On Thu, Jun 3, 2021 at 11:32 PM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> 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.
>
> All other parties accessing the BPF prog are under RTNL protection, so
> need no changes.
>
> Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>.
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> ---
> 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@xxxxxxxxxx>
> */
>
> +#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;
> + goto out;
> + }
> +
> + old_prog = prog->filter;
> +
> + if (new_prog == old_prog) {
> + ret = 0;
So the contract is that if update is successful, new_prog's refcount
taken by link_update() in kernel/bpf/syscall.c is transferred here. On
error, it will be bpf_prog_put() by link_update(). So here you don't
need extra refcnt, but it's also not an error, so you need to
bpf_prog_put(new_prog) explicitly to balance out refcnt. See how it's
done for XDP, for example.
> + 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);
and you don't need this, you already got the reference from link_update()
> + /* 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
>
