815 lines
27 KiB
ReStructuredText
815 lines
27 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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====================
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Filesystem Mount API
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====================
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.. CONTENTS
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(1) Overview.
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(2) The filesystem context.
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(3) The filesystem context operations.
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(4) Filesystem context security.
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(5) VFS filesystem context API.
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(6) Superblock creation helpers.
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(7) Parameter description.
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(8) Parameter helper functions.
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Overview
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========
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The creation of new mounts is now to be done in a multistep process:
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(1) Create a filesystem context.
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(2) Parse the parameters and attach them to the context. Parameters are
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expected to be passed individually from userspace, though legacy binary
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parameters can also be handled.
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(3) Validate and pre-process the context.
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(4) Get or create a superblock and mountable root.
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(5) Perform the mount.
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(6) Return an error message attached to the context.
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(7) Destroy the context.
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To support this, the file_system_type struct gains two new fields::
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int (*init_fs_context)(struct fs_context *fc);
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const struct fs_parameter_description *parameters;
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The first is invoked to set up the filesystem-specific parts of a filesystem
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context, including the additional space, and the second points to the
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parameter description for validation at registration time and querying by a
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future system call.
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Note that security initialisation is done *after* the filesystem is called so
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that the namespaces may be adjusted first.
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The Filesystem context
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======================
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The creation and reconfiguration of a superblock is governed by a filesystem
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context. This is represented by the fs_context structure::
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struct fs_context {
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const struct fs_context_operations *ops;
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struct file_system_type *fs_type;
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void *fs_private;
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struct dentry *root;
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struct user_namespace *user_ns;
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struct net *net_ns;
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const struct cred *cred;
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char *source;
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char *subtype;
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void *security;
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void *s_fs_info;
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unsigned int sb_flags;
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unsigned int sb_flags_mask;
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unsigned int s_iflags;
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unsigned int lsm_flags;
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enum fs_context_purpose purpose:8;
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...
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};
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The fs_context fields are as follows:
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* ::
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const struct fs_context_operations *ops
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These are operations that can be done on a filesystem context (see
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below). This must be set by the ->init_fs_context() file_system_type
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operation.
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* ::
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struct file_system_type *fs_type
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A pointer to the file_system_type of the filesystem that is being
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constructed or reconfigured. This retains a reference on the type owner.
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* ::
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void *fs_private
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A pointer to the file system's private data. This is where the filesystem
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will need to store any options it parses.
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* ::
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struct dentry *root
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A pointer to the root of the mountable tree (and indirectly, the
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superblock thereof). This is filled in by the ->get_tree() op. If this
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is set, an active reference on root->d_sb must also be held.
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* ::
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struct user_namespace *user_ns
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struct net *net_ns
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There are a subset of the namespaces in use by the invoking process. They
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retain references on each namespace. The subscribed namespaces may be
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replaced by the filesystem to reflect other sources, such as the parent
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mount superblock on an automount.
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* ::
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const struct cred *cred
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The mounter's credentials. This retains a reference on the credentials.
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* ::
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char *source
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This specifies the source. It may be a block device (e.g. /dev/sda1) or
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something more exotic, such as the "host:/path" that NFS desires.
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* ::
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char *subtype
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This is a string to be added to the type displayed in /proc/mounts to
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qualify it (used by FUSE). This is available for the filesystem to set if
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desired.
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* ::
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void *security
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A place for the LSMs to hang their security data for the superblock. The
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relevant security operations are described below.
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* ::
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void *s_fs_info
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The proposed s_fs_info for a new superblock, set in the superblock by
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sget_fc(). This can be used to distinguish superblocks.
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* ::
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unsigned int sb_flags
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unsigned int sb_flags_mask
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Which bits SB_* flags are to be set/cleared in super_block::s_flags.
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* ::
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unsigned int s_iflags
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These will be bitwise-OR'd with s->s_iflags when a superblock is created.
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* ::
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enum fs_context_purpose
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This indicates the purpose for which the context is intended. The
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available values are:
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========================== ======================================
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FS_CONTEXT_FOR_MOUNT, New superblock for explicit mount
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FS_CONTEXT_FOR_SUBMOUNT New automatic submount of extant mount
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FS_CONTEXT_FOR_RECONFIGURE Change an existing mount
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========================== ======================================
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The mount context is created by calling vfs_new_fs_context() or
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vfs_dup_fs_context() and is destroyed with put_fs_context(). Note that the
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structure is not refcounted.
