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

Hi Thomas,

On Thu, Jul 2, 2026 at 10:05=E2=80=AFPM Thomas Gleixner <[email protected]> w=
rote:
>
> On Thu, Jul 02 2026 at 21:46, Thomas Gleixner wrote:
> > On Thu, Jul 02 2026 at 13:15, Peter Zijlstra wrote:
> >> A quick test with folio_zero_user() seems to suggest the
> >> ASM_CALL_CONSTRAINT movement (your patch) generates the same code for
> >> defconfg as does the unmodified code. The change from the thread linke=
d
> >> above (input constraint on __builtin_frame_address(0)) generates
> >> different code.
> >>
> >> A third option is to force FRAME_POINTER=3Dy for KMSAN builds -- for n=
ow,
> >> until clang is taught to be less insane.
> >
> > With FRAME_POINTER=3Dy I get now with clang-22
> >
> > vmlinux.o: warning: objtool: folio_zero_user+0xa7b: stack state mismatc=
h: reg1[4]=3D-1+0 reg2[4]=3D-2-336
> > vmlinux.o: warning: objtool: set_ftrace_ops_ro+0x64: relocation to !END=
BR: machine_kexec_prepare+0x8f0
> >
> > That's with your objtool patch applied. When I remove that then the
> > folio_zero_user one goes away. The relocation to !ENDBR stays.
> >
> > Oh well...
>
> And none of this boots when KMSAN=3Dy ....

Thanks for checking this!
I wonder if we are debugging clang here because I just retested the patch w=
ith
CONFIG_FRAME_POINTER on the current Linus' tree and it works in my env:

clang --version
clang version 22.1.6

git pull -r
git show -s --oneline -2
21f3d2b2069e (HEAD -> master) x86/mm: fix objtool failure with KMSAN enable=
d
d2c9a99135da (origin/master, origin/HEAD) Merge tag 'device-id-rework'
of git://git.kernel.org/pub/scm/linux/kernel/git/ukleinek/linux

make mrproper
make LLVM=3D1 defconfig
./scripts/config \
  -e CONFIG_KMSAN \
  -e CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT \
  -e UNWINDER_FRAME_POINTER
make LLVM=3D1 olddefconfig
make LLVM=3D1 W=3Dce -j(nproc) bzImage

grep -e CONFIG_KMSAN=3D -e CONFIG_FRAME_POINTER \
  -e CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT .config

CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=3Dy
CONFIG_FRAME_POINTER=3Dy
CONFIG_KMSAN=3Dy

qemu-system-x86_64 \
  -kernel arch/x86/boot/bzImage \
  -accel kvm \
  -append "console=3DttyS0" \
  -nographic \
  -m 8G -smp 4 \
  -serial file:serial_out.log

grep -e 'Linux version' -A1 -e KernelMemorySanitizer -e 'append a
correct' serial_out.log

[    0.000000] Linux version 7.2.0-rc1-00009-gb0b29f0286be
(voyt@voyt-laptop-dell) (clang version 22.1.6, LLD 22.1.6) #1 SMP
PREEMPT_DYNAMIC Fri Jul  3 11:55:02 CEST 2026
[    0.000000] Command line: console=3DttyS0
--
[    0.218380] Starting KernelMemorySanitizer
[    0.218382] ATTENTION: KMSAN is a debugging tool! Do not use it on
production machines!
--
[    4.037384] Please append a correct "root=3D" boot option; here are
the available partitions:
[    4.037772] 0b00         1048575 sr0


Could another CONFIG_ be the culprit on your side? Or, has your clang
version some different (buggy?) behaviour?