Series comparison

The issue of unbootable VMs with CONFIG_PVH due to CONFIG_KASAN is back.

Booting directly from vmlinux (instead of bzImage) now fails with gcc-14/15
(but works with gcc-12/13) if CONFIG_KASAN_GENERIC is set, on Ubuntu 25.10.

The PVH code is required/supposed not to hit the KASAN memory access check
in the kernel entry point as KASAN has not yet been setup, or an exception
is hit and the boot fails.

This was previously described and addressed with __builtin_mem{cmp,set}():
- commit 661362e3dcab ("xen, pvh: fix unbootable VMs (PVH + KASAN - AMD_MEM_ENCRYPT)")
- commit 416a33c9afce ("x86/cpu: fix unbootable VMs by inlining memcmp() in hypervisor_cpuid_base()")
- commit fbe5a6dfe492 ("xen, pvh: fix unbootable VMs by inlining memset() in xen_prepare_pvh()")

However, even with __builtin the compiler may decide to use the out of line
function instead of the inline implementation. So, that does not really fix
the issue unconditionally, as it's being seen (details below).

In order to address this, it's required to switch to inline implementations
that do not depend on the compiler.

There's such a memset in <asm/string.h> and memcmp in 'boot/string.c', now
exposed in <asm/string.h> too. Use them instead of builtins in PVH entry.

Testing:

- Booting from vmlinux (fixed) and bzImage (still works) using
  allnoconfig + CONFIG_PVH + CONFIG_KASAN with gcc-12/13/14/15.

- Building allyesconfig (check for issues with <asm/string.h>).

Details/Debugging:

- Only CONFIG_PVH (works):

make allnoconfig
  ./scripts/config \
    -e 64BIT -e HYPERVISOR_GUEST -e PVH \
    -e SERIAL_8250 -e SERIAL_8250_CONSOLE
  make olddefconfig
  make -j$(nproc) vmlinux

qemu-system-x86_64 \
    -accel kvm -nodefaults -nographic -serial stdio \
    -kernel vmlinux -append 'console=ttyS0'
  ...
  SeaBIOS (version ...)
  Booting from ROM...
  Linux version ...
  ...
  <Ctrl-C>

- With CONFIG_KASAN (fails)
  
  ./scripts/config -e KASAN
  make olddefconfig
  make -j$(nproc) vmlinux

qemu-system-x86_64 \
    -accel kvm -nodefaults -nographic -serial stdio \
    -kernel vmlinux -append 'console=ttyS0'
  ...
  SeaBIOS (version ...)
  Booting from ROM...
  <QEMU reboot loop, flashing the text above>

- Debugging:

Enable debug info and rebuild.

QEMU: enable and wait for GDB, stop rebooting, remain running.

qemu-system-x86_64 \
    -s -S -no-reboot -no-shutdown \
    <other options>

gdb vmlinux
  (gdb) target remote localhost:1234
  ...
  (gdb) c
  ...
  Thread 2 received signal SIGQUIT, Quit.
  ...
  (gdb) info threads
    Id   Target Id                    Frame
    1    Thread 1.1 (CPU#0 [running]) bytes_is_nonzero (
      start=0xfffffbfff031eebe <error: Cannot access memory at address 0xfffffbfff031eebe>, size=1)
      at .../linux/mm/kasan/generic.c:98
  * 2    Thread 1.2 (CPU#1 [halted ]) 0x00000000000fd0a9 in ?? ()
  ...
  (gdb) thr 1
  ...
  (gdb) bt
  #0  bytes_is_nonzero (start=0xfffffbfff031eebe <error: Cannot access memory at address 0xfffffbfff031eebe>, size=1)
      at .../linux/mm/kasan/generic.c:98
  #1  memory_is_nonzero (start=0xfffffbfff031eebe, end=0xfffffbfff031eebf) at .../linux/mm/kasan/generic.c:115
  #2  memory_is_poisoned_n (addr=0xffffffff818f75f0, size=8) at .../linux/mm/kasan/generic.c:140
  #3  memory_is_poisoned (addr=0xffffffff818f75f0, size=8) at .../linux/mm/kasan/generic.c:172
  #4  check_region_inline (addr=0xffffffff818f75f0, size=8, write=false, ret_ip=18446744071585002062)
      at .../linux/mm/kasan/generic.c:191
  #5  kasan_check_range (addr=addr@entry=0xffffffff818f75f0, size=size@entry=8, write=write@entry=false,
      ret_ip=18446744071585002062) at .../linux/mm/kasan/generic.c:200
  #6  0xffffffff813eb283 in __asan_loadN (addr=addr@entry=0xffffffff818f75f0, size=size@entry=8)
      at .../linux/mm/kasan/generic.c:278
  #7  0xffffffff815df24e in memcmp (cs=cs@entry=0xffffffff818f75f0, ct=ct@entry=0x1be2fe4, count=<optimized out>,
      count@entry=12) at .../linux/lib/string.c:683
  #8  0xffffffff81ba2323 in cpuid_base_hypervisor (sig=0xffffffff818f75f0 "XenVMMXenVMM", leaves=2)
      at .../linux/arch/x86/include/asm/cpuid/api.h:206
  #9  xen_cpuid_base () at .../linux/arch/x86/include/asm/xen/hypervisor.h:46
  #10 xen_prepare_pvh () at .../linux/arch/x86/platform/pvh/enlighten.c:119
  #11 0x0000000001ba2588 in ?? ()
  #12 0x0000000000000000 in ?? ()
  (gdb)

