Re: [PATCH bpf-next v3 2/4] bpf, netns: Keep attached programs in bpf_prog_array

Prepare for having multi-prog attachments for new netns attach types by
storing programs to run in a bpf_prog_array, which is well suited for
iterating over programs and running them in sequence.

After this change bpf(PROG_QUERY) may block to allocate memory in
bpf_prog_array_copy_to_user() for collected program IDs. This forces a
change in how we protect access to the attached program in the query
callback. Because bpf_prog_array_copy_to_user() can sleep, we switch from
an RCU read lock to holding a mutex that serializes updaters.

Because we allow only one BPF flow_dissector program to be attached to
netns at all times, the bpf_prog_array pointed by net->bpf.run_array is
always either detached (null) or one element long.

No functional changes intended.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
---
 include/net/netns/bpf.h    |   5 +-
 kernel/bpf/net_namespace.c | 120 +++++++++++++++++++++++++------------
 net/core/flow_dissector.c  |  19 +++---
 3 files changed, 96 insertions(+), 48 deletions(-)

diff --git a/include/net/netns/bpf.h b/include/net/netns/bpf.h
index a8dce2a380c8..a5015bda9979 100644
--- a/include/net/netns/bpf.h
+++ b/include/net/netns/bpf.h

@@ -9,9 +9,12 @@
 #include <linux/bpf-netns.h>
 
 struct bpf_prog;
+struct bpf_prog_array;
 
 struct netns_bpf {
-	struct bpf_prog __rcu *progs[MAX_NETNS_BPF_ATTACH_TYPE];
+	/* Array of programs to run compiled from progs or links */
+	struct bpf_prog_array __rcu *run_array[MAX_NETNS_BPF_ATTACH_TYPE];
+	struct bpf_prog *progs[MAX_NETNS_BPF_ATTACH_TYPE];
 	struct bpf_link *links[MAX_NETNS_BPF_ATTACH_TYPE];