[PATCH v7 2/5] x86/asm: add volatile, clobbers and zero-length check in inline memcmp

[Mauricio Faria de Oliveira]

Add the volatile qualifier and clobbers parameter to prevent bugs with
instruction reordering and optimization.

Also check the zero-length case, as the 'repe' prefix does not run the
'cmpsb' instruction if the 'count' register is zero, which doesn't set
the condition-code/zero flag, so the result is based on a stale flag.

Those are pre-existing issues found by Sashiko.

Link: https://sashiko.dev/#/patchset/20260701-pvh-kasan-inline-v6-0-ba99045dfa9f%40igalia.com
Signed-off-by: Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>
---
 arch/x86/include/asm/shared/string.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/shared/string.h b/arch/x86/include/asm/shared/string.h
index 02b92927553f7b8e1c87e6122bbaa70439e57ea7..166274e44f3cb49e3dccab3cdac281d67aef5d44 100644
--- a/arch/x86/include/asm/shared/string.h
+++ b/arch/x86/include/asm/shared/string.h
@@ -11,8 +11,12 @@ static __always_inline int __inline_memcmp(const void *s1, const void *s2, size_
 {
 	bool diff;
 
-	asm("repe cmpsb"
-	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
+	if (len == 0)
+		return 0;
+
+	asm volatile("repe cmpsb"
+		     : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len)
+		     : : "cc", "memory");
 
 	return diff;
 }

-- 
2.47.3