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From: Jakub Sitnicki <jakub@cloudflare.com>
To: bpf@vger.kernel.org
Cc: netdev@vger.kernel.org, kernel-team@cloudflare.com,
	Stanislav Fomichev <sdf@fomichev.me>
Subject: [PATCH bpf-next v2 1/2] flow_dissector: Allow updating the flow dissector program atomically
Date: Thu, 10 Oct 2019 20:17:49 +0200
Message-ID: <20191010181750.5964-2-jakub@cloudflare.com> (raw)
In-Reply-To: <20191010181750.5964-1-jakub@cloudflare.com>

It is currently not possible to detach the flow dissector program and
attach a new one in an atomic fashion, that is with a single syscall.
Attempts to do so will be met with EEXIST error.

This makes updates to flow dissector program hard. Traffic steering that
relies on BPF-powered flow dissection gets disrupted while old program has
been already detached but the new one has not been attached yet.

There is also a window of opportunity to attach a flow dissector to a
non-root namespace while updating the root flow dissector, thus blocking
the update.

Lastly, the behavior is inconsistent with cgroup BPF programs, which can be
replaced with a single bpf(BPF_PROG_ATTACH, ...) syscall without any

Allow attaching a new flow dissector program when another one is already
present with a restriction that it can't be the same program.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
 net/core/flow_dissector.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 6b4b88d1599d..dbf502c18656 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -128,6 +128,8 @@ int skb_flow_dissector_bpf_prog_attach(const union bpf_attr *attr,
 		struct net *ns;
 		for_each_net(ns) {
+			if (ns == &init_net)
+				continue;
 			if (rcu_access_pointer(ns->flow_dissector_prog)) {
 				ret = -EEXIST;
 				goto out;
@@ -145,12 +147,14 @@ int skb_flow_dissector_bpf_prog_attach(const union bpf_attr *attr,
 	attached = rcu_dereference_protected(net->flow_dissector_prog,
-	if (attached) {
-		/* Only one BPF program can be attached at a time */
-		ret = -EEXIST;
+	if (attached == prog) {
+		/* The same program cannot be attached twice */
+		ret = -EINVAL;
 		goto out;
 	rcu_assign_pointer(net->flow_dissector_prog, prog);
+	if (attached)
+		bpf_prog_put(attached);
 	return ret;

  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-10 18:17 [PATCH bpf-next v2 0/2] Atomic flow dissector updates Jakub Sitnicki
2019-10-10 18:17 ` Jakub Sitnicki [this message]
2019-10-10 19:41   ` [PATCH bpf-next v2 1/2] flow_dissector: Allow updating the flow dissector program atomically Martin Lau
2019-10-10 18:17 ` [PATCH bpf-next v2 2/2] selftests/bpf: Check that flow dissector can be re-attached Jakub Sitnicki
2019-10-10 19:42   ` Martin Lau
2019-10-10 19:57 ` [PATCH bpf-next v2 0/2] Atomic flow dissector updates Stanislav Fomichev
2019-10-10 23:38 ` Daniel Borkmann
2019-10-11  8:31   ` Jakub Sitnicki

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