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.
