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/[email protected]/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.
