vfs: Introduce the basic header for the new mount API's filesystem context

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.

Also introduce a struct for typed key=value parameter concept with which
configuration data will be passed to filesystems.  This will allow not only
for ordinary string values, but also make it possible to pass more exotic
values such as binary blobs, paths and fds with greater ease.

Signed-off-by: David Howells <dhowells@redhat.com>

diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
new file mode 100644 (file)
index 0000000..56e406a
--- /dev/null
+++ b/include/linux/fs_context.h
@@ -0,0 +1,108 @@
+/* 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>
+
+struct cred;
+struct dentry;
+struct file_operations;
+struct file_system_type;
+struct mnt_namespace;
+struct net;
+struct pid_namespace;
+struct super_block;
+struct user_namespace;
+struct vfsmount;
+struct path;
+
+enum fs_context_purpose {
+       FS_CONTEXT_FOR_USER_MOUNT,      /* New superblock for user-specified mount */
+       FS_CONTEXT_FOR_KERNEL_MOUNT,    /* New superblock for kernel-internal mount */
+       FS_CONTEXT_FOR_SUBMOUNT,        /* New superblock for automatic submount */
+       FS_CONTEXT_FOR_ROOT_MOUNT,      /* New superblock for internal root mount */
+       FS_CONTEXT_FOR_RECONFIGURE,     /* Superblock reconfiguration (remount) */
+       FS_CONTEXT_FOR_UMOUNT,          /* Reconfiguration to R/O for unmount */
+       FS_CONTEXT_FOR_EMERGENCY_RO,    /* Emergency reconfiguration to R/O */
+};
+
+/*
+ * Type of parameter value.
+ */
+enum fs_value_type {
+       fs_value_is_undefined,
+       fs_value_is_flag,               /* Value not given a value */
+       fs_value_is_string,             /* Value is a string */
+       fs_value_is_blob,               /* Value is a binary blob */
+       fs_value_is_filename,           /* Value is a filename* + dirfd */
+       fs_value_is_filename_empty,     /* Value is a filename* + dirfd + AT_EMPTY_PATH */
+       fs_value_is_file,               /* Value is a file* */
+};
+
+/*
+ * Configuration parameter.
+ */
+struct fs_parameter {
+       const char              *key;           /* Parameter name */
+       enum fs_value_type      type:8;         /* The type of value here */
+       union {
+               char            *string;
+               void            *blob;
+               struct filename *name;
+               struct file     *file;
+       };
+       size_t  size;
+       int     dirfd;
+};
+
+/*
+ * 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 {
+       const struct fs_context_operations *ops;
+       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 */
+       char                    *source;        /* The source name (eg. dev path) */
+       char                    *subtype;       /* The subtype to set on the superblock */
+       void                    *security;      /* The LSM context */
+       void                    *s_fs_info;     /* Proposed s_fs_info */
+       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                    sloppy:1;       /* T if unrecognised options are okay */
+       bool                    silent:1;       /* T if "o silent" specified */
+       bool                    need_free:1;    /* Need to call ops->free() */
+};
+
+struct fs_context_operations {
+       void (*free)(struct fs_context *fc);
+       int (*dup)(struct fs_context *fc, struct fs_context *src_fc);
+       int (*parse_param)(struct fs_context *fc, struct fs_parameter *param);
+       int (*parse_monolithic)(struct fs_context *fc, void *data, size_t data_size);
+       int (*validate)(struct fs_context *fc);
+       int (*get_tree)(struct fs_context *fc);
+       int (*reconfigure)(struct fs_context *fc);
+};
+
+#endif /* _LINUX_FS_CONTEXT_H */