net: Change sock_getsockopt() to take the sk ptr instead of the sock ptr

A latter patch refactors bpf_getsockopt(SOL_SOCKET) with the
sock_getsockopt() to avoid code duplication and code
drift between the two duplicates.

The current sock_getsockopt() takes sock ptr as the argument.
The very first thing of this function is to get back the sk ptr
by 'sk = sock->sk'.

bpf_getsockopt() could be called when the sk does not have
the sock ptr created.  Meaning sk->sk_socket is NULL.  For example,
when a passive tcp connection has just been established but has yet
been accept()-ed.  Thus, it cannot use the sock_getsockopt(sk->sk_socket)
or else it will pass a NULL ptr.

This patch moves all sock_getsockopt implementation to the newly
added sk_getsockopt().  The new sk_getsockopt() takes a sk ptr
and immediately gets the sock ptr by 'sock = sk->sk_socket'

The existing sock_getsockopt(sock) is changed to call
sk_getsockopt(sock->sk).  All existing callers have both sock->sk
and sk->sk_socket pointer.

The latter patch will make bpf_getsockopt(SOL_SOCKET) call
sk_getsockopt(sk) directly.  The bpf_getsockopt(SOL_SOCKET) does
not use the optnames that require sk->sk_socket, so it will
be safe.
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20220902002756.2887884-1-kafai@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

net/core/sock.c

@@ -1580,10 +1580,10 @@ static int groups_to_user(gid_t __user *dst, const struct group_info *src)
 	return 0;
 }
 
-int sock_getsockopt(struct socket *sock, int level, int optname,
-		    char __user *optval, int __user *optlen)
+static int sk_getsockopt(struct sock *sk, int level, int optname,
+			 char __user *optval, int __user *optlen)
 {
-	struct sock *sk = sock->sk;
+	struct socket *sock = sk->sk_socket;
 
 	union {
 		int val;
@@ -1947,6 +1947,12 @@ int sock_getsockopt(struct socket *sock, int level, int optname,
 	return 0;
 }
 
+int sock_getsockopt(struct socket *sock, int level, int optname,
+		    char __user *optval, int __user *optlen)
+{
+	return sk_getsockopt(sock->sk, level, optname, optval, optlen);
+}
+
 /*
  * Initialize an sk_lock.
  *