new helpers: vfs_create_mount(), fc_mount()

Create a new helper, vfs_create_mount(), that creates a detached vfsmount
object from an fs_context that has a superblock attached to it.

Almost all uses will be paired with immediately preceding vfs_get_tree();
add a helper for such combination.

Switch vfs_kern_mount() to use this.

NOTE: mild behaviour change; passing NULL as 'device name' to
something like procfs will change /proc/*/mountstats - "device none"
instead on "no device".  That is consistent with /proc/mounts et.al.

[do'h - EXPORT_SYMBOL_GPL slipped in by mistake; removed]
[AV -- remove confused comment from vfs_create_mount()]
[AV -- removed the second argument]

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

2 files changed, 55 insertions, 24 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 3f2fd7a34733..156771f5745a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -941,12 +941,59 @@ static struct mount *skip_mnt_tree(struct mount *p)
 	return p;
 }
 
+/**
+ * vfs_create_mount - Create a mount for a configured superblock
+ * @fc: The configuration context with the superblock attached
+ *
+ * Create a mount to an already configured superblock.  If necessary, the
+ * caller should invoke vfs_get_tree() before calling this.
+ *
+ * Note that this does not attach the mount to anything.
+ */
+struct vfsmount *vfs_create_mount(struct fs_context *fc)
+{
+	struct mount *mnt;
+
+	if (!fc->root)
+		return ERR_PTR(-EINVAL);
+
+	mnt = alloc_vfsmnt(fc->source ?: "none");
+	if (!mnt)
+		return ERR_PTR(-ENOMEM);
+
+	if (fc->sb_flags & SB_KERNMOUNT)
+		mnt->mnt.mnt_flags = MNT_INTERNAL;
+
+	atomic_inc(&fc->root->d_sb->s_active);
+	mnt->mnt.mnt_sb		= fc->root->d_sb;
+	mnt->mnt.mnt_root	= dget(fc->root);
+	mnt->mnt_mountpoint	= mnt->mnt.mnt_root;
+	mnt->mnt_parent		= mnt;
+
+	lock_mount_hash();
+	list_add_tail(&mnt->mnt_instance, &mnt->mnt.mnt_sb->s_mounts);
+	unlock_mount_hash();
+	return &mnt->mnt;
+}
+EXPORT_SYMBOL(vfs_create_mount);
+
+struct vfsmount *fc_mount(struct fs_context *fc)
+{
+	int err = vfs_get_tree(fc);
+	if (!err) {
+		up_write(&fc->root->d_sb->s_umount);
+		return vfs_create_mount(fc);
+	}
+	return ERR_PTR(err);
+}
+EXPORT_SYMBOL(fc_mount);
+
 struct vfsmount *vfs_kern_mount(struct file_system_type *type,
 				int flags, const char *name,
 				void *data)
 {
 	struct fs_context *fc;
-	struct mount *mnt;
+	struct vfsmount *mnt;
 	int ret = 0;
 
 	if (!type)
@@ -964,31 +1011,12 @@ struct vfsmount *vfs_kern_mount(struct file_system_type *type,
 	if (!ret)
 		ret = parse_monolithic_mount_data(fc, data);
 	if (!ret)
-		ret = vfs_get_tree(fc);
-	if (ret) {
-		put_fs_context(fc);
-		return ERR_PTR(ret);
-	}
-	up_write(&fc->root->d_sb->s_umount);
-	mnt = alloc_vfsmnt(name);
-	if (!mnt) {
-		put_fs_context(fc);
-		return ERR_PTR(-ENOMEM);
-	}
-
-	if (flags & SB_KERNMOUNT)
-		mnt->mnt.mnt_flags = MNT_INTERNAL;
+		mnt = fc_mount(fc);
+	else
+		mnt = ERR_PTR(ret);
 
-	atomic_inc(&fc->root->d_sb->s_active);
-	mnt->mnt.mnt_root = dget(fc->root);
-	mnt->mnt.mnt_sb = fc->root->d_sb;
-	mnt->mnt_mountpoint = mnt->mnt.mnt_root;
-	mnt->mnt_parent = mnt;
-	lock_mount_hash();
-	list_add_tail(&mnt->mnt_instance, &fc->root->d_sb->s_mounts);
-	unlock_mount_hash();
 	put_fs_context(fc);
-	return &mnt->mnt;
+	return mnt;
 }
 EXPORT_SYMBOL_GPL(vfs_kern_mount);
 
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 037eed52164b..9197ddbf35fb 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -21,6 +21,7 @@ struct super_block;
 struct vfsmount;
 struct dentry;
 struct mnt_namespace;
+struct fs_context;
 
 #define MNT_NOSUID	0x01
 #define MNT_NODEV	0x02
@@ -88,6 +89,8 @@ struct path;
 extern struct vfsmount *clone_private_mount(const struct path *path);
 
 struct file_system_type;
+extern struct vfsmount *fc_mount(struct fs_context *fc);
+extern struct vfsmount *vfs_create_mount(struct fs_context *fc);
 extern struct vfsmount *vfs_kern_mount(struct file_system_type *type,
 				      int flags, const char *name,
 				      void *data);