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 <>
Reviewed-by: Stanislav Fomichev <>
Acked-by: Martin KaFai Lau <>
 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 
                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;

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