Re: objtool: undefined stack state in folio_zero_user()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jun 22, 2026 at 04:23:46PM +0300, Dmitry Antipov wrote:
> As of ef0c9f75a195 ("lib: Add stale 'raid6' directory to .gitignore file")
> with clang 22.1.8 and KMSAN enabled, objtool stucks in folio_zero_user():
> 
> $ ./tools/objtool/objtool --hacks=jump_label --hacks=noinstr \
>   --hacks=skylake --ibt --prefix=16 --orc --retpoline --rethunk \
>   --static-call --uaccess --no-unreachable --noinstr  --unret --link \
>   vmlinux.o
> vmlinux.o: warning: objtool: folio_zero_user+0x947: undefined stack state
> vmlinux.o: error: objtool: folio_zero_user+0x947: unknown CFA base reg -1
> 
> Dmitry

> 0000000001533940 <folio_zero_user>:

>  1534272:	48 89 e1             	mov    %rsp,%rcx
>  1534275:	48 85 ed             	test   %rbp,%rbp
>  1534278:	8b 54 24 1c          	mov    0x1c(%rsp),%edx
>  153427c:	0f 85 c2 00 00 00    	jne    1534344 <folio_zero_user+0xa04>
>  1534282:	31 c0                	xor    %eax,%eax
>  1534284:	48 89 cc             	mov    %rcx,%rsp
>  1534287:	4c 89 f7             	mov    %r14,%rdi                        ;; HERE

...
>  1534327:	48 89 64 24 78       	mov    %rsp,0x78(%rsp)
...
>  153433a:	48 8b 4c 24 78       	mov    0x78(%rsp),%rcx
>  153433f:	e9 31 ff ff ff       	jmp    1534275 <folio_zero_user+0x935>


This is well insane codegen, and I cannot blame objtool for hating on it
-- in fact, I hate on it too.

Let me try and figure out how best to fix this insane compiler output.