[PATCH 6.18 126/396] x86/boot: Add volatile, clobbers and zero-length test in memcmp()

6.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mauricio Faria de Oliveira <[email protected]>

[ Upstream commit a8c171c107c0b61a5e7e10cedab0fb72aeaf640d ]

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

Also add TEST for the zero-length case to set ZF, as, if the count register
is zero, the REPE prefix does not run the CMPSB instruction, leaving the ZF
flag undetermined.

  [ bp: Add a comment about the len==0 case. ]

Fixes: 62bd0337d0c4 ("Top header file for new x86 setup code")
Closes: https://sashiko.dev/#/patchset/20260701-pvh-kasan-inline-v6-0-ba99045dfa9f%40igalia.com
Suggested-by: Borislav Petkov <[email protected]>
Signed-off-by: Mauricio Faria de Oliveira <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Link: https://lore.kernel.org/all/[email protected]
Signed-off-by: Sasha Levin <[email protected]>
---
 arch/x86/boot/string.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
index b25c6a9303b73..3a2bba7c25e9d 100644
--- a/arch/x86/boot/string.c
+++ b/arch/x86/boot/string.c
@@ -32,8 +32,15 @@
 int memcmp(const void *s1, const void *s2, size_t len)
 {
 	bool diff;
-	asm("repe cmpsb"
-	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
+
+	/*
+	 * Make sure ZF is properly set in the len==0 case because in it,
+	 * RCX==0 and the REPE; CMPSB won't get executed.
+	 */
+	asm volatile("test %3, %3\n\t"
+		     "repe cmpsb"
+		     : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len)
+		     : : "cc", "memory");
 	return diff;
 }
 
-- 
2.53.0