RFC: changed error code when binding unix socket twice

[Michal Kubecek <mkubecek@suse.cz>]

Hello,

commit 0fb44559ffd6 ("af_unix: move unix_mknod() out of bindlock") moves
the special file creation in unix_bind() before u->bindlock is taken in
order to avoid an ABBA deadlock with do_splice(). As a side effect, it
also moves the check for existence of the special file (which would
result in -EADDRINUSE) before the check of u->addr (which would result
in -EINVAL if socket is already bound). This means that the error
returned for an attempt to bind a unix socket to the same path twice
changed from -EINVAL to -EADDRINUSE with this commit.

One way to restore the old error code is indicated below but before
submitting it, I would like to ask if we need/want it.

Pro:
  - in general, we do not want to change return code for given testcase
  - old error (-EINVAL) is consistent with AF_INET(6)
Con:
  - both POSIX and Linux man page only list error conditions without
    stating which should take precedence if more of them apply so
    neither of them seems wrong, strictly speaking


diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 1a0c961f4ffe..509292bdf7ed 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -992,7 +992,7 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 	struct unix_sock *u = unix_sk(sk);
 	struct sockaddr_un *sunaddr = (struct sockaddr_un *)uaddr;
 	char *sun_path = sunaddr->sun_path;
-	int err;
+	int err, mknod_err;
 	unsigned int hash;
 	struct unix_address *addr;
 	struct hlist_head *list;
@@ -1016,12 +1016,10 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 	if (sun_path[0]) {
 		umode_t mode = S_IFSOCK |
 		       (SOCK_INODE(sock)->i_mode & ~current_umask());
-		err = unix_mknod(sun_path, mode, &path);
-		if (err) {
-			if (err == -EEXIST)
-				err = -EADDRINUSE;
-			goto out;
-		}
+		mknod_err = unix_mknod(sun_path, mode, &path);
+		/* do not exit on error until after u->addr check */
+		if (mknod_err == -EEXIST)
+			mknod_err = -EADDRINUSE;
 	}
 
 	err = mutex_lock_interruptible(&u->bindlock);
@@ -1031,6 +1029,10 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 	err = -EINVAL;
 	if (u->addr)
 		goto out_up;
+	if (mknod_err) {
+		err = mknod_err;
+		goto out_up;
+	}
 
 	err = -ENOMEM;
 	addr = kmalloc(sizeof(*addr)+addr_len, GFP_KERNEL);