Message-ID: <22bc3062-e860-5266-7a2d-9f5eff8b8559@schaufler-ca.com>
Date:   Wed, 14 Jun 2017 13:03:43 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     David Howells <dhowells@...hat.com>, mszeredi@...hat.com,
        viro@...iv.linux.org.uk
Cc:     linux-nfs@...r.kernel.org, jlayton@...hat.com,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 08/27] VFS: Introduce the structs and doc for a filesystem
 context [ver #5]

On 6/14/2017 8:16 AM, David Howells wrote:
> 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) Device name.
>
>  (4) Superblock flags (MS_*).
>
>  (5) Security details.
>
>  (6) Filesystem-specific data, as set by the mount options.
>
> Signed-off-by: David Howells <dhowells@...hat.com>
> ---
>
>  Documentation/filesystems/mounting.txt |  436 ++++++++++++++++++++++++++++++++
>  include/linux/fs_context.h             |   72 +++++
>  2 files changed, 508 insertions(+)
>  create mode 100644 Documentation/filesystems/mounting.txt
>  create mode 100644 include/linux/fs_context.h
>
> diff --git a/Documentation/filesystems/mounting.txt b/Documentation/filesystems/mounting.txt
> new file mode 100644
> index 000000000000..315a5a4ff5cc
> --- /dev/null
> +++ b/Documentation/filesystems/mounting.txt
> @@ -0,0 +1,436 @@
> +			      ===================
> +			      FILESYSTEM MOUNTING
> +			      ===================
> +
> +CONTENTS
> +
> + (1) Overview.
> +
> + (2) The filesystem context.
> +
> + (3) The filesystem context operations.
> +
> + (4) Filesystem context security.
> +
> + (5) VFS filesystem context operations.
> +
> +
> +========
> +OVERVIEW
> +========
> +
> +The creation of new mounts is now to be done in a multistep process:
> +
> + (1) Create a filesystem context.
> +
> + (2) Parse the options and attach them to the context.  Options may be passed
> +     individually from userspace.
> +
> + (3) Validate and pre-process the context.
> +
> + (4) Get or create a superblock and mountable root.
> +
> + (5) Perform the mount.
> +
> + (6) Return an error message attached to the context.
> +
> + (7) Destroy the context.
> +
> +To support this, the file_system_type struct gains two new fields:
> +
> +	unsigned short fs_context_size;
> +
> +which indicates the total amount of space that should be allocated for context
> +data (see the Filesystem Context section), and:
> +
> +	int (*init_fs_context)(struct fs_context *fc, struct super_block *src_sb);
> +
> +which is invoked to set up the filesystem-specific parts of a filesystem
> +context, including the additional space.  The src_sb parameter is used to
> +convey the superblock from which the filesystem may draw extra information
> +(such as namespaces), for submount (FS_CONTEXT_FOR_SUBMOUNT) or remount
> +(FS_CONTEXT_FOR_REMOUNT) purposes or it will be NULL.
> +
> +Note that security initialisation is done *after* the filesystem is called so
> +that the namespaces may be adjusted first.
> +
> +And the super_operations struct gains one:
> +
> +	int (*remount_fs_fc) (struct super_block *, struct fs_context *);
> +
> +This shadows the ->remount_fs() operation and takes a prepared filesystem
> +context instead of the mount flags and data page.  It may modify the ms_flags
> +in the context for the caller to pick up.
> +
> +[NOTE] remount_fs_fc is intended as a replacement for remount_fs.
> +
> +
> +======================
> +THE FILESYSTEM CONTEXT
> +======================
> +
> +The creation and reconfiguration of a superblock is governed by a filesystem
> +context.  This is represented by the fs_context structure:
> +
> +	struct fs_context {
> +		const struct fs_context_operations *ops;
> +		struct file_system_type *fs;
> +		struct dentry		*root;
> +		struct user_namespace	*user_ns;
> +		struct net		*net_ns;
> +		const struct cred	*cred;
> +		char			*device;
> +		void			*security;
> +		unsigned int		sb_flags;
> +		bool			sloppy;
> +		bool			silent;
> +		bool			degraded;
> +		enum fs_context_purpose	purpose : 8;
> +	};

Could you namespace the fields of this structure?
e.g. fs_cred, fs_security
It makes it so much easier to determine which ->cred
you're looking at.

