On Fri, Nov 24, 2023 at 1:20 PM Dmitry Dolgov <9erthalion6@xxxxxxxxx> wrote:
>
> > On Thu, Nov 23, 2023 at 11:24:34PM -0800, Song Liu wrote:
> > > Following the corresponding discussion [1], the reason for that is to
> > > avoid tracing progs call cycles without introducing more complex
> > > solutions. Relax "no same type" requirement to "no progs that are
> > > already an attach target themselves" for the tracing type. In this way
> > > only a standalone tracing program (without any other progs attached to
> > > it) could be attached to another one, and no cycle could be formed. To
> >
> > If prog B attached to prog A, and prog C attached to prog B, then we
> > detach B. At this point, can we re-attach B to A?
>
> Nope, with the proposed changes it still wouldn't be possible to
> reattach B to A (if we're talking about tracing progs of course),
> because this time B is an attachment target on its own.
>
> > > + if (tgt_prog) {
> > > + /* Bookkeeping for managing the prog attachment chain. */
> > > + tgt_prog->aux->follower_cnt++;
> > > + prog->aux->attach_depth = tgt_prog->aux->attach_depth + 1;
> > > + }
> > > +
> >
> > attach_depth is calculated at attach time, so...
> >
> > > struct bpf_prog_aux *aux = tgt_prog->aux;
> > >
> > > + if (aux->attach_depth >= 32) {
> > > + bpf_log(log, "Target program attach depth is %d. Too large\n",
> > > + aux->attach_depth);
> > > + return -EINVAL;
> > > + }
> > > +
> >
> > (continue from above) attach_depth is always 0 at program load time, no?
>
> Right, it's going to be always 0 for the just loaded program -- but here
> in verifier we check attach_depth of the target program, which is
> calculated at some point before. Or were you asking about something else?
Actually, I was wrong. attach_depth is checked at BPF_LINK_CREATE. So
never mind about this one.