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VFS, security and filesystem mount options are set individually with
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vfs_parse_mount_option(). Options provided by the old mount(2) system call as
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a page of data can be parsed with generic_parse_monolithic().
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When mounting, the filesystem is allowed to take data from any of the pointers
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and attach it to the superblock (or whatever), provided it clears the pointer
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in the mount context.
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The filesystem is also allowed to allocate resources and pin them with the
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mount context. For instance, NFS might pin the appropriate protocol version
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module.
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The Filesystem Context Operations
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=================================
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The filesystem context points to a table of operations::
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struct fs_context_operations {
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void (*free)(struct fs_context *fc);
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int (*dup)(struct fs_context *fc, struct fs_context *src_fc);
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int (*parse_param)(struct fs_context *fc,
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struct fs_parameter *param);
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int (*parse_monolithic)(struct fs_context *fc, void *data);
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int (*get_tree)(struct fs_context *fc);
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int (*reconfigure)(struct fs_context *fc);
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};
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These operations are invoked by the various stages of the mount procedure to
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manage the filesystem context. They are as follows:
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* ::
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void (*free)(struct fs_context *fc);
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Called to clean up the filesystem-specific part of the filesystem context
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when the context is destroyed. It should be aware that parts of the
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context may have been removed and NULL'd out by ->get_tree().
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* ::
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int (*dup)(struct fs_context *fc, struct fs_context *src_fc);
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Called when a filesystem context has been duplicated to duplicate the
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filesystem-private data. An error may be returned to indicate failure to
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do this.
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.. Warning::
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Note that even if this fails, put_fs_context() will be called
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immediately thereafter, so ->dup() *must* make the
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filesystem-private data safe for ->free().
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* ::
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int (*parse_param)(struct fs_context *fc,
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struct fs_parameter *param);
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Called when a parameter is being added to the filesystem context. param
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points to the key name and maybe a value object. VFS-specific options
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will have been weeded out and fc->sb_flags updated in the context.
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Security options will also have been weeded out and fc->security updated.
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The parameter can be parsed with fs_parse() and fs_lookup_param(). Note
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that the source(s) are presented as parameters named "source".
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If successful, 0 should be returned or a negative error code otherwise.
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* ::
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int (*parse_monolithic)(struct fs_context *fc, void *data);
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Called when the mount(2) system call is invoked to pass the entire data
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page in one go. If this is expected to be just a list of "key[=val]"
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items separated by commas, then this may be set to NULL.
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The return value is as for ->parse_param().
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If the filesystem (e.g. NFS) needs to examine the data first and then
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finds it's the standard key-val list then it may pass it off to
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generic_parse_monolithic().
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* ::
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int (*get_tree)(struct fs_context *fc);
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Called to get or create the mountable root and superblock, using the
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information stored in the filesystem context (reconfiguration goes via a
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different vector). It may detach any resources it desires from the
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filesystem context and transfer them to the superblock it creates.
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On success it should set fc->root to the mountable root and return 0. In
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the case of an error, it should return a negative error code.
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The phase on a userspace-driven context will be set to only allow this to
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be called once on any particular context.
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* ::
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int (*reconfigure)(struct fs_context *fc);
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Called to effect reconfiguration of a superblock using information stored
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in the filesystem context. It may detach any resources it desires from
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the filesystem context and transfer them to the superblock. The
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superblock can be found from fc->root->d_sb.
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On success it should return 0. In the case of an error, it should return
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a negative error code.
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.. Note:: reconfigure is intended as a replacement for remount_fs.
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Filesystem context Security
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===========================
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The filesystem context contains a security pointer that the LSMs can use for
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building up a security context for the superblock to be mounted. There are a
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number of operations used by the new mount code for this purpose:
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* ::
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int security_fs_context_alloc(struct fs_context *fc,
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struct dentry *reference);
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Called to initialise fc->security (which is preset to NULL) and allocate
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any resources needed. It should return 0 on success or a negative error
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code on failure.
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reference will be non-NULL if the context is being created for superblock
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reconfiguration (FS_CONTEXT_FOR_RECONFIGURE) in which case it indicates
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the root dentry of the superblock to be reconfigured. It will also be
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non-NULL in the case of a submount (FS_CONTEXT_FOR_SUBMOUNT) in which case
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it indicates the automount point.