> +
> +When the VFS creates this, it allocates ->fs_context_size bytes (as specified
> +by the file_system_type object) to hold both the fs_context struct and any
> +extra data required by the filesystem.  The fs_context struct is placed at the
> +beginning of this space.  Any extra space beyond that is for use by the
> +filesystem.  The filesystem should wrap the struct in its own, e.g.:
> +
> +	struct nfs_fs_context {
> +		struct fs_context fc;
> +		...
> +	};
> +
> +placing the fs_context struct first.  container_of() can then be used.  The
> +file_system_type would be initialised thus:
> +
> +	struct file_system_type nfs = {
> +		...
> +		.fs_context_size	= sizeof(struct nfs_fs_context),
> +		.init_fs_context	= nfs_init_fs_context,
> +		...
> +	};
> +
> +The fs_context fields are as follows:
> +
> + (*) const struct fs_context_operations *ops
> +
> +     These are operations that can be done on a filesystem context (see
> +     below).  This must be set by the ->init_fs_context() file_system_type
> +     operation.
> +
> + (*) struct file_system_type *fs
> +
> +     A pointer to the file_system_type of the filesystem that is being
> +     constructed or reconfigured.  This retains a ref on the type owner.
> +
> + (*) struct dentry *root
> +
> +     A pointer to the root of the mountable tree (and indirectly, the
> +     superblock thereof).  This is filled in by the ->get_tree() op.
> +
> + (*) struct user_namespace *user_ns
> + (*) struct net *net_ns
> +
> +     This is a subset of the namespaces in use by the invoking process.  This
> +     retains a ref on each namespace.  The subscribed namespaces may be
> +     replaced by the filesystem to reflect other sources, such as the parent
> +     mount superblock on an automount.
> +
> + (*) struct cred *cred
> +
> +     The mounter's credentials.  This retains a ref on the credentials.
> +
> + (*) char *device
> +
> +     This is the device to be mounted.  It may be a block device
> +     (e.g. /dev/sda1) or something more exotic, such as the "host:/path" that
> +     NFS desires.
> +
> + (*) void *security
> +
> +     A place for the LSMs to hang their security data for the superblock.  The
> +     relevant security operations are described below.
> +
> + (*) unsigned int sb_flags
> +
> +     This holds the MS_* flags mount flags.
> +
> + (*) bool sloppy
> + (*) bool silent
> +
> +     These are set if the sloppy or silent mount options are given.
> +
> +     [NOTE] sloppy is probably unnecessary when userspace passes over one
> +     option at a time since the error can just be ignored if userspace deems it
> +     to be unimportant.
> +
> +     [NOTE] silent is probably redundant with ms_flags & MS_SILENT.
> +
> + (*) bool degraded
> +
> +     This is set if any preallocated resources in the context have been used
> +     up, thereby rendering it unreusable for the ->get_tree() op.
> +
> + (*) enum fs_context_purpose
> +
> +     This indicates the purpose for which the context is intended.  The
> +     available values are:
> +
> +	FS_CONTEXT_FOR_NEW	-- New mount
> +	FS_CONTEXT_FOR_SUBMOUNT	-- New automatic submount of extant mount
> +	FS_CONTEXT_FOR_REMOUNT	-- Change an existing mount
> +
> +The mount context is created by calling vfs_new_fs_context(), vfs_sb_reconfig()
> +or vfs_dup_fs_context() and is destroyed with put_fs_context().  Note that the
> +structure is not refcounted.
> +
> +VFS, security and filesystem mount options are set individually with
> +vfs_parse_mount_option().  Options provided by the old mount(2) system call as
> +a page of data can be parsed with generic_monolithic_mount_data().
> +
> +When mounting, the filesystem is allowed to take data from any of the pointers
> +and attach it to the superblock (or whatever), provided it clears the pointer
> +in the mount context.
> +
> +The filesystem is also allowed to allocate resources and pin them with the
> +mount context.  For instance, NFS might pin the appropriate protocol version
> +module.
