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

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Message-ID: <20210403182155.upi6267fh3gsdvrq@ast-mbp>
Date:   Sat, 3 Apr 2021 11:21:55 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [RFC PATCH bpf-next 1/4] bpf: Allow trampoline re-attach

On Sat, Apr 03, 2021 at 01:24:12PM +0200, Toke Høiland-Jørgensen wrote:
> >  	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? :)

The feature makes sense to me as well.
I don't quite see how it would get here with attach_btf == NULL.
Maybe WARN_ON then?
Also if we're allowing re-attach this way why exclude PROG_EXT and LSM?