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

[Ilya Leoshkevich]

On 7/1/26 14:51, 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.
> 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>
> ---
>   arch/x86/include/asm/page_64.h | 13 +++++++------
>   1 file changed, 7 insertions(+), 6 deletions(-)

FWIW I just bumped into this build issue; this patch helped and the
resulting kernel booted.

Config: https://pastebin.com/B6GedVDL


Tested-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>