vfs: syscall: Add open_tree(2) to reference or clone a mount

open_tree(dfd, pathname, flags)

Returns an O_PATH-opened file descriptor or an error.
dfd and pathname specify the location to open, in usual
fashion (see e.g. fstatat(2)).  flags should be an OR of
some of the following:
	* AT_PATH_EMPTY, AT_NO_AUTOMOUNT, AT_SYMLINK_NOFOLLOW -
same meanings as usual
	* OPEN_TREE_CLOEXEC - make the resulting descriptor
close-on-exec
	* OPEN_TREE_CLONE or OPEN_TREE_CLONE | AT_RECURSIVE -
instead of opening the location in question, create a detached
mount tree matching the subtree rooted at location specified by
dfd/pathname.  With AT_RECURSIVE the entire subtree is cloned,
without it - only the part within in the mount containing the
location in question.  In other words, the same as mount --rbind
or mount --bind would've taken.  The detached tree will be
dissolved on the final close of obtained file.  Creation of such
detached trees requires the same capabilities as doing mount --bind.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-api@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 6 insertions, 3 deletions

diff --git a/fs/file_table.c b/fs/file_table.c
index 155d7514a094..3f9c1b452c1d 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -255,6 +255,7 @@ static void __fput(struct file *file)
 	struct dentry *dentry = file->f_path.dentry;
 	struct vfsmount *mnt = file->f_path.mnt;
 	struct inode *inode = file->f_inode;
+	fmode_t mode = file->f_mode;
 
 	if (unlikely(!(file->f_mode & FMODE_OPENED)))
 		goto out;
@@ -277,18 +278,20 @@ static void __fput(struct file *file)
 	if (file->f_op->release)
 		file->f_op->release(inode, file);
 	if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL &&
-		     !(file->f_mode & FMODE_PATH))) {
+		     !(mode & FMODE_PATH))) {
 		cdev_put(inode->i_cdev);
 	}
 	fops_put(file->f_op);
 	put_pid(file->f_owner.pid);
-	if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
+	if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
 		i_readcount_dec(inode);
-	if (file->f_mode & FMODE_WRITER) {
+	if (mode & FMODE_WRITER) {
 		put_write_access(inode);
 		__mnt_drop_write(mnt);
 	}
 	dput(dentry);
+	if (unlikely(mode & FMODE_NEED_UNMOUNT))
+		dissolve_on_fput(mnt);
 	mntput(mnt);
 out:
 	file_free(file);