Re: [PATCH v2 13/13] objtool: Validate kCFI calls

Peter Zijlstra
2025-04-30T19:03:47Z

On Wed, Apr 30, 2025 at 08:59:53AM -0700, Josh Poimboeuf wrote:
> On Wed, Apr 30, 2025 at 01:07:47PM +0200, Peter Zijlstra wrote:
> > + case ANNOTYPE_NOCFI:
> > + sym = insn->sym;
> > + if (!sym) {
> > + ERROR_INSN(insn, "dodgy NOCFI annotation");
> > + break;
>
> return -1;

Oh right.

> > + /*
> > + * kCFI call sites look like:
> > + *
> > + * movl $(-0x12345678), %r10d
> > + * addl -4(%r11), %r10d
> > + * jz 1f
> > + * ud2
> > + * 1: cs call __x86_indirect_thunk_r11
> > + *
> > + * Verify all indirect calls are kCFI adorned by checking for the
> > + * UD2. Notably, doing __nocfi calls to regular (cfi) functions is
> > + * broken.
> > + */
> > + list_for_each_entry(insn, &file->retpoline_call_list, call_node) {
> > + struct symbol *sym = insn->sym;
> > +
> > + if (sym && sym->type == STT_FUNC && !sym->nocfi) {
> > + struct instruction *prev =
> > + prev_insn_same_sym(file, insn);
> > +
> > + if (!prev || prev->type != INSN_BUG) {
> > + WARN_INSN(insn, "no-cfi indirect call!");
> > + warnings++;
>
> Do we not care about indirect calls from !STT_FUNC?

Let me try, see what happens.