net: Remove redundant calls of sk_tx_queue_clear().

The commit 41b14fb8 ("net: Do not clear the sock TX queue in
sk_set_socket()") removes sk_tx_queue_clear() from sk_set_socket() and adds
it instead in sk_alloc() and sk_clone_lock() to fix an issue introduced in
the commit e022f0b4 ("net: Introduce sk_tx_queue_mapping"). On the
other hand, the original commit had already put sk_tx_queue_clear() in
sk_prot_alloc(): the callee of sk_alloc() and sk_clone_lock(). Thus
sk_tx_queue_clear() is called twice in each path.

If we remove sk_tx_queue_clear() in sk_alloc() and sk_clone_lock(), it
currently works well because (i) sk_tx_queue_mapping is defined between
sk_dontcopy_begin and sk_dontcopy_end, and (ii) sock_copy() called after
sk_prot_alloc() in sk_clone_lock() does not overwrite sk_tx_queue_mapping.
However, if we move sk_tx_queue_mapping out of the no copy area, it
introduces a bug unintentionally.

Therefore, this patch adds a compile-time check to take care of the order
of sock_copy() and sk_tx_queue_clear() and removes sk_tx_queue_clear() from
sk_prot_alloc() so that it does the only allocation and its callers
initialize fields.

CC: Boris Pismenny <borisp@mellanox.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Acked-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20210128150217.6060-1-kuniyu@amazon.co.jpSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/sock.c

@@ -1657,6 +1657,16 @@ static void sock_copy(struct sock *nsk, const struct sock *osk)
 #ifdef CONFIG_SECURITY_NETWORK
 	void *sptr = nsk->sk_security;
 #endif
+
+	/* If we move sk_tx_queue_mapping out of the private section,
+	 * we must check if sk_tx_queue_clear() is called after
+	 * sock_copy() in sk_clone_lock().
+	 */
+	BUILD_BUG_ON(offsetof(struct sock, sk_tx_queue_mapping) <
+		     offsetof(struct sock, sk_dontcopy_begin) ||
+		     offsetof(struct sock, sk_tx_queue_mapping) >=
+		     offsetof(struct sock, sk_dontcopy_end));
+
 	memcpy(nsk, osk, offsetof(struct sock, sk_dontcopy_begin));
 
 	memcpy(&nsk->sk_dontcopy_end, &osk->sk_dontcopy_end,
@@ -1690,7 +1700,6 @@ static struct sock *sk_prot_alloc(struct proto *prot, gfp_t priority,
 
 		if (!try_module_get(prot->owner))
 			goto out_free_sec;
-		sk_tx_queue_clear(sk);
 	}
 
 	return sk;