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

[Alexander Potapenko]

On Wed, Jul 1, 2026 at 4:45 PM Dmitry Voytik <voytikd@xxxxxxxxx> wrote:
>
> Hi Alexander,
>
> On Wed, Jul 1, 2026 at 3:18 PM Alexander Potapenko <glider@xxxxxxxxxx> wrote:
> >
> > 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.
>
> I only tested the fix with defconfig + CONFIG_KMSAN +
> CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT + olddefconfig.
>
> qemu-system-x86_64 \
>     -kernel arch/x86/boot/bzImage \
>     -append "console=ttyS0" \
>     -nographic \
>     -m 8G -smp 4 \
>     -serial file:serial_output.log
>
> Booting from ROM..
> [    0.000000] Linux version 7.1.0-14280-g3dd2bc904bda
> (voyt@voyt-laptop-dell) (clang version 22.1.6, LLD 22.1.6) #2 SMP
> PREEMPT_DYNAMIC Wed Jul  1 16:25:28 CEST 2026
> ...

In fact I think the hangs are being caused by CONFIG_UNWINDER_ORC=y,
sorry for the noise.