netdev - [PATCH 6/8] unix_bind_bsd(): move done_path_create() call after dealing with ->bindlock

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Date:   Sat, 19 Jun 2021 03:50:31 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     netdev@...r.kernel.org
Cc:     David Miller <davem@...emloft.net>
Subject: [PATCH 6/8] unix_bind_bsd(): move done_path_create() call after dealing with ->bindlock

Final preparations for doing unlink on failure past the successful
mknod.  We can't hold ->bindlock over ->mknod() or ->unlink(), since
either might do sb_start_write() (e.g. on overlayfs).  However, we
can do it while holding filesystem and VFS locks - doing
	kern_path_create()
	vfs_mknod()
	grab ->bindlock
	if u->addr had been set
		drop ->bindlock
		done_path_create
		return -EINVAL
	else
		assign the address to socket
		drop ->bindlock
		done_path_create
		return 0
would be deadlock-free.  Here we massage unix_bind_bsd() to that
form.  We are still doing equivalent transformations.

Next commit will *not* be an equivalent transformation - it will
add a call of vfs_unlink() before done_path_create() in "alread bound"
case.

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
 net/unix/af_unix.c | 26 +++++++++++---------------
 1 file changed, 11 insertions(+), 15 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index d5ec0abd389d..3c389a6bf670 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -988,8 +988,8 @@ static int unix_bind_bsd(struct sock *sk, struct unix_address *addr)
 	struct unix_sock *u = unix_sk(sk);
 	umode_t mode = S_IFSOCK |
 	       (SOCK_INODE(sk->sk_socket)->i_mode & ~current_umask());
-	struct path parent, path;
 	struct user_namespace *ns; // barf...
+	struct path parent;
 	struct dentry *dentry;
 	unsigned int hash;
 	int err;
@@ -1007,36 +1007,32 @@ static int unix_bind_bsd(struct sock *sk, struct unix_address *addr)
 	 * All right, let's create it.
 	 */
 	err = security_path_mknod(&parent, dentry, mode, 0);
-	if (!err) {
+	if (!err)
 		err = vfs_mknod(ns, d_inode(parent.dentry), dentry, mode, 0);
-		if (!err) {
-			path.mnt = mntget(parent.mnt);
-			path.dentry = dget(dentry);
-		}
-	}
-	done_path_create(&parent, dentry);
-	if (err)
+	if (err) {
+		done_path_create(&parent, dentry);
 		return err;
-
+	}
 	err = mutex_lock_interruptible(&u->bindlock);
 	if (err) {
-		path_put(&path);
+		done_path_create(&parent, dentry);
 		return err;
 	}
-
 	if (u->addr) {
 		mutex_unlock(&u->bindlock);
-		path_put(&path);
+		done_path_create(&parent, dentry);
 		return -EINVAL;
 	}
 
 	addr->hash = UNIX_HASH_SIZE;
-	hash = d_backing_inode(path.dentry)->i_ino & (UNIX_HASH_SIZE - 1);
+	hash = d_backing_inode(dentry)->i_ino & (UNIX_HASH_SIZE - 1);
 	spin_lock(&unix_table_lock);
-	u->path = path;
+	u->path.mnt = mntget(parent.mnt);
+	u->path.dentry = dget(dentry);
 	__unix_set_addr(sk, addr, hash);
 	spin_unlock(&unix_table_lock);
 	mutex_unlock(&u->bindlock);
+	done_path_create(&parent, dentry);
 	return 0;
 }
 
-- 
2.11.0