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

Peter Zijlstra
2025-05-06T07:31:21Z

On Sat, May 03, 2025 at 11:28:37AM -0700, Josh Poimboeuf wrote:
> 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

Yes, that should work. But I wanted to have a poke at objtool, so it
will properly complain about the mistake first.