vfs: Implement a filesystem superblock creation/configuration context

[AV - unfuck kern_mount_data(); we want non-NULL ->mnt_ns on long-living
mounts]
[AV - reordering fs/namespace.c is badly overdue, but let's keep it
separate from that series]
[AV - drop simple_pin_fs() change]
[AV - clean vfs_kern_mount() failure exits up]

Implement a filesystem context concept to be used during superblock
creation for mount and superblock reconfiguration for remount.

The mounting procedure then becomes:

 (1) Allocate new fs_context context.

 (2) Configure the context.

 (3) Create superblock.

 (4) Query the superblock.

 (5) Create a mount for the superblock.

 (6) Destroy the context.

Rather than calling fs_type->mount(), an fs_context struct is created and
fs_type->init_fs_context() is called to set it up.  Pointers exist for the
filesystem and LSM to hang their private data off.

A set of operations has to be set by ->init_fs_context() to provide
freeing, duplication, option parsing, binary data parsing, validation,
mounting and superblock filling.

Legacy filesystems are supported by the provision of a set of legacy
fs_context operations that build up a list of mount options and then invoke
fs_type->mount() from within the fs_context ->get_tree() operation.  This
allows all filesystems to be accessed using fs_context.

It should be noted that, whilst this patch adds a lot of lines of code,
there is quite a bit of duplication with existing code that can be
eliminated should all filesystems be converted over.

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

1 files changed, 9 insertions, 16 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 931228d8518a..1a1ed2528f47 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -997,17 +997,15 @@ struct vfsmount *vfs_kern_mount(struct file_system_type *type,
 	int ret = 0;
 
 	if (!type)
-		return ERR_PTR(-ENODEV);
+		return ERR_PTR(-EINVAL);
 
 	fc = fs_context_for_mount(type, flags);
 	if (IS_ERR(fc))
 		return ERR_CAST(fc);
 
-	if (name) {
-		fc->source = kstrdup(name, GFP_KERNEL);
-		if (!fc->source)
-			ret = -ENOMEM;
-	}
+	if (name)
+		ret = vfs_parse_fs_string(fc, "source",
+					  name, strlen(name));
 	if (!ret)
 		ret = parse_monolithic_mount_data(fc, data);
 	if (!ret)
@@ -2611,16 +2609,11 @@ static int do_new_mount(struct path *path, const char *fstype, int sb_flags,
 	if (IS_ERR(fc))
 		return PTR_ERR(fc);
 
-	if (subtype) {
-		fc->subtype = kstrdup(subtype, GFP_KERNEL);
-		if (!fc->subtype)
-			err = -ENOMEM;
-	}
-	if (!err && name) {
-		fc->source = kstrdup(name, GFP_KERNEL);
-		if (!fc->source)
-			err = -ENOMEM;
-	}
+	if (subtype)
+		err = vfs_parse_fs_string(fc, "subtype",
+					  subtype, strlen(subtype));
+	if (!err && name)
+		err = vfs_parse_fs_string(fc, "source", name, strlen(name));
 	if (!err)
 		err = parse_monolithic_mount_data(fc, data);
 	if (!err)