[PATCH v2] x86/boot: add volatile, clobbers and zero-length test in memcmp()

[Mauricio Faria de Oliveira]

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.

Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
Closes: https://sashiko.dev/#/patchset/20260701-pvh-kasan-inline-v6-0-ba99045dfa9f%40igalia.com
Link: https://lore.kernel.org/all/20260718150230.GAaluVhlozNI__9IsA@fat_crate.local/
Fixes: 62bd0337d0c4 ("Top header file for new x86 setup code")
Link: https://lore.kernel.org/all/20260722170334.GCamD35gwrCng30WH1@fat_crate.local/
Signed-off-by: Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>
---
Sending this patch separately from its previous series ("v1" below) as requested [1].

The backports for 6.12 and earlier will be slightly different due to
commit c6c973dbfa5e ("x86/asm: Remove code depending on __GCC_ASM_FLAG_OUTPUTS__").
I'd be happy to handle it.

The disassembly of memcmp() in arch/x86/boot/string.o is as expected:

  32:   66 85 c9                test   %cx,%cx
  35:   f3 a6                   repz cmpsb %es:(%edi),%ds:(%esi)
  37:   0f 95 c0                setne  %al

And bzImage still boots.

[1] https://lore.kernel.org/all/20260722170334.GCamD35gwrCng30WH1@fat_crate.local/

Changes in v2:
- Remove this patch from "v1" series (Borislav Petkov).
- Change some words' capitalization/quotes (Borislav Petkov).
- Change zero-length check from C to asm (H. Peter Anvin).
- Link to "v1": https://lore.kernel.org/all/20260721-pvh-kasan-inline-v7-2-38979a50cef0@xxxxxxxxxx/
---
 arch/x86/boot/string.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
index ac0f900ebc47efa81c92e1bb2010ea41677899c4..6827551720dd9b96afe8c55a051401d6cdc6f05e 100644
--- a/arch/x86/boot/string.c
+++ b/arch/x86/boot/string.c
@@ -32,8 +32,10 @@
 int memcmp(const void *s1, const void *s2, size_t len)
 {
 	bool diff;
-	asm("repe cmpsb"
-	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
+	asm volatile("test %3, %3\n\t"
+		     "repe cmpsb"
+		     : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len)
+		     : : "cc", "memory");
 	return diff;
 }
 

---
base-commit: b4515cf4156356e8f4fe6e0fdc17f59adab9772f
change-id: 20260723-x86-memcmp-asm-082937e9c391

Best regards,
-- 
Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>