> +
> +
> +=================================
> +THE FILESYSTEM CONTEXT OPERATIONS
> +=================================
> +
> +The filesystem context points to a table of operations:
> +
> +	struct fs_context_operations {
> +		void (*free)(struct fs_context *fc);
> +		int (*dup)(struct fs_context *fc, struct fs_context *src_fc);
> +		int (*parse_option)(struct fs_context *fc, char *p);
> +		int (*monolithic_mount_data)(struct fs_context *fc, void *data);
> +		int (*validate)(struct fs_context *fc);
> +		int (*get_tree)(struct fs_context *fc);
> +	};
> +
> +These operations are invoked by the various stages of the mount procedure to
> +manage the filesystem context.  They are as follows:
> +
> + (*) void (*free)(struct fs_context *fc);
> +
> +     Called to clean up the filesystem-specific part of the filesystem context
> +     when the context is destroyed.  It should be aware that parts of the
> +     context may have been removed and NULL'd out by ->get_tree().
> +
> + (*) int (*dup)(struct fs_context *fc, struct fs_context *src_fc);
> +
> +     Called when a filesystem context has been duplicated to get any refs or
> +     copy any non-referenced resources held in the filesystem-specific part of
> +     the filesystem context.  An error may be returned to indicate failure to
> +     do this.
> +
> +     [!] Note that even if this fails, put_fs_context() will be called
> +	 immediately thereafter, so ->dup() *must* make the
> +	 filesystem-specific part safe for ->free().
> +
> + (*) int (*parse_option)(struct fs_context *fc, char *p);
> +
> +     Called when an option is to be added to the filesystem context.  p points
> +     to the option string, likely in "key[=val]" format.  VFS-specific options
> +     will have been weeded out and fc->sb_flags updated in the context.
> +     Security options will also have been weeded out and fc->security updated.
> +
> +     If successful, 0 should be returned and a negative error code otherwise.
> +     If an ambiguous error (such as -EINVAL) is returned, sb_cfg_error() or
> +     sb_cfg_inval() should be used to provide a string that provides more
> +     information.
> +
> + (*) int (*monolithic_mount_data)(struct fs_context *fc, void *data);
> +
> +     Called when the mount(2) system call is invoked to pass the entire data
> +     page in one go.  If this is expected to be just a list of "key[=val]"
> +     items separated by commas, then this may be set to NULL.
> +
> +     The return value is as for ->parse_option().
> +
> +     If the filesystem (eg. NFS) needs to examine the data first and then finds
> +     it's the standard key-val list then it may pass it off to
> +     generic_monolithic_mount_data().
> +
> + (*) int (*validate)(struct fs_context *fc);
> +
> +     Called when all the options have been applied and the mount is about to
> +     take place.  It is should check for inconsistencies from mount options and
> +     it is also allowed to do preliminary resource acquisition.  For instance,
> +     the core NFS module could load the NFS protocol module here.
> +
> +     Note that if fc->mount_type == FS_CONTEXT_FOR_REMOUNT, some of the options
> +     necessary for a new mount may not be set.
> +
> +     The return value is as for ->parse_option().
> +
> + (*) int (*get_tree)(struct fs_context *fc);
> +
> +     Called to get or create the mountable root and superblock, using the
> +     information stored in the filesystem context (remounts go
> +     via a different vector).  It may detach any resources it desires from the
> +     filesystem context and transfer them to the superblock it
> +     creates.
> +
> +     On success it should set fc->root to the mountable root.
> +
> +     In the case of an error, it should return a negative error code and
> +     consider invoking sb_cfg_inval() or sb_cfg_error().
> +
> +
> +=========================================
> +FILESYSTEM CONTEXT SECURITY
> +========================================
> +
> +The filesystem context contains a security points that the LSMs
> +can use for building up a security context for the superblock to be mounted.
> +There are a number of operations used by the new mount code for this purpose:
> +
> + (*) int security_fs_context_alloc(struct fs_context *fc,
> +				  struct super_block *src_sb);
> +
> +     Called to initialise fc->security (which is preset to NULL) and allocate
> +     any resources needed.  It should return 0 on success and a negative error
> +     code on failure.
