Subject: Re: [RFC PATCH bpf-next 1/4] bpf: Allow trampoline re-attach

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Sat, 03 Apr 2021 13:24:12 +0200

Jiri Olsa <[email-masked]> writes:

> Currently we don't allow re-attaching of trampolines. Once
> it's detached, it can't be re-attach even when the program
> is still loaded.
>
> Adding the possibility to re-attach the loaded tracing
> kernel program.

Hmm, yeah, didn't really consider this case when I added the original
disallow. But don't see why not, so (with one nit below):

Acked-by: Toke Høiland-Jørgensen <[email-masked]>

> Signed-off-by: Jiri Olsa <[email-masked]>
> ---
>  kernel/bpf/syscall.c    | 25 +++++++++++++++++++------
>  kernel/bpf/trampoline.c |  2 +-
>  2 files changed, 20 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 9603de81811a..e14926b2e95a 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -2645,14 +2645,27 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
>  	 *   target_btf_id using the link_create API.
>  	 *
>  	 * - if tgt_prog == NULL when this function was called using the old
> -         *   raw_tracepoint_open API, and we need a target from prog->aux
> -         *
> -         * The combination of no saved target in prog->aux, and no target
> -         * specified on load is illegal, and we reject that here.
> +	 *   raw_tracepoint_open API, and we need a target from prog->aux
> +	 *
> +	 * The combination of no saved target in prog->aux, and no target
> +	 * specified on is legal only for tracing programs re-attach, rest
> +	 * is illegal, and we reject that here.
>  	 */
>  	if (!prog->aux->dst_trampoline && !tgt_prog) {
> -		err = -ENOENT;
> -		goto out_unlock;
> +		/*
> +		 * Allow re-attach for tracing programs, if it's currently
> +		 * linked, bpf_trampoline_link_prog will fail.
> +		 */
> +		if (prog->type != BPF_PROG_TYPE_TRACING) {
> +			err = -ENOENT;
> +			goto out_unlock;
> +		}
> +		if (!prog->aux->attach_btf) {
> +			err = -EINVAL;
> +			goto out_unlock;
> +		}

I'm wondering about the two different return codes here. Under what
circumstances will aux->attach_btf be NULL, and why is that not an
ENOENT error? :)

-Toke