bpf: Use hlist_add_head_rcu when linking to local_storage

The local_storage->list will be traversed by rcu reader in parallel.
Thus, hlist_add_head_rcu() is needed in bpf_selem_link_storage_nolock().
This patch fixes it.

This part of the code has recently been refactored in bpf-next
and this patch makes changes to the new file "bpf_local_storage.c".
Instead of using the original offending commit in the Fixes tag,
the commit that created the file "bpf_local_storage.c" is used.

A separate fix has been provided to the bpf tree.

Fixes: 450af8d0f6 ("bpf: Split bpf_local_storage to bpf_sk_storage")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200916204453.2003915-1-kafai@fb.com

kernel/bpf/bpf_local_storage.c

@@ -159,7 +159,7 @@ void bpf_selem_link_storage_nolock(struct bpf_local_storage *local_storage,
 				   struct bpf_local_storage_elem *selem)
 {
 	RCU_INIT_POINTER(selem->local_storage, local_storage);
-	hlist_add_head(&selem->snode, &local_storage->list);
+	hlist_add_head_rcu(&selem->snode, &local_storage->list);
 }
 
 void bpf_selem_unlink_map(struct bpf_local_storage_elem *selem)