Re: [PATCH 4/7] x86: ban 32-bit sigreturn when user IBT enabled

[Richard Patel]

On Mon, May 18, 2026 at 01:22:19PM -0700, H. Peter Anvin wrote:
> On May 17, 2026 11:30:21 AM PDT, Richard Patel <ripatel@xxxxxxx> wrote:
> >IBT is not implemented for 32-bit and cannot be enabled using a
> >32-bit syscall. However, a 64-bit thread could far jump into 32-bit.
> >Therefore, 32-bit sigreturn must be banned until IBT supports that
> >environment.
> Dumb question: is there any reason not to just enable it for 32 bits? It doesn't seem that it would be that big of a delta to Just Do It.™
> 
> That being said, I suspect the number of users will be very small if any.

Yes, partially, good call. It'd be tricky for legacy IA32 signals.
sigframe_ia32 would need to be modified I think. For rt_sigframe_ia32
(rt_sigreturn) we can put it in ucontext_ia32::uc_flags.

I'll fix it in v2 later this week and add tests.