Re: [PATCH v2 00/13] objtool: Detect and warn about indirect calls in __nocfi functions

Peter Zijlstra
2025-05-28T16:30:42Z

On Wed, May 28, 2025 at 09:44:52AM +0200, Peter Zijlstra wrote:
> On Tue, May 06, 2025 at 12:18:49PM -0700, Josh Poimboeuf wrote:
>
> > Weird, I'm not seeing that.
>
> I Ate'nt Crazeh...
>
> https://lore.kernel.org/all/202505280410.2qfTQCRt-lkp@intel.com/T/#u
>
> I'll go poke at it, see if today is the day I can figure out WTF
> happens.

It manages to trip the CFI_UNDEFINED case in op->dest.reg == cfa->base
in update_cfi_state().

I figured it ought to tickle the regular 'mov %rbp, %rsp' case above
there, but it doesn't, for some reason it has cfa.base == SP at this
point.

This happens... /me looks in scrollback ... at POP_REGS 'pop
%rbp'. ARGH!!

So the sequence of fail is:

push %rbp
mov %rsp, %rbp # cfa.base = BP

SAVE
...
push %rbp
...
pop %rbp # cfa.base = SP
...
mov %rbp, %rsp # UNDEF
nop # FAIL
RESTORE

Note that the MOV+NOP is the 4 bytes ERETS needs.