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

Josh Poimboeuf
2025-05-03T18:28:40Z

On Sat, May 03, 2025 at 11:50:23AM +0200, Peter Zijlstra wrote:
> > +++ b/arch/x86/entry/entry_64_fred.S
> > @@ -116,7 +116,8 @@ SYM_FUNC_START(asm_fred_entry_from_kvm)
> > movq %rsp, %rdi /* %rdi -> pt_regs */
> > call __fred_entry_from_kvm /* Call the C entry point */
> > POP_REGS
> > - ERETS
> > +
> > + ALTERNATIVE "", __stringify(ERETS), X86_FEATURE_FRED
> > 1:
> > /*
> > * Objtool doesn't understand what ERETS does, this hint tells it that
> > @@ -124,7 +125,7 @@ SYM_FUNC_START(asm_fred_entry_from_kvm)
> > * isn't strictly needed, but it's the simplest form.
> > */
> > UNWIND_HINT_RESTORE
> > - pop %rbp
> > + leave
> > RET
>
> So this, while clever, might be a problem with ORC unwinding. Because
> now the stack is different depending on the alternative, and we can't
> deal with that.
>
> Anyway, I'll go have a poke on Monday (or Tuesday if Monday turns out to
> be a bank holiday :-).

Can we just adjust the stack in the alternative?

ALTERNATIVE "add $64 %rsp", __stringify(ERETS), X86_FEATURE_FRED
1:
UNWIND_HINT_RESTORE
pop %rbp
RET

--
Josh