Frames #7-#8 show the non-builtin memcmp() (lib/string.c) was called
  even with __builtin_memcmp() being used in cpuid_base_hypervisor().

Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
---
Mauricio Faria de Oliveira (3):
      x86/asm, x86/boot: Expose inline memcmp
      x86/cpuid: fix unbootable VMs by really inlining memcmp() in hypervisor_cpuid_base()
      x86/pvh: fix unbootable VMs by really inlining memset() in xen_prepare_pvh()

arch/x86/boot/string.c            |  6 ++----
 arch/x86/include/asm/cpuid/api.h  |  2 +-
 arch/x86/include/asm/string.h     | 11 +++++++++++
 arch/x86/platform/pvh/enlighten.c |  3 ++-
 4 files changed, 16 insertions(+), 6 deletions(-)
---
base-commit: 6596a02b207886e9e00bb0161c7fd59fea53c081
change-id: 20260422-pvh-kasan-inline-6efac77f1b27

Best regards,
-- 
Mauricio Faria de Oliveira <mfo@igalia.com>

Move the inline memcmp function, currently only available to boot code
(boot/string.c), into the header with similar inline string functions
(include/asm/string.h) so it may be reused.

Add a _SETUP guard in string.h so not to include the 32/64-bit specific
string headers in boot/ code (16-bit, real mode) and avoid build errors.

Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
---
 arch/x86/boot/string.c        |  6 ++----
 arch/x86/include/asm/string.h | 11 +++++++++++
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
index XXXXXXX..XXXXXXX 100644
--- a/arch/x86/boot/string.c
+++ b/arch/x86/boot/string.c
@@ -XXX,XX +XXX,XX @@
 #include <linux/errno.h>
 #include <linux/limits.h>
 #include <asm/asm.h>
+#include <asm/string.h>
 #include "ctype.h"
 #include "string.h"
 
@@ -XXX,XX +XXX,XX @@
 
 int memcmp(const void *s1, const void *s2, size_t len)
 {
-	bool diff;
-	asm("repe cmpsb"
-	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
-	return diff;
+	return __inline_memcmp(s1, s2, len);
 }
 
 /*
diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
index XXXXXXX..XXXXXXX 100644
--- a/arch/x86/include/asm/string.h
+++ b/arch/x86/include/asm/string.h
@@ -XXX,XX +XXX,XX @@
 #ifndef _ASM_X86_STRING_H
 #define _ASM_X86_STRING_H
 
+#ifndef _SETUP
 #ifdef CONFIG_X86_32
 # include <asm/string_32.h>
 #else
 # include <asm/string_64.h>
 #endif
+#endif
 
 static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len)
 {
@@ -XXX,XX +XXX,XX @@ static __always_inline void *__inline_memset(void *s, int v, size_t n)
 	return ret;
 }
 
+static __always_inline int __inline_memcmp(const void *s1, const void *s2, size_t len)
+{
+	bool diff;
+
+	asm("repe cmpsb"
+	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
+	return diff;
+}
+
 #endif /* _ASM_X86_STRING_H */

-- 
2.51.0

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.

For testing purposes, if the size argument is reduced from 12 to 8 then the
compiler decides to use the inline implementation; that shows results vary.

Switch the builtin to the inline implementation to address it.

Fixes: 416a33c9afce ("x86/cpu: fix unbootable VMs by inlining memcmp() in hypervisor_cpuid_base()")
Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
---
 arch/x86/include/asm/cpuid/api.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpuid/api.h b/arch/x86/include/asm/cpuid/api.h
index XXXXXXX..XXXXXXX 100644
--- a/arch/x86/include/asm/cpuid/api.h
+++ b/arch/x86/include/asm/cpuid/api.h
@@ -XXX,XX +XXX,XX @@ 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;
 	}

-- 
2.51.0

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

This particular one (still) generated the inline implementation as expected
(at least in these compiler versions), but this is not guaranteed to remain
the case, as seen with the previous commit.

