Re: [PATCH bpf-next v2 4/7] net: sched: add lightweight update path for cls_bpf

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

On Fri, Jun 04, 2021 at 11:24:28PM IST, Alexei Starovoitov wrote:
> On Fri, Jun 04, 2021 at 12:01:13PM +0530, Kumar Kartikeya Dwivedi wrote:
> > This is used by BPF_LINK_UPDATE to replace the attach SCHED_CLS bpf prog
> > effectively changing the classifier implementation for a given filter
> > owned by a bpf_link.
> >
> > Note that READ_ONCE suffices in this case as the ordering for loads from
> > the filter are implicitly provided by the data dependency on BPF prog
> > pointer.
> >
> > On the writer side we can just use a relaxed WRITE_ONCE store to make
> > sure one or the other value is visible to a reader in cls_bpf_classify.
> > Lifetime is managed using RCU so bpf_prog_put path should wait until
> > readers are done for old_prog.
>
> Should those be rcu_deref and rcu_assign_pointer ?
> Typically the pointer would be __rcu annotated which would be
> another small change in struct cls_bpf_prog.
> That would make the life time easier to follow?
>

True, I'll make that change.

> > All other parties accessing the BPF prog are under RTNL protection, so
> > need no changes.
> >
> > Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>.
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> > ---
> >  net/sched/cls_bpf.c | 55 +++++++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 53 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/sched/cls_bpf.c b/net/sched/cls_bpf.c
> > index bf61ffbb7fd0..f23304685c48 100644
> > --- a/net/sched/cls_bpf.c
> > +++ b/net/sched/cls_bpf.c
> > @@ -9,6 +9,7 @@
> >   * (C) 2013 Daniel Borkmann <dborkman@redhat.com>
> >   */
> >
> > +#include <linux/atomic.h>
> >  #include <linux/module.h>
> >  #include <linux/types.h>
> >  #include <linux/skbuff.h>
> > @@ -104,11 +105,11 @@ static int cls_bpf_classify(struct sk_buff *skb, const struct tcf_proto *tp,
> >  			/* It is safe to push/pull even if skb_shared() */
> >  			__skb_push(skb, skb->mac_len);
> >  			bpf_compute_data_pointers(skb);
> > -			filter_res = BPF_PROG_RUN(prog->filter, skb);
> > +			filter_res = BPF_PROG_RUN(READ_ONCE(prog->filter), skb);
> >  			__skb_pull(skb, skb->mac_len);
> >  		} else {
> >  			bpf_compute_data_pointers(skb);
> > -			filter_res = BPF_PROG_RUN(prog->filter, skb);
> > +			filter_res = BPF_PROG_RUN(READ_ONCE(prog->filter), skb);
> >  		}
> >
> >  		if (prog->exts_integrated) {
> > @@ -775,6 +776,55 @@ static int cls_bpf_link_detach(struct bpf_link *link)
> >  	return 0;
> >  }
> >
> > +static int cls_bpf_link_update(struct bpf_link *link, struct bpf_prog *new_prog,
> > +			       struct bpf_prog *old_prog)
> > +{
> > +	struct cls_bpf_link *cls_link;
> > +	struct cls_bpf_prog cls_prog;
> > +	struct cls_bpf_prog *prog;
> > +	int ret;
> > +
> > +	rtnl_lock();
> > +
> > +	cls_link = container_of(link, struct cls_bpf_link, link);
> > +	if (!cls_link->prog) {
> > +		ret = -ENOLINK;
> > +		goto out;
> > +	}
> > +
> > +	prog = cls_link->prog;
> > +
> > +	/* BPF_F_REPLACEing? */
> > +	if (old_prog && prog->filter != old_prog) {
> > +		ret = -EINVAL;
>
> Other places like cgroup_bpf_replace and bpf_iter_link_replace
> return -EPERM in such case.
>

Ok, will change.

> > +		goto out;
> > +	}
> > +
> > +	old_prog = prog->filter;
> > +
> > +	if (new_prog == old_prog) {
> > +		ret = 0;
> > +		goto out;
> > +	}
> > +
> > +	cls_prog = *prog;
> > +	cls_prog.filter = new_prog;
> > +
> > +	ret = cls_bpf_offload(prog->tp, &cls_prog, prog, NULL);
> > +	if (ret < 0)
> > +		goto out;
> > +
> > +	WRITE_ONCE(prog->filter, new_prog);
> > +
> > +	bpf_prog_inc(new_prog);
> > +	/* release our reference */
> > +	bpf_prog_put(old_prog);
> > +
> > +out:
> > +	rtnl_unlock();
> > +	return ret;
> > +}
> > +
> >  static void __bpf_fill_link_info(struct cls_bpf_link *link,
> >  				 struct bpf_link_info *info)
> >  {
> > @@ -859,6 +909,7 @@ static const struct bpf_link_ops cls_bpf_link_ops = {
> >  	.show_fdinfo = cls_bpf_link_show_fdinfo,
> >  #endif
> >  	.fill_link_info = cls_bpf_link_fill_link_info,
> > +	.update_prog = cls_bpf_link_update,
> >  };
> >
> >  static inline char *cls_bpf_link_name(u32 prog_id, const char *name)
> > --
> > 2.31.1
> >
>
> --

--
Kartikeya