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* ::
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int security_fs_context_dup(struct fs_context *fc,
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struct fs_context *src_fc);
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Called to initialise fc->security (which is preset to NULL) and allocate
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any resources needed. The original filesystem context is pointed to by
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src_fc and may be used for reference. It should return 0 on success or a
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negative error code on failure.
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* ::
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void security_fs_context_free(struct fs_context *fc);
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Called to clean up anything attached to fc->security. Note that the
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contents may have been transferred to a superblock and the pointer cleared
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during get_tree.
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* ::
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int security_fs_context_parse_param(struct fs_context *fc,
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struct fs_parameter *param);
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Called for each mount parameter, including the source. The arguments are
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as for the ->parse_param() method. It should return 0 to indicate that
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the parameter should be passed on to the filesystem, 1 to indicate that
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the parameter should be discarded or an error to indicate that the
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parameter should be rejected.
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The value pointed to by param may be modified (if a string) or stolen
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(provided the value pointer is NULL'd out). If it is stolen, 1 must be
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returned to prevent it being passed to the filesystem.
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* ::
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int security_fs_context_validate(struct fs_context *fc);
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Called after all the options have been parsed to validate the collection
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as a whole and to do any necessary allocation so that
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security_sb_get_tree() and security_sb_reconfigure() are less likely to
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fail. It should return 0 or a negative error code.
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In the case of reconfiguration, the target superblock will be accessible
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via fc->root.
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* ::
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int security_sb_get_tree(struct fs_context *fc);
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Called during the mount procedure to verify that the specified superblock
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is allowed to be mounted and to transfer the security data there. It
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should return 0 or a negative error code.
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* ::
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void security_sb_reconfigure(struct fs_context *fc);
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Called to apply any reconfiguration to an LSM's context. It must not
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fail. Error checking and resource allocation must be done in advance by
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the parameter parsing and validation hooks.
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* ::
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int security_sb_mountpoint(struct fs_context *fc,
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struct path *mountpoint,
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unsigned int mnt_flags);
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Called during the mount procedure to verify that the root dentry attached
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to the context is permitted to be attached to the specified mountpoint.
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It should return 0 on success or a negative error code on failure.
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VFS Filesystem context API
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==========================
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There are four operations for creating a filesystem context and one for
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destroying a context:
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* ::
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struct fs_context *fs_context_for_mount(struct file_system_type *fs_type,
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unsigned int sb_flags);
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Allocate a filesystem context for the purpose of setting up a new mount,
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whether that be with a new superblock or sharing an existing one. This
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sets the superblock flags, initialises the security and calls
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fs_type->init_fs_context() to initialise the filesystem private data.
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fs_type specifies the filesystem type that will manage the context and
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sb_flags presets the superblock flags stored therein.
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* ::
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struct fs_context *fs_context_for_reconfigure(
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struct dentry *dentry,
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unsigned int sb_flags,
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unsigned int sb_flags_mask);
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Allocate a filesystem context for the purpose of reconfiguring an
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existing superblock. dentry provides a reference to the superblock to be
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configured. sb_flags and sb_flags_mask indicate which superblock flags
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need changing and to what.
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* ::
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struct fs_context *fs_context_for_submount(
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struct file_system_type *fs_type,
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struct dentry *reference);
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Allocate a filesystem context for the purpose of creating a new mount for
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an automount point or other derived superblock. fs_type specifies the
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filesystem type that will manage the context and the reference dentry
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supplies the parameters. Namespaces are propagated from the reference
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dentry's superblock also.
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Note that it's not a requirement that the reference dentry be of the same
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filesystem type as fs_type.
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* ::
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struct fs_context *vfs_dup_fs_context(struct fs_context *src_fc);
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Duplicate a filesystem context, copying any options noted and duplicating
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or additionally referencing any resources held therein. This is available
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for use where a filesystem has to get a mount within a mount, such as NFS4
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does by internally mounting the root of the target server and then doing a
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private pathwalk to the target directory.
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The purpose in the new context is inherited from the old one.
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* ::
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void put_fs_context(struct fs_context *fc);
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Destroy a filesystem context, releasing any resources it holds. This
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calls the ->free() operation. This is intended to be called by anyone who
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created a filesystem context.
