Re: [PATCH bpf-next v5 03/21] bpf: Add the bpf_unwind_resume() kfunc

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

On Wed Sep 23, 2026 at 6:59 AM CEST, Yonghong Song wrote:
> A compiler-emitted cleanup landing pad ends with a call to
> _Unwind_Resume(), which carries the unwind on once that frame's cleanups
> have run. The kernel provides the same terminator as a kfunc, named
> bpf_unwind_resume() to keep the 'bpf_' prefix kfunc convention; a later
> libbpf patch resolves the compiler's name to it.
>
> The kfunc is defined here but not registered with any program type yet. A
> call to it is only ever valid inside a landing pad, and the verifier cannot
> say that until it knows what a landing pad is, so registration waits for
> the patch that adds the rule.
>
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> ---
>  kernel/bpf/helpers.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 301b35bd85c8..249418e60ee4 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -3445,6 +3445,19 @@ __bpf_kfunc void bpf_throw(u64 cookie)
>  	WARN(1, "A call to BPF exception callback should never return\n");
>  }
>
> +/*
> + * Terminator of a compiler-emitted cleanup landing pad. The compiler names
> + * this _Unwind_Resume, the base unwind ABI's entry point for carrying an
> + * unwind on once a frame's cleanups have run. To match kernel kfunc
> + * convention, the kernel calls it bpf_unwind_resume and libbpf maps the
> + * compiler's name onto it.
> + */
> +__bpf_kfunc void bpf_unwind_resume(void)
> +{
> +	/* Never reached: a JIT emits the way out of a landing pad instead. */
> +	WARN_ONCE(1, "A JIT should have replaced the exception cleanup resume\n");
> +}
> +

Can we change this to void bpf_unwind_resume(void *ptr__ign)?

Itanium ABI passes the exception descriptor to _Unwind_Resume. We do not need it
right now, but I feel like it might be better to take and ignore it. Changing it
later might be more expensive, while it costs nothing to change for now.