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 for 'for ()' trick that is also used in the implementation of the guarded scope helpers.

Cc Will Deacon <[email protected]>,

Cc Catalin Marinas <[email protected]>,

Cc Kees Cook <[email protected]>

Cc Eric Biggers <[email protected]>

Signed-off-by Ard Biesheuvel <[email protected]>

---
 arch/arm64/include/asm/simd.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

arch/arm64/include/asm/simd.h +0 -1

index 0941f6f58a14..69ecbd69ca8c 100644
@@ -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){})