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.. Warning::
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filesystem contexts are not refcounted, so this causes unconditional
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destruction.
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In all the above operations, apart from the put op, the return is a mount
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context pointer or a negative error code.
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For the remaining operations, if an error occurs, a negative error code will be
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returned.
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* ::
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int vfs_parse_fs_param(struct fs_context *fc,
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struct fs_parameter *param);
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Supply a single mount parameter to the filesystem context. This includes
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the specification of the source/device which is specified as the "source"
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parameter (which may be specified multiple times if the filesystem
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supports that).
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param specifies the parameter key name and the value. The parameter is
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first checked to see if it corresponds to a standard mount flag (in which
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case it is used to set an SB_xxx flag and consumed) or a security option
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(in which case the LSM consumes it) before it is passed on to the
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filesystem.
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The parameter value is typed and can be one of:
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==================== =============================
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fs_value_is_flag Parameter not given a value
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fs_value_is_string Value is a string
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fs_value_is_blob Value is a binary blob
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fs_value_is_filename Value is a filename* + dirfd
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fs_value_is_file Value is an open file (file*)
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==================== =============================
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If there is a value, that value is stored in a union in the struct in one
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of param->{string,blob,name,file}. Note that the function may steal and
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clear the pointer, but then becomes responsible for disposing of the
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object.
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* ::
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int vfs_parse_fs_string(struct fs_context *fc, const char *key,
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const char *value, size_t v_size);
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A wrapper around vfs_parse_fs_param() that copies the value string it is
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passed.
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* ::
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int generic_parse_monolithic(struct fs_context *fc, void *data);
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Parse a sys_mount() data page, assuming the form to be a text list
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consisting of key[=val] options separated by commas. Each item in the
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list is passed to vfs_mount_option(). This is the default when the
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->parse_monolithic() method is NULL.
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* ::
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int vfs_get_tree(struct fs_context *fc);
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Get or create the mountable root and superblock, using the parameters in
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the filesystem context to select/configure the superblock. This invokes
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the ->get_tree() method.
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* ::
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struct vfsmount *vfs_create_mount(struct fs_context *fc);
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Create a mount given the parameters in the specified filesystem context.
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Note that this does not attach the mount to anything.
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Superblock Creation Helpers
|
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===========================
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A number of VFS helpers are available for use by filesystems for the creation
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or looking up of superblocks.
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* ::
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struct super_block *
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sget_fc(struct fs_context *fc,
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int (*test)(struct super_block *sb, struct fs_context *fc),
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int (*set)(struct super_block *sb, struct fs_context *fc));
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This is the core routine. If test is non-NULL, it searches for an
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existing superblock matching the criteria held in the fs_context, using
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the test function to match them. If no match is found, a new superblock
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is created and the set function is called to set it up.
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Prior to the set function being called, fc->s_fs_info will be transferred
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to sb->s_fs_info - and fc->s_fs_info will be cleared if set returns
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success (ie. 0).
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The following helpers all wrap sget_fc():
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(1) vfs_get_single_super
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|
|
Only one such superblock may exist in the system. Any further
|
|
attempt to get a new superblock gets this one (and any parameter
|
|
differences are ignored).
|
|
|
|
(2) vfs_get_keyed_super
|
|
|
|
Multiple superblocks of this type may exist and they're keyed on
|
|
their s_fs_info pointer (for example this may refer to a
|
|
namespace).
|
|
|
|
(3) vfs_get_independent_super
|
|
|
|
Multiple independent superblocks of this type may exist. This
|
|
function never matches an existing one and always creates a new
|
|
one.
|
|
|
|
|
|
Parameter Description
|
|
=====================
|
|
|
|
Parameters are described using structures defined in linux/fs_parser.h.
