Re: [BUG] CONFIG_KMSAN breaks objtool and building of the kernel

[Dmitry Voytik]

Hi Peter,

On Wed, Jun 17, 2026 at 9:48 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Jun 17, 2026 at 12:14:26AM +0200, Dmitry Voytik wrote:
> > Enabling CONFIG_KMSAN=y triggers the following build error:
> > make LLVM=1 -j(nproc)
> >   SYNC    include/config/auto.conf
> >   DESCEND objtool
> >   INSTALL libsubcmd_headers
> >   LD      vmlinux.o
> > vmlinux.o: warning: objtool: intel_pt_interrupt+0xde9: undefined stack state
> > vmlinux.o: warning: objtool: folio_zero_user+0x7cc: undefined stack state
> > vmlinux.o: error: objtool: intel_pt_interrupt+0xde4: unknown cfa base reg -1
> > make[2]: *** [scripts/Makefile.vmlinux_o:76: vmlinux.o] Error 255
> > make[2]: *** Deleting file 'vmlinux.o'
> >
> > ↪ git log --oneline | head -n1
> > 062871f1371b (HEAD -> master, origin/master, origin/HEAD) Merge tag
> > 'pinctrl-v7.1-3' of
> > git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-pinctrl
> >
> > ↪ clang --version
> > clang version 22.1.6
> > Target: x86_64-pc-linux-gnu
> >
> > Please let me know if you need my .config (basically, it's the default
> > Arch Linux config + make oldconfg)
>
> Using v7.1-2765-g6b5a2b7d9bc1, clang-22.1.0 (debian) and defconfig+KMSAN, I
> cannot reproduce :/

Thanks for double checking that.
Indeed, defconfig + KMSAN is OK.

It took me quite some time to narrow down the following combinations:
defconfig + CONFIG_KMSAN + CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
defconfig + CONFIG_KMSAN + CONFIG_DEBUG_INFO_DWARF4
defconfig + CONFIG_KMSAN + CONFIG_DEBUG_INFO_DWARF5

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

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
make[2]: *** Deleting file 'vmlinux.o'
make[1]: *** [/home/voyt/p/linux/Makefile:1326: vmlinux_o] Error 2
make: *** [Makefile:248: __sub-make] Error 2