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flow_dissector: Allow updating the flow dissector program atomically 

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
restrictions.

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>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20191011082946.22695-2-jakub@cloudflare.com
This commit is contained in:
Jakub Sitnicki 2019-10-11 10:29:45 +02:00 committed by Daniel Borkmann
parent 84a081f60d
commit 719b78a567
1 changed files with 7 additions and 3 deletions
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10
net/core/flow_dissector.c
View File

@ -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,
lockdep_is_held(&flow_dissector_mutex));
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);
out:
mutex_unlock(&flow_dissector_mutex);
return ret;