> +
> +     src_sb is non-NULL in the case of a remount (FS_CONTEXT_FOR_REMOUNT) in
> +     which case it indicates the superblock to be remounted or in the case of a
> +     submount (FS_CONTEXT_FOR_SUBMOUNT) in which case it indicates the parent
> +     superblock.
> +
> + (*) int security_fs_context_dup(struct fs_context *fc,
> +				struct fs_context *src_mc);
> +
> +     Called to initialise fc->security (which is preset to NULL) and allocate
> +     any resources needed.  The original filesystem context is
> +     pointed to by src_mc and may be used for reference.  It should return 0 on
> +     success and a negative error code on failure.
> +
> + (*) void security_fs_context_free(struct fs_context *fc);
> +
> +     Called to clean up anything attached to fc->security.  Note that the
> +     contents may have been transferred to a superblock and the pointer NULL'd
> +     out during mount.
> +
> + (*) int security_fs_context_parse_option(struct fs_context *fc, char *opt);
> +
> +     Called for each mount option.  The mount options are in "key[=val]" form.
> +     An active LSM may reject one with an error, pass one over and return 0 or
> +     consume one and return 1.  If consumed, the option isn't passed on to the
> +     filesystem.
> +
> +     If it returns an error, more information can be returned with
> +     sb_cfg_inval() or sb_cfg_error().
> +
> + (*) int security_sb_get_tree(struct fs_context *fc);
> +
> +     Called during the mount procedure to verify that the specified superblock
> +     is allowed to be mounted and to transfer the security data there.
> +
> +     On success, it should return 0; otherwise it should return an error and
> +     perhaps call invalf() or errorf() to indicate the problem.  It should not
> +     return -ENOMEM as this should be taken care of in advance.
> +
> +     [NOTE] Should I add a security_fs_context_validate() operation so that the
> +     LSM has the opportunity to allocate stuff and check the options as a
> +     whole?
> +
> +
> +=================================
> +VFS FILESYSTEM CONTEXT OPERATIONS
> +=================================
> +
> +There are four operations for creating a filesystem context and
> +one for destroying a context:
> +
> + (*) struct fs_context *__vfs_new_fs_context(struct file_system_type *fs_type,
> +					     struct super_block *src_sb;
> +					     unsigned int ms_flags);
> +
> +     Create a filesystem context given a filesystem type pointer.
> +     This allocates the filesystem context, sets the flags,
> +     initialises the security and calls fs_type->init_fs_context() to initialise
> +     the filesystem context.
> +
> +     src_sb can be NULL or it may indicate a superblock that is going to be
> +     remounted (FS_CONTEXT_FOR_REMOUNT) or a superblock that is the parent of a
> +     submount (FS_CONTEXT_FOR_SUBMOUNT).  This superblock is provided as a
> +     source of namespace information.
> +
> + (*) struct fs_context *vfs_sb_reconfig(struct vfsmount *mnt,
> +					unsigned int ms_flags);
> +
> +     Create a filesystem context from the same filesystem as an
> +     extant mount and initialise the mount parameters from the superblock
> +     underlying that mount.  This is for use by remount.
> +
> + (*) struct fs_context *vfs_new_fs_context(const char *fs_name);
> +
> +     Create a filesystem context given a filesystem name.  It is assumed that
> +     the mount flags will be passed in as text options or set directly later.
> +     This is intended to be called from sys_mount() or sys_fsopen().  This
> +     copies current's namespaces to the context.
> +
> + (*) struct fs_context *vfs_dup_fs_context(struct fs_context *src_fc);
> +
> +     Duplicate a filesystem context, copying any options noted and duplicating
> +     or additionally referencing any resources held therein.  This is
> +     available for use where a filesystem has to get a mount within a mount,
> +     such as NFS4 does by internally mounting the root of the target server
> +     and then doing a private pathwalk to the target directory.
> +
> + (*) void put_fs_context(struct fs_context *fc);
> +
> +     Destroy a filesystem context, releasing any resources it holds.  This
> +     calls the ->free() operation.  This is intended to be called by anyone
> +     who created a filesystem context.
> +
> +     [!] filesystem contexts are not refcounted, so this causes unconditional
> +	 destruction.
