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

[Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>]

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;
+		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