p=thirdparty%2Flinux.git arm64/simd: Avoid pointless clearing of FP/SIMD buffer 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 the 'for ()' trick that is also used in the implementation of the guarded scope helpers. Cc: Will Deacon Cc: Catalin Marinas Cc: Kees Cook Cc: Eric Biggers Signed-off-by: Ard Biesheuvel Fixes: 4fa617cc6851 ("arm64/fpsimd: Allocate kernel mode FP/SIMD buffers on the stack") Link: https://lore.kernel.org/r/20251209054848.998878-2-ardb@kernel.org Signed-off-by: Eric Biggers --- diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h index 0941f6f58a146..69ecbd69ca8cc 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