Re: [PATCH] x86/mm: fix objtool failure with KMSAN enabled

[Alexander Potapenko]

On Wed, Jul 1, 2026 at 2:51 PM Dmitry Voytik <voytikd@xxxxxxxxx> 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.
> Additionally, reintroduce asm_inline to prevent potential compiler
> rejection of inlining.
>
> Link: https://lore.kernel.org/CAAX90H2_RPnZL_dFYN7cQF6yt-wAweKKSx2=6e2aZ0kv+Pm+NQ@xxxxxxxxxxxxxx
> Signed-off-by: Dmitry Voytik <voytikd@xxxxxxxxx>
Have you tried running KMSAN with this fix?

I tried running with the config that Thomas posted at
https://lore.kernel.org/all/87tsqjq3i3.ffs@fw13/
(https://tglx.de/~tglx/config.fail), and it still hangs for me.