From a8c171c107c0b61a5e7e10cedab0fb72aeaf640d Mon Sep 17 00:00:00 2001 From: Mauricio Faria de Oliveira Date: Thu, 23 Jul 2026 20:08:04 -0300 Subject: [PATCH] x86/boot: Add volatile, clobbers and zero-length test in memcmp() 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 Signed-off-by: Mauricio Faria de Oliveira Signed-off-by: Borislav Petkov (AMD) Link: https://lore.kernel.org/all/20260721-pvh-kasan-inline-v7-2-38979a50cef0@igalia.com --- 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 ac0f900ebc47..1632d40e1f54 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; }