[PATCH v10 4/4] x86/cpuid: fix unbootable VMs by really inlining memcmp() in cpuid_base_hypervisor() and xen_prepare_pvh()

[Mauricio Faria de Oliveira]

Even with __builtin the compiler may decide to use the out of line function
instead of the inline implementation.

The existing code is broken with gcc-14/15 but not gcc-12/13 (Ubuntu 25.10)
and vmlinux no longer boots with CONFIG_PVH if CONFIG_KASAN_GENERIC is set.
The instrumented out of line function performs a memory access to a region
not yet initialized by KASAN, as it is still in the PVH kernel entry point.

For testing purposes, if the size argument in cpuid_base_hypervisor() is
reduced from 12 to 8 the compiler decides to use the inline implementation.
In xen_prepare_pvh(), it (still) decides to use the inline implementation
(at least in these compiler versions), but it is not guaranteed to remain.

Switch the builtin to the inline implementation to address this.

Fixes: 416a33c9afce ("x86/cpu: fix unbootable VMs by inlining memcmp() in hypervisor_cpuid_base()")
Fixes: fbe5a6dfe492 ("xen, pvh: fix unbootable VMs by inlining memset() in xen_prepare_pvh()")
Signed-off-by: Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
---
 arch/x86/include/asm/cpuid/api.h  | 2 +-
 arch/x86/platform/pvh/enlighten.c | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/cpuid/api.h b/arch/x86/include/asm/cpuid/api.h
index 82eddfa2347b32b76c2ea9b85f005ca5416ac71f..2d9f3d4d63de6e721f275d9e80d372edbdfedf30 100644
--- a/arch/x86/include/asm/cpuid/api.h
+++ b/arch/x86/include/asm/cpuid/api.h
@@ -204,7 +204,7 @@ static inline u32 cpuid_base_hypervisor(const char *sig, u32 leaves)
 		 * from PVH early boot code before instrumentation is set up
 		 * and memcmp() itself may be instrumented.
 		 */
-		if (!__builtin_memcmp(sig, signature, 12) &&
+		if (!__inline_memcmp(sig, signature, 12) &&
 		    (leaves == 0 || ((eax - base) >= leaves)))
 			return base;
 	}
diff --git a/arch/x86/platform/pvh/enlighten.c b/arch/x86/platform/pvh/enlighten.c
index f2053cbe9b0ce3d2178938269607c652ae8f528e..cb442cbd9d828619421babb281bfe9759edbca8a 100644
--- a/arch/x86/platform/pvh/enlighten.c
+++ b/arch/x86/platform/pvh/enlighten.c
@@ -8,6 +8,7 @@
 #include <asm/hypervisor.h>
 #include <asm/e820/api.h>
 #include <asm/x86_init.h>
+#include <asm/string.h>
 
 #include <asm/xen/interface.h>
 
@@ -129,7 +130,7 @@ void __init xen_prepare_pvh(void)
 	 * This must not compile to "call memset" because memset() may be
 	 * instrumented.
 	 */
-	__builtin_memset(&pvh_bootparams, 0, sizeof(pvh_bootparams));
+	__inline_memset(&pvh_bootparams, 0, sizeof(pvh_bootparams));
 
 	hypervisor_specific_init(xen_guest);
 

-- 
2.47.3