vfs: Introduce fs_context, switch vfs_kern_mount() to it.

Introduce a filesystem context concept to be used during superblock
creation for mount and superblock reconfiguration for remount.  This is
allocated at the beginning of the mount procedure and into it is placed:

 (1) Filesystem type.

 (2) Namespaces.

 (3) Source/Device names (there may be multiple).

 (4) Superblock flags (SB_*).

 (5) Security details.

 (6) Filesystem-specific data, as set by the mount options.

Accessor functions are then provided to set up a context, parameterise it
from monolithic mount data (the data page passed to mount(2)) and tear it
down again.

A legacy wrapper is provided that implements what will be the basic
operations, wrapping access to filesystems that aren't yet aware of the
fs_context.

Finally, vfs_kern_mount() is changed to make use of the fs_context and
mount_fs() is replaced by vfs_get_tree(), called from vfs_kern_mount().
[AV -- add missing kstrdup()]
[AV -- put_cred() can be unconditional - fc->cred can't be NULL]
[AV -- take legacy_validate() contents into legacy_parse_monolithic()]
[AV -- merge KERNEL_MOUNT and USER_MOUNT]
[AV -- don't unlock superblock on success return from vfs_get_tree()]
[AV -- kill 'reference' argument of init_fs_context()]

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

1 files changed, 23 insertions, 27 deletions

diff --git a/fs/super.c b/fs/super.c
index 48e25eba8465..fc3887277ad1 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -35,6 +35,7 @@
 #include <linux/fsnotify.h>
 #include <linux/lockdep.h>
 #include <linux/user_namespace.h>
+#include <linux/fs_context.h>
 #include <uapi/linux/mount.h>
 #include "internal.h"
 
@@ -1241,27 +1242,24 @@ struct dentry *mount_single(struct file_system_type *fs_type,
 }
 EXPORT_SYMBOL(mount_single);
 
-struct dentry *
-mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
+/**
+ * vfs_get_tree - Get the mountable root
+ * @fc: The superblock configuration context.
+ *
+ * The filesystem is invoked to get or create a superblock which can then later
+ * be used for mounting.  The filesystem places a pointer to the root to be
+ * used for mounting in @fc->root.
+ */
+int vfs_get_tree(struct fs_context *fc)
 {
-        struct dentry *root;
         struct super_block *sb;
-        int error = -ENOMEM;
-        void *sec_opts = NULL;
+        int error;
 
-        if (data && !(type->fs_flags & FS_BINARY_MOUNTDATA)) {
-                error = security_sb_eat_lsm_opts(data, &sec_opts);
-                if (error)
-                        return ERR_PTR(error);
-        }
+        error = legacy_get_tree(fc);
+        if (error < 0)
+                return error;
 
-        root = type->mount(type, flags, name, data);
-        if (IS_ERR(root)) {
-                error = PTR_ERR(root);
-                goto out_free_secdata;
-        }
-        sb = root->d_sb;
-        BUG_ON(!sb);
+        sb = fc->root->d_sb;
         WARN_ON(!sb->s_bdi);
 
         /*
@@ -1273,11 +1271,11 @@ mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
         smp_wmb();
         sb->s_flags |= SB_BORN;
 
-        error = security_sb_set_mnt_opts(sb, sec_opts, 0, NULL);
+        error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
         if (error)
                 goto out_sb;
 
-        if (!(flags & (MS_KERNMOUNT|MS_SUBMOUNT))) {
+        if (!(fc->sb_flags & (MS_KERNMOUNT|MS_SUBMOUNT))) {
                 error = security_sb_kern_mount(sb);
                 if (error)
                         goto out_sb;
@@ -1290,18 +1288,16 @@ mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
          * violate this rule.
          */
         WARN((sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
-                "negative value (%lld)\n", type->name, sb->s_maxbytes);
+                "negative value (%lld)\n", fc->fs_type->name, sb->s_maxbytes);
 
-        up_write(&sb->s_umount);
-        security_free_mnt_opts(&sec_opts);
-        return root;
+        return 0;
 out_sb:
-        dput(root);
+        dput(fc->root);
+        fc->root = NULL;
         deactivate_locked_super(sb);
-out_free_secdata:
-        security_free_mnt_opts(&sec_opts);
-        return ERR_PTR(error);
+        return error;
 }
+EXPORT_SYMBOL(vfs_get_tree);
 
 /*
  * Setup private BDI for given superblock. It gets automatically cleaned up