|
|
There's a core description struct that links everything together::
|
|
|
|
struct fs_parameter_description {
|
|
const struct fs_parameter_spec *specs;
|
|
const struct fs_parameter_enum *enums;
|
|
};
|
|
|
|
For example::
|
|
|
|
enum {
|
|
Opt_autocell,
|
|
Opt_bar,
|
|
Opt_dyn,
|
|
Opt_foo,
|
|
Opt_source,
|
|
};
|
|
|
|
static const struct fs_parameter_description afs_fs_parameters = {
|
|
.specs = afs_param_specs,
|
|
.enums = afs_param_enums,
|
|
};
|
|
|
|
The members are as follows:
|
|
|
|
(1) ::
|
|
|
|
const struct fs_parameter_specification *specs;
|
|
|
|
Table of parameter specifications, terminated with a null entry, where the
|
|
entries are of type::
|
|
|
|
struct fs_parameter_spec {
|
|
const char *name;
|
|
u8 opt;
|
|
enum fs_parameter_type type:8;
|
|
unsigned short flags;
|
|
};
|
|
|
|
The 'name' field is a string to match exactly to the parameter key (no
|
|
wildcards, patterns and no case-independence) and 'opt' is the value that
|
|
will be returned by the fs_parser() function in the case of a successful
|
|
match.
|
|
|
|
The 'type' field indicates the desired value type and must be one of:
|
|
|
|
======================= ======================= =====================
|
|
TYPE NAME EXPECTED VALUE RESULT IN
|
|
======================= ======================= =====================
|
|
fs_param_is_flag No value n/a
|
|
fs_param_is_bool Boolean value result->boolean
|
|
fs_param_is_u32 32-bit unsigned int result->uint_32
|
|
fs_param_is_u32_octal 32-bit octal int result->uint_32
|
|
fs_param_is_u32_hex 32-bit hex int result->uint_32
|
|
fs_param_is_s32 32-bit signed int result->int_32
|
|
fs_param_is_u64 64-bit unsigned int result->uint_64
|
|
fs_param_is_enum Enum value name result->uint_32
|
|
fs_param_is_string Arbitrary string param->string
|
|
fs_param_is_blob Binary blob param->blob
|
|
fs_param_is_blockdev Blockdev path * Needs lookup
|
|
fs_param_is_path Path * Needs lookup
|
|
fs_param_is_fd File descriptor result->int_32
|
|
======================= ======================= =====================
|
|
|
|
Note that if the value is of fs_param_is_bool type, fs_parse() will try
|
|
to match any string value against "0", "1", "no", "yes", "false", "true".
|
|
|
|
Each parameter can also be qualified with 'flags':
|
|
|
|
======================= ================================================
|
|
fs_param_v_optional The value is optional
|
|
fs_param_neg_with_no result->negated set if key is prefixed with "no"
|
|
fs_param_neg_with_empty result->negated set if value is ""
|
|
fs_param_deprecated The parameter is deprecated.
|
|
======================= ================================================
|
|
|
|
These are wrapped with a number of convenience wrappers:
|
|
|
|
======================= ===============================================
|
|
MACRO SPECIFIES
|
|
======================= ===============================================
|
|
fsparam_flag() fs_param_is_flag
|
|
fsparam_flag_no() fs_param_is_flag, fs_param_neg_with_no
|
|
fsparam_bool() fs_param_is_bool
|
|
fsparam_u32() fs_param_is_u32
|
|
fsparam_u32oct() fs_param_is_u32_octal
|
|
fsparam_u32hex() fs_param_is_u32_hex
|
|
fsparam_s32() fs_param_is_s32
|
|
fsparam_u64() fs_param_is_u64
|
|
fsparam_enum() fs_param_is_enum
|
|
fsparam_string() fs_param_is_string
|
|
fsparam_blob() fs_param_is_blob
|
|
fsparam_bdev() fs_param_is_blockdev
|
|
fsparam_path() fs_param_is_path
|
|
fsparam_fd() fs_param_is_fd
|
|
======================= ===============================================
|
|
|
|
all of which take two arguments, name string and option number - for
|
|
example::
|
|
|
|
static const struct fs_parameter_spec afs_param_specs[] = {
|
|
fsparam_flag ("autocell", Opt_autocell),
|
|
fsparam_flag ("dyn", Opt_dyn),
|
|
fsparam_string ("source", Opt_source),
|
|
fsparam_flag_no ("foo", Opt_foo),
|
|
{}
|
|
};
|
|
|
|
An addition macro, __fsparam() is provided that takes an additional pair
|
|
of arguments to specify the type and the flags for anything that doesn't
|
|
match one of the above macros.
