Re: [PATCH 24/29] x86/text-patching: Make text_gen_insn() IBT aware

[Peter Zijlstra]

On Wed, Feb 23, 2022 at 05:18:04PM -0800, Joao Moreira wrote:
> > +#ifdef CONFIG_X86_IBT
> > +	if (is_endbr(dest))
> > +		dest += 4;
> > +#endif
> 
> Hi, FWIIW I saw this snippet trigger a bug in the jump_label infra where the
> target displacement would not fit in a JMP8 operand.

Bah, I was afraid of seening that :/

> For these cases perhaps we can verify if the displacement fits the operand
> and, if not, simply ignore and lose the decode cycle which may not be a huge
> problem and remains semantically correct. Seems more sensible than padding
> jump tables with nops. In the meantime I'll investigate clang's behavior and
> if it is really a bug, I'll work on a patch.

Urgh, trouble is, we're going to be re-writing a bunch of ENDBR to be
UD1 0x0(%eax),%eax, and you really don't want to try and execute those.