Re: [PATCH bpf-next v2 07/11] bpf, sockmap: Return socket cookie on lookup from syscall

Tooling that populates the SOCKMAP with sockets from user-space needs a way
to inspect its contents. Returning the struct sock * that SOCKMAP holds to
user-space is neither safe nor useful. An approach established by
REUSEPORT_SOCKARRAY is to return a socket cookie (a unique identifier)
instead.

Since socket cookies are u64 values SOCKMAP needs to support such a value
size for lookup to be possible. This requires special handling on update,
though. Attempts to do a lookup on SOCKMAP holding u32 values will be met
with ENOSPC error.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
---
 net/core/sock_map.c | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index d1a91e41ff82..3731191a7d1e 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c

@@ -10,6 +10,7 @@
 #include <linux/skmsg.h>
 #include <linux/list.h>
 #include <linux/jhash.h>
+#include <linux/sock_diag.h>
 
 struct bpf_stab {
 	struct bpf_map map;

@@ -31,7 +32,8 @@ static struct bpf_map *sock_map_alloc(union bpf_attr *attr)
 		return ERR_PTR(-EPERM);
 	if (attr->max_entries == 0 ||
 	    attr->key_size    != 4 ||
-	    attr->value_size  != 4 ||
+	    (attr->value_size != sizeof(u32) &&
+	     attr->value_size != sizeof(u64)) ||
 	    attr->map_flags & ~SOCK_CREATE_FLAG_MASK)
 		return ERR_PTR(-EINVAL);
 

@@ -298,6 +300,23 @@ static void *sock_map_lookup(struct bpf_map *map, void *key)
 	return ERR_PTR(-EOPNOTSUPP);
 }
 
+static void *sock_map_lookup_sys(struct bpf_map *map, void *key)
+{
+	struct sock *sk;
+
+	WARN_ON_ONCE(!rcu_read_lock_held());