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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
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. 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; 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 related to <asm/string.h>, 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$(nproc) vmlinux qemu-system-x86_64 \ -accel kvm -nodefaults -nographic -serial stdio \ -kernel vmlinux -append 'console=ttyS0' ... SeaBIOS (version ...) Booting from ROM... Linux version ... ... <Ctrl-C> - With CONFIG_KASAN (fails) ./scripts/config -e KASAN make olddefconfig make -j$(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> --- 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 (4): x86/asm: move inline string functions to <asm/string_inline.h> 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 | 21 +------------------- arch/x86/include/asm/string_inline.h | 37 ++++++++++++++++++++++++++++++++++++ arch/x86/platform/pvh/enlighten.c | 3 ++- 5 files changed, 43 insertions(+), 26 deletions(-) --- base-commit: d387b06f7c15b4639244ad66b4b0900c6a02b430 change-id: 20260422-pvh-kasan-inline-6efac77f1b27 Best regards, -- Mauricio Faria de Oliveira <mfo@igalia.com>
In a future patch, 'boot/string.c' will include inline string functions. Using the header <asm/string.h> is problematic for a couple of reasons (i.e., build errors), which can be addressed, but introduce unnecessary complexity and regression risk (beyond these _found_ couple of reasons). Using a new header <asm/string_inline.h> is simpler and transparent to existing users of <asm/string.h>, with less changes to 'boot/string.c' and its users (eg 'boot/compressed/string.c' and 'purgatory/purgatory.ro'), which minimize regression risk. No functional change intended. Reported-by: kernel test robot <lkp@intel.com> Closes: https://lore.kernel.org/oe-kbuild-all/202605140922.q7IlUv7o-lkp@intel.com/ Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com> Reviewed-by: Juergen Gross <jgross@suse.com> --- (*) Reasons not to include <asm/string.h> in 'boot/string.c': 1) 'boot/string.c' is built for 16-bit/real mode thus some type and word size errors happen when <asm/string.h> include, e.g., <asm/string_64.h>. This can be addressed with '#ifndef _SETUP' (defined by 'boot/Makefile'). 2) 'boot/string.c' is included by 'boot/compressed/string.c' which is the source of 'purgatory/string.o', linked by 'purgatory/purgatory.ro' (CONFIG_KEXEC_FILE). In 64BIT, <asm/string.h> includes <asm/string_64.h>, which references __memset() and __memmove() with KCFI_REFERENCE(), ie, __ADDRESSABLE(); however, 'purgatory/purgatory.ro' is not linked with implementations. So, CONFIG_KEXEC_FILE and CONFIG_CFI without CONFIG_KASAN hit errors: >> ld.lld: error: undefined symbol: __memset >>> referenced by string.c >>> arch/x86/purgatory/purgatory.ro:\ (__UNIQUE_ID_addressable___memset_0) -- >> ld.lld: error: undefined symbol: __memmove >>> referenced by string.c >>> arch/x86/purgatory/purgatory.ro:\ (__UNIQUE_ID_addressable___memmove_1) (Note: this is not hit with CONFIG_KASAN because 'boot/compressed/string.c' adds aliases __memset()/__memmove() to memset()/memmove() in that case.) This can be addressed with 'CFLAGS_string.o := -D__DISABLE_EXPORTS' so to disable KCFI_REFERENCE() in 'purgatory/Makefile' (it removes CC_FLAGS_CFI anyway). ... However, since a change in this series would need more changes to address errors it causes, it is reasonable to change the series not to cause them, by using a separate header with _just_ inline string functions. --- arch/x86/include/asm/string.h | 21 +-------------------- arch/x86/include/asm/string_inline.h | 26 ++++++++++++++++++++++++++ 2 files changed, 27 insertions(+), 20 deletions(-) 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 @@ # include <asm/string_64.h> #endif -static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len) -{ - void *ret = to; - - asm volatile("rep movsb" - : "+D" (to), "+S" (from), "+c" (len) - : : "memory"); - return ret; -} - -static __always_inline void *__inline_memset(void *s, int v, size_t n) -{ - void *ret = s; - - asm volatile("rep stosb" - : "+D" (s), "+c" (n) - : "a" ((uint8_t)v) - : "memory"); - return ret; -} +#include <asm/string_inline.h> #endif /* _ASM_X86_STRING_H */ diff --git a/arch/x86/include/asm/string_inline.h b/arch/x86/include/asm/string_inline.h new file mode 100644 index XXXXXXX..XXXXXXX --- /dev/null +++ b/arch/x86/include/asm/string_inline.h @@ -XXX,XX +XXX,XX @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _ASM_X86_STRING_INLINE_H +#define _ASM_X86_STRING_INLINE_H + +static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len) +{ + void *ret = to; + + asm volatile("rep movsb" + : "+D" (to), "+S" (from), "+c" (len) + : : "memory"); + return ret; +} + +static __always_inline void *__inline_memset(void *s, int v, size_t n) +{ + void *ret = s; + + asm volatile("rep stosb" + : "+D" (s), "+c" (n) + : "a" ((uint8_t)v) + : "memory"); + return ret; +} + +#endif /* _ASM_X86_STRING_INLINE_H */ -- 2.51.0
Move the inline memcmp function currently only available in 'boot/string.c' and its users (eg, 'boot/compressed/string.c' and 'purgatory/purgatory.ro') into the inline string functions header <asm/string_inline.h> to be reused. Note that the inline memcmp() returns 0/1, not -1/0/1 as regular memcmp() (reported by David Laight <david.laight.linux@gmail.com>), which is not caused or changed by this commit; this will be addressed separately. Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com> Reviewed-by: Juergen Gross <jgross@suse.com> --- arch/x86/boot/string.c | 6 ++---- arch/x86/include/asm/string_inline.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_inline.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_inline.h b/arch/x86/include/asm/string_inline.h index XXXXXXX..XXXXXXX 100644 --- a/arch/x86/include/asm/string_inline.h +++ b/arch/x86/include/asm/string_inline.h @@ -XXX,XX +XXX,XX @@ static __always_inline void *__inline_memset(void *s, int v, size_t n) return ret; } +/* 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_STRING_INLINE_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> Reviewed-by: Juergen Gross <jgross@suse.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> Reviewed-by: Juergen Gross <jgross@suse.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