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, 64 insertions, 0 deletions

diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
new file mode 100644
index 000000000000..9805514444c9
--- /dev/null
+++ b/include/linux/fs_context.h
@@ -0,0 +1,64 @@
+/* Filesystem superblock creation and reconfiguration context.
+ *
+ * Copyright (C) 2018 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public Licence
+ * as published by the Free Software Foundation; either version
+ * 2 of the Licence, or (at your option) any later version.
+ */
+
+#ifndef _LINUX_FS_CONTEXT_H
+#define _LINUX_FS_CONTEXT_H
+
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/security.h>
+
+struct cred;
+struct dentry;
+struct file_operations;
+struct file_system_type;
+struct net;
+struct user_namespace;
+
+enum fs_context_purpose {
+	FS_CONTEXT_FOR_MOUNT,		/* New superblock for explicit mount */
+};
+
+/*
+ * Filesystem context for holding the parameters used in the creation or
+ * reconfiguration of a superblock.
+ *
+ * Superblock creation fills in ->root whereas reconfiguration begins with this
+ * already set.
+ *
+ * See Documentation/filesystems/mounting.txt
+ */
+struct fs_context {
+	struct file_system_type	*fs_type;
+	void			*fs_private;	/* The filesystem's context */
+	struct dentry		*root;		/* The root and superblock */
+	struct user_namespace	*user_ns;	/* The user namespace for this mount */
+	struct net		*net_ns;	/* The network namespace for this mount */
+	const struct cred	*cred;		/* The mounter's credentials */
+	const char		*source;	/* The source name (eg. dev path) */
+	const char		*subtype;	/* The subtype to set on the superblock */
+	void			*security;	/* Linux S&M options */
+	unsigned int		sb_flags;	/* Proposed superblock flags (SB_*) */
+	unsigned int		sb_flags_mask;	/* Superblock flags that were changed */
+	enum fs_context_purpose	purpose:8;
+	bool			need_free:1;	/* Need to call ops->free() */
+};
+
+/*
+ * fs_context manipulation functions.
+ */
+extern struct fs_context *fs_context_for_mount(struct file_system_type *fs_type,
+						unsigned int sb_flags);
+
+extern int vfs_get_tree(struct fs_context *fc);
+extern void put_fs_context(struct fs_context *fc);
+
+#endif /* _LINUX_FS_CONTEXT_H */