Switch the builtin to the inline implementation to prevent a similar issue.

Fixes: fbe5a6dfe492 ("xen, pvh: fix unbootable VMs by inlining memset() in xen_prepare_pvh()")
Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
---
 arch/x86/platform/pvh/enlighten.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/platform/pvh/enlighten.c b/arch/x86/platform/pvh/enlighten.c
index XXXXXXX..XXXXXXX 100644
--- a/arch/x86/platform/pvh/enlighten.c
+++ b/arch/x86/platform/pvh/enlighten.c
@@ -XXX,XX +XXX,XX @@
 #include <asm/hypervisor.h>
 #include <asm/e820/api.h>
 #include <asm/x86_init.h>
+#include <asm/string.h>
 
 #include <asm/xen/interface.h>
 
@@ -XXX,XX +XXX,XX @@ 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);

-- 
2.51.0

[RESEND v5 (2026-06-01): address feedback from Borislav Petkov in Patch 1.]

The issue of unbootable VMs with CONFIG_PVH due to CONFIG_KASAN is back.

Booting directly from vmlinux (instead of bzImage) now fails with gcc-14/15
(but works with gcc-12/13) if CONFIG_KASAN_GENERIC is set, on Ubuntu 25.10.

The PVH code is required/supposed not to use the KASAN memory access check
in the kernel entry point as KASAN has not yet been setup, or an exception
is hit and the boot fails.

However, even with __builtin the compiler may decide to use the out of line
function instead of the inline implementation. So, that does not really fix
the issue unconditionally; see details below.

In order to address this, it's required to switch to inline implementations
that do not depend on the compiler.

There's such a memset() in <asm/string.h> and memcmp() in 'boot/string.c'.
Use them instead of builtins in PVH entry.

Testing:

- Booting from vmlinux (fixed) and bzImage (still works) using
  allnoconfig + CONFIG_PVH + CONFIG_KASAN with gcc-12/13/14/15.

- Building with CONFIG_KEXEC_FILE, CONFIG_CFI and !CONFIG_KASAN with LLVM 20
  (check for a build error not caught previously).

Details/Debugging:

- Only CONFIG_PVH (works):

make allnoconfig
  ./scripts/config \
    -e 64BIT -e HYPERVISOR_GUEST -e PVH \
    -e SERIAL_8250 -e SERIAL_8250_CONSOLE
  make olddefconfig
  make -j$(nproc) vmlinux

- With CONFIG_KASAN (fails)

./scripts/config -e KASAN
  make olddefconfig
  make -j$(nproc) vmlinux

- Debugging:

Enable debug info and rebuild.

QEMU: enable and wait for GDB, stop rebooting, remain running.

qemu-system-x86_64 \
    -s -S -no-reboot -no-shutdown \
    <other options>

Frames #7-#8 show the non-builtin memcmp() (lib/string.c) was called
  even with __builtin_memcmp() being used in cpuid_base_hypervisor().

Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
---
Changes in v5:
- Create a minimal separate header in <asm/shared/string.h> instead,
  to be used by 'boot/setup.c' and <asm/string.h> (Borislav Petkov).
- Patch 1 (in v4/v3) is no longer needed; removed.
- Patch 1 (in v5):
  - Briefly mention there are issues with <asm/string.h>.
  - Remove 'Reviewed-by: Jurgen Gross' to be conservative
    (same code change and result, but the means changed).
- Link to v4: https://lore.kernel.org/r/20260526-pvh-kasan-inline-v4-0-a310e6a25ecd@igalia.com

Changes in v4:
- Patch 1: address Juergen's feedback:
  - s/In next patch/In a future patch/.
  - Move footnote (Reasons not to include...) after "---".
- Add 'Reviewed-by: Juergen Gross' in patches 1 and 2 as well.
- Link to v3: https://lore.kernel.org/r/20260520-pvh-kasan-inline-v3-0-bede769c6ec7@igalia.com

Changes in v3:
- Create and use a separate header for inline string functions
  to fix a build error reported by kernel test robot (patch 1).
- That also removes '#ifndef _SETUP/#endif' in <asm/string.h>.
- Link to v2: https://lore.kernel.org/r/20260427-pvh-kasan-inline-v2-0-2c57b8dcff6a@igalia.com

