Re: [PATCH bpf-next 1/2] bpf, x86: patch tail-call fentry slot on non-IBT JITs

[Takeru Hayasaka]

Hi Alexei

Thanks, and Sorry, I sent an older changelog from while I was still
iterating on this, and it described the issue incorrectly.

My changelog made this sound like an IBT/non-IBT-specific issue, but
that was wrong. On current kernels, fentry on tail-called programs is
not supported in either case. Only the regular fentry patch site is
patched; there is no tail-call landing patching in either case, so
disabling IBT does not make it work.

What this series was trying to do was add support for fentry on
tail-called x86 programs. The non-IBT part was only about a bug in my
initial implementation of that support, not the underlying motivation.

The motivation is observability of existing tailcall-heavy BPF/XDP
programs, where tail-called leaf programs are currently a blind spot for
fentry-based debugging.

If supporting fentry on tail-called programs is still not something
you'd want upstream, I understand. If I resend this, I'll fix the
changelog/cover letter to describe it correctly.