> +
> +In all the above operations, apart from the put op, the return is a mount
> +context pointer or a negative error code.  No error string is saved as the
> +error string is only guaranteed as long as the file_system_type is pinned (and
> +thus the module).
> +
> +In the remaining operations, if an error occurs, a negative error code is
> +returned and, if not obvious, fc->error_msg may have been set to point to a
> +useful string.  This string should not be freed.
> +
> + (*) int vfs_get_tree(struct fs_context *fc);
> +
> +     Get or create the mountable root and superblock, using the parameters in
> +     the filesystem context to select/configure the superblock.  This invokes
> +     the ->validate() op and then the ->get_tree() op.
> +
> +     [NOTE] ->validate() can probably be rolled into ->get_tree() and
> +     ->remount_fs_fc().
> +
> + (*) struct vfsmount *vfs_kern_mount_fc(struct fs_context *fc);
> +
> +     Create a mount given the parameters in the specified filesystem context.
> +
> + (*) struct vfsmount *vfs_submount_fc(const struct dentry *mountpoint,
> +				      struct fs_context *fc);
> +
> +     Create a mount given a filesystem context and set MS_SUBMOUNT on it.  A
> +     wrapper around vfs_kern_mount_fc().  This is intended to be called from
> +     filesystems that have automount points (NFS, AFS, ...).
> +
> + (*) int vfs_parse_mount_option(struct fs_context *fc, char *data);
> +
> +     Supply a single mount option to the filesystem context.  The mount option
> +     should likely be in a "key[=val]" string form.  The option is first
> +     checked to see if it corresponds to a standard mount flag (in which case
> +     it is used to mark an MS_xxx flag and consumed) or a security option (in
> +     which case the LSM consumes it) before it is passed on to the filesystem.
> +
> + (*) int generic_monolithic_mount_data(struct fs_context *fc, void *data);
> +
> +     Parse a sys_mount() data page, assuming the form to be a text list
> +     consisting of key[=val] options separated by commas.  Each item in the
> +     list is passed to vfs_mount_option().  This is the default when the
> +     ->monolithic_mount_data() operation is NULL.
> diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
> new file mode 100644
> index 000000000000..429c40be2c9e
> --- /dev/null
> +++ b/include/linux/fs_context.h
> @@ -0,0 +1,72 @@
> +/* Filesystem superblock creation and reconfiguration context.
> + *
> + * Copyright (C) 2017 Red Hat, Inc. All Rights Reserved.
> + * Written by David Howells (dhowells@...hat.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;
> +
> +enum fs_context_purpose {
> +	FS_CONTEXT_FOR_NEW,	/* New superblock for direct mount */
> +	FS_CONTEXT_FOR_SUBMOUNT,	/* New superblock for automatic submount */
> +	FS_CONTEXT_FOR_REMOUNT,	/* Superblock reconfiguration for remount */
> +};
> +
> +/*
> + * Filesystem context as allocated and constructed by the ->init_fs_context()
> + * file_system_type operation.  The size of the object allocated is specified
> + * in struct file_system_type::fs_context_size and this must include sufficient
> + * space for the fs_context struct.
> + *
> + * 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;
> +	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			*device;	/* The device name or mount target */
> +	char			*subtype;	/* The subtype to set on the superblock */
> +	void			*security;	/* The LSM context */
> +	unsigned int		sb_flags;	/* The superblock flags (MS_*) */
> +	bool			sloppy;		/* Unrecognised options are okay */
> +	bool			silent;
> +	bool			degraded;	/* True if the context can't be reused */
> +	enum fs_context_purpose	purpose : 8;
> +};
> +
> +struct fs_context_operations {
> +	void (*free)(struct fs_context *fc);
> +	int (*dup)(struct fs_context *fc, struct fs_context *src_fc);
> +	int (*parse_option)(struct fs_context *fc, char *p);
> +	int (*monolithic_mount_data)(struct fs_context *fc, void *data);
> +	int (*validate)(struct fs_context *fc);
> +	int (*get_tree)(struct fs_context *fc);
> +};
> +
> +#endif /* _LINUX_FS_CONTEXT_H */
>
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