Changes in v2:
- Add comment about the return value of __inline_memcmp() in patch 1. (v3: now 2)
- Add 'Reviewed-by: Juergen Gross' in patches 2 and 3 (v3: now 3 and 4).
- Link to v1: https://lore.kernel.org/r/20260422-pvh-kasan-inline-v1-0-7e6194344c92@igalia.com

---
Mauricio Faria de Oliveira (3):
      x86/asm, x86/boot: expose inline memcmp
      x86/cpuid: fix unbootable VMs by really inlining memcmp() in hypervisor_cpuid_base()
      x86/pvh: fix unbootable VMs by really inlining memset() in xen_prepare_pvh()

arch/x86/boot/string.c               |  6 ++----
 arch/x86/include/asm/cpuid/api.h     |  2 +-
 arch/x86/include/asm/shared/string.h | 16 ++++++++++++++++
 arch/x86/include/asm/string.h        |  1 +
 arch/x86/platform/pvh/enlighten.c    |  3 ++-
 5 files changed, 22 insertions(+), 6 deletions(-)
---
base-commit: 7de6ae9e12207ec146f2f3f1e58d1a99317e88bc
change-id: 20260422-pvh-kasan-inline-6efac77f1b27

Best regards,
-- 
Mauricio Faria de Oliveira <mfo@igalia.com>

Move the inline memcmp function currently only available in 'boot/string.c'
into the shared string function header <asm/shared/string.h> to be reused.

This is not done through <asm/string.h> to avoid pulling unnecessary code
in 'boot/string.c' that causes build errors in 'boot/compressed/string.c'
and 'purgatory/purgatory.ro'.

Note that the inline memcmp() returns 0/1, not -1/0/1 as regular memcmp()
(reported by David Laight <david.laight.linux@gmail.com>).

Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
---
 arch/x86/boot/string.c               |  6 ++----
 arch/x86/include/asm/shared/string.h | 16 ++++++++++++++++
 arch/x86/include/asm/string.h        |  1 +
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
index XXXXXXX..XXXXXXX 100644
--- a/arch/x86/boot/string.c
+++ b/arch/x86/boot/string.c
@@ -XXX,XX +XXX,XX @@
 #include <linux/errno.h>
 #include <linux/limits.h>
 #include <asm/asm.h>
+#include <asm/shared/string.h>
 #include "ctype.h"
 #include "string.h"
 
@@ -XXX,XX +XXX,XX @@
 
 int memcmp(const void *s1, const void *s2, size_t len)
 {
-	bool diff;
-	asm("repe cmpsb"
-	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
-	return diff;
+	return __inline_memcmp(s1, s2, len);
 }
 
 /*
diff --git a/arch/x86/include/asm/shared/string.h b/arch/x86/include/asm/shared/string.h
new file mode 100644
index XXXXXXX..XXXXXXX
--- /dev/null
+++ b/arch/x86/include/asm/shared/string.h
@@ -XXX,XX +XXX,XX @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_SHARED_STRING_H
+#define _ASM_X86_SHARED_STRING_H
+
+/* Note: this memcmp() returns 0/1, not -1/0/1 as regular memcmp(). */
+static __always_inline int __inline_memcmp(const void *s1, const void *s2, size_t len)
+{
+	bool diff;
+
+	asm("repe cmpsb"
+	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
+
+	return diff;
+}
+
+#endif /* _ASM_X86_SHARED_STRING_H */
diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
index XXXXXXX..XXXXXXX 100644
--- a/arch/x86/include/asm/string.h
+++ b/arch/x86/include/asm/string.h
@@ -XXX,XX +XXX,XX @@
 #else
 # include <asm/string_64.h>
 #endif
+#include <asm/shared/string.h>
 
 static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len)
 {

-- 
2.47.3

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.

For testing purposes, if the size argument is reduced from 12 to 8 then the
compiler decides to use the inline implementation; that shows results vary.

Switch the builtin to the inline implementation to address it.

Fixes: 416a33c9afce ("x86/cpu: fix unbootable VMs by inlining memcmp() in hypervisor_cpuid_base()")
Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
---
 arch/x86/include/asm/cpuid/api.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

-- 
2.47.3

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

This particular one (still) generated the inline implementation as expected
(at least in these compiler versions) but this is not guaranteed to remain.

Switch the builtin to the inline implementation to address it.

Fixes: fbe5a6dfe492 ("xen, pvh: fix unbootable VMs by inlining memset() in xen_prepare_pvh()")
Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
---
 arch/x86/platform/pvh/enlighten.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

-- 
2.47.3