bpf, sockmap: Add locking annotations to iterator

The sparse checker currently outputs the following warnings:

    include/linux/rcupdate.h:632:9: sparse: sparse: context imbalance in 'sock_hash_seq_start' - wrong count at exit
    include/linux/rcupdate.h:632:9: sparse: sparse: context imbalance in 'sock_map_seq_start' - wrong count at exit

Add the necessary __acquires and __release annotations to make the
iterator locking schema palatable to sparse. Also add __must_hold
for good measure.

The kernel codebase uses both __acquires(rcu) and __acquires(RCU).
I couldn't find any guidance which one is preferred, so I used
what is easier to type out.

Fixes: 0365351 ("net: Allow iterating sockmap and sockhash")
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Lorenz Bauer <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: John Fastabend <[email protected]>
Acked-by: Jakub Sitnicki <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: lmb

net/core/sock_map.c

@@ -745,6 +745,7 @@ static void *sock_map_seq_lookup_elem(struct sock_map_seq_info *info)
 }
 
 static void *sock_map_seq_start(struct seq_file *seq, loff_t *pos)
+	__acquires(rcu)
 {
 	struct sock_map_seq_info *info = seq->private;
 
@@ -757,6 +758,7 @@ static void *sock_map_seq_start(struct seq_file *seq, loff_t *pos)
 }
 
 static void *sock_map_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+	__must_hold(rcu)
 {
 	struct sock_map_seq_info *info = seq->private;
 
@@ -767,6 +769,7 @@ static void *sock_map_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 }
 
 static int sock_map_seq_show(struct seq_file *seq, void *v)
+	__must_hold(rcu)
 {
 	struct sock_map_seq_info *info = seq->private;
 	struct bpf_iter__sockmap ctx = {};
@@ -789,6 +792,7 @@ static int sock_map_seq_show(struct seq_file *seq, void *v)
 }
 
 static void sock_map_seq_stop(struct seq_file *seq, void *v)
+	__releases(rcu)
 {
 	if (!v)
 		(void)sock_map_seq_show(seq, NULL);
@@ -1353,6 +1357,7 @@ static void *sock_hash_seq_find_next(struct sock_hash_seq_info *info,
 }
 
 static void *sock_hash_seq_start(struct seq_file *seq, loff_t *pos)
+	__acquires(rcu)
 {
 	struct sock_hash_seq_info *info = seq->private;
 
@@ -1365,6 +1370,7 @@ static void *sock_hash_seq_start(struct seq_file *seq, loff_t *pos)
 }
 
 static void *sock_hash_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+	__must_hold(rcu)
 {
 	struct sock_hash_seq_info *info = seq->private;
 
@@ -1373,6 +1379,7 @@ static void *sock_hash_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 }
 
 static int sock_hash_seq_show(struct seq_file *seq, void *v)
+	__must_hold(rcu)
 {
 	struct sock_hash_seq_info *info = seq->private;
 	struct bpf_iter__sockmap ctx = {};
@@ -1396,6 +1403,7 @@ static int sock_hash_seq_show(struct seq_file *seq, void *v)
 }
 
 static void sock_hash_seq_stop(struct seq_file *seq, void *v)
+	__releases(rcu)
 {
 	if (!v)
 		(void)sock_hash_seq_show(seq, NULL);