Re: [PATCH] x86/mm: fix objtool failure with KMSAN enabled
From: Borislav Petkov <bp@alien8.de>
Date: 2026-07-02 02:54:21
Also in:
lkml, llvm
On Wed, Jul 01, 2026 at 02:51:51PM +0200, Dmitry Voytik wrote:
This patch fixes broken builds with defconfig + CONFIG_KMSAN +
CONFIG_DEBUG_INFO_*.
To reproduce the issue before the fix:
make mrproper
make LLVM=1 defconfig
./scripts/config -e CONFIG_KMSAN \
-e CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
make LLVM=1 olddefconfig
make LLVM=1 -j(nproc) vmlinux
...
LD vmlinux.o
vmlinux.o: warning: objtool: folio_zero_user+0x801: undefined stack state
vmlinux.o: error: objtool: folio_zero_user+0x801: unknown CFA base reg -1
make[2]: *** [scripts/Makefile.vmlinux_o:76: vmlinux.o] Error 255
objtool in verbose mode shows how the frame pointer is omitted:
make LLVM=1 OBJTOOL_VERBOSE=1 -j(nproc) vmlinux
...
b15a2c: folio_zero_user+0x7fc xor %eax,%eax
b15a2e: folio_zero_user+0x7fe mov %rcx,%rsp
b15a31: folio_zero_user+0x801 mov %r14,%rdi
b15a34: folio_zero_user+0x804 mov %rbx,%rcx
b15a37: folio_zero_user+0x807 call 0xb15a3c <__clear_pages_unrol
After the fix, the frame pointer is back:
b15a37: 31 c0 xor %eax,%eax
b15a39: 48 89 ec mov %rbp,%rsp
b15a3c: 4c 89 f7 mov %r14,%rdi
b15a3f: 48 89 d9 mov %rbx,%rcx
b15a42: e8 00 00 00 00 call b15a47 <folio_zero_user+0x817>
It seems the issue was introduced by
commit 54a6b89a3db2 ("x86/mm: simplify clear_page_*")
The actual fix is to revert the change how ASM_CALL_CONSTRAINT is
positioned.Why? Where does it say that the current stack ptr dependency needs to be the first asm input operand? If that were the case, we have a bunch more of those bugs around the tree. Anyway, + linux-toolchains.
quoted hunk
Additionally, reintroduce asm_inline to prevent potential compiler rejection of inlining. Link: https://lore.kernel.org/CAAX90H2_RPnZL_dFYN7cQF6yt-wAweKKSx2=6e2aZ0kv+Pm+NQ@mail.gmail.com (local) Signed-off-by: Dmitry Voytik <redacted> --- arch/x86/include/asm/page_64.h | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h index 1895c207f629..de8604f0a3a1 100644 --- a/arch/x86/include/asm/page_64.h +++ b/arch/x86/include/asm/page_64.h@@ -100,12 +100,13 @@ static inline void clear_pages(void *addr, unsigned int npages) * __clear_pages_unrolled() are part of the inline asm register * specification. */ - asm volatile(ALTERNATIVE_2("call __clear_pages_unrolled", - "shrq $3, %%rcx; rep stosq", X86_FEATURE_REP_GOOD, - "rep stosb", X86_FEATURE_ERMS) - : "+c" (len), "+D" (addr), ASM_CALL_CONSTRAINT - : "a" (0) - : "cc", "memory"); + asm_inline volatile( + ALTERNATIVE_2("call __clear_pages_unrolled", + "shrq $3, %%rcx; rep stosq", X86_FEATURE_REP_GOOD, + "rep stosb", X86_FEATURE_ERMS) + : ASM_CALL_CONSTRAINT, "+c"(len), "+D"(addr) + : "a"(0) + : "cc", "memory"); } #define clear_pages clear_pages
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette