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

From: Jiri Olsa <[email-masked]>
Date: Mon, 5 Apr 2021 23:58:25 +0200

On Mon, Apr 05, 2021 at 04:15:54PM +0200, Toke Høiland-Jørgensen wrote:
> Jiri Olsa <[email-masked]> writes:
> 
> > On Sat, Apr 03, 2021 at 11:21:55AM -0700, Alexei Starovoitov wrote:
> >> 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?
> >
> > right, that should be always there
> >
> >> Also if we're allowing re-attach this way why exclude PROG_EXT and LSM?
> >> 
> >
> > I was enabling just what I needed for the test, which is so far
> > the only use case.. I'll see if I can enable that for all of them
> 
> How would that work? For PROG_EXT we clear the destination on the first
> attach (to avoid keeping a ref on it), so re-attach can only be done
> with an explicit target (which already works just fine)...

right, I'm just looking on it ;-) extensions already seem allow for that,
I'll check LSM

jirka