|
|
|
|
(2) ::
|
|
|
|
const struct fs_parameter_enum *enums;
|
|
|
|
Table of enum value names to integer mappings, terminated with a null
|
|
entry. This is of type::
|
|
|
|
struct fs_parameter_enum {
|
|
u8 opt;
|
|
char name[14];
|
|
u8 value;
|
|
};
|
|
|
|
Where the array is an unsorted list of { parameter ID, name }-keyed
|
|
elements that indicate the value to map to, e.g.::
|
|
|
|
static const struct fs_parameter_enum afs_param_enums[] = {
|
|
{ Opt_bar, "x", 1},
|
|
{ Opt_bar, "y", 23},
|
|
{ Opt_bar, "z", 42},
|
|
};
|
|
|
|
If a parameter of type fs_param_is_enum is encountered, fs_parse() will
|
|
try to look the value up in the enum table and the result will be stored
|
|
in the parse result.
|
|
|
|
The parser should be pointed to by the parser pointer in the file_system_type
|
|
struct as this will provide validation on registration (if
|
|
CONFIG_VALIDATE_FS_PARSER=y) and will allow the description to be queried from
|
|
userspace using the fsinfo() syscall.
|
|
|
|
|
|
Parameter Helper Functions
|
|
==========================
|
|
|
|
A number of helper functions are provided to help a filesystem or an LSM
|
|
process the parameters it is given.
|
|
|
|
* ::
|
|
|
|
int lookup_constant(const struct constant_table tbl[],
|
|
const char *name, int not_found);
|
|
|
|
Look up a constant by name in a table of name -> integer mappings. The
|
|
table is an array of elements of the following type::
|
|
|
|
struct constant_table {
|
|
const char *name;
|
|
int value;
|
|
};
|
|
|
|
If a match is found, the corresponding value is returned. If a match
|
|
isn't found, the not_found value is returned instead.
|
|
|
|
* ::
|
|
|
|
bool validate_constant_table(const struct constant_table *tbl,
|
|
size_t tbl_size,
|
|
int low, int high, int special);
|
|
|
|
Validate a constant table. Checks that all the elements are appropriately
|
|
ordered, that there are no duplicates and that the values are between low
|
|
and high inclusive, though provision is made for one allowable special
|
|
value outside of that range. If no special value is required, special
|
|
should just be set to lie inside the low-to-high range.
|
|
|
|
If all is good, true is returned. If the table is invalid, errors are
|
|
logged to the kernel log buffer and false is returned.
|
|
|
|
* ::
|
|
|
|
bool fs_validate_description(const struct fs_parameter_description *desc);
|
|
|
|
This performs some validation checks on a parameter description. It
|
|
returns true if the description is good and false if it is not. It will
|
|
log errors to the kernel log buffer if validation fails.
|
|
|
|
* ::
|
|
|
|
int fs_parse(struct fs_context *fc,
|
|
const struct fs_parameter_description *desc,
|
|
struct fs_parameter *param,
|
|
struct fs_parse_result *result);
|
|
|
|
This is the main interpreter of parameters. It uses the parameter
|
|
description to look up a parameter by key name and to convert that to an
|
|
option number (which it returns).
|
|
|
|
If successful, and if the parameter type indicates the result is a
|
|
boolean, integer or enum type, the value is converted by this function and
|
|
the result stored in result->{boolean,int_32,uint_32,uint_64}.
|
|
|
|
If a match isn't initially made, the key is prefixed with "no" and no
|
|
value is present then an attempt will be made to look up the key with the
|
|
prefix removed. If this matches a parameter for which the type has flag
|
|
fs_param_neg_with_no set, then a match will be made and result->negated
|
|
will be set to true.
|
|
|
|
If the parameter isn't matched, -ENOPARAM will be returned; if the
|
|
parameter is matched, but the value is erroneous, -EINVAL will be
|
|
returned; otherwise the parameter's option number will be returned.
|
|
|
|
* ::
|
|
|
|
int fs_lookup_param(struct fs_context *fc,
|
|
struct fs_parameter *value,
|
|
bool want_bdev,
|
|
unsigned int flags,
|
|
struct path *_path);
|
|
|
|
This takes a parameter that carries a string or filename type and attempts
|
|
to do a path lookup on it. If the parameter expects a blockdev, a check
|
|
is made that the inode actually represents one.
|
|
|
|
Returns 0 if successful and ``*_path`` will be set; returns a negative
|
|
error code if not.
|