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

[Alexei Starovoitov]

On Fri, Mar 27, 2026 at 8:12 AM Takeru Hayasaka <hayatake396@xxxxxxxxx> wrote:
>
> 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.

I get that, but I'd rather not open this can of worms.
We had enough headaches when tailcalls, fentry, subprogs are combined.
Like this set:
https://lore.kernel.org/all/20230912150442.2009-1-hffilwlqm@xxxxxxxxx/
and the followups.