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Commit 24dcb3d9 authored by David Howells's avatar David Howells Committed by Al Viro
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vfs: syscall: Add fsopen() to prepare for superblock creation 



Provide an fsopen() system call that starts the process of preparing to
create a superblock that will then be mountable, using an fd as a context
handle.  fsopen() is given the name of the filesystem that will be used:

	int mfd = fsopen(const char *fsname, unsigned int flags);

where flags can be 0 or FSOPEN_CLOEXEC.

For example:

	sfd = fsopen("ext4", FSOPEN_CLOEXEC);
	fsconfig(sfd, FSCONFIG_SET_PATH, "source", "/dev/sda1", AT_FDCWD);
	fsconfig(sfd, FSCONFIG_SET_FLAG, "noatime", NULL, 0);
	fsconfig(sfd, FSCONFIG_SET_FLAG, "acl", NULL, 0);
	fsconfig(sfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
	fsconfig(sfd, FSCONFIG_SET_STRING, "sb", "1", 0);
	fsconfig(sfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
	fsinfo(sfd, NULL, ...); // query new superblock attributes
	mfd = fsmount(sfd, FSMOUNT_CLOEXEC, MS_RELATIME);
	move_mount(mfd, "", sfd, AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);

	sfd = fsopen("afs", -1);
	fsconfig(fd, FSCONFIG_SET_STRING, "source",
		 "#grand.central.org:root.cell", 0);
	fsconfig(fd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
	mfd = fsmount(sfd, 0, MS_NODEV);
	move_mount(mfd, "", sfd, AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);

If an error is reported at any step, an error message may be available to be
read() back (ENODATA will be reported if there isn't an error available) in
the form:

	"e <subsys>:<problem>"
	"e SELinux:Mount on mountpoint not permitted"

Once fsmount() has been called, further fsconfig() calls will incur EBUSY,
even if the fsmount() fails.  read() is still possible to retrieve error
information.

The fsopen() syscall creates a mount context and hangs it of the fd that it
returns.

Netlink is not used because it is optional and would make the core VFS
dependent on the networking layer and also potentially add network
namespace issues.

Note that, for the moment, the caller must have SYS_CAP_ADMIN to use
fsopen().

Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
cc: linux-api@vger.kernel.org
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent dadd2299

arch/x86/entry/syscalls/syscall_32.tbl

+2 −1
Original line number Diff line number Diff line
@@ -400,7 +400,8 @@ 
386	i386	rseq			sys_rseq			__ia32_sys_rseq

387	i386	open_tree		sys_open_tree			__ia32_sys_open_tree

388	i386	move_mount		sys_move_mount			__ia32_sys_move_mount

# don't use numbers 389 through 392, add new calls at the end

389	i386	fsopen			sys_fsopen			__ia32_sys_fsopen

# don't use numbers 390 through 392, add new calls at the end

393	i386	semget			sys_semget    			__ia32_sys_semget

394	i386	semctl			sys_semctl    			__ia32_compat_sys_semctl

395	i386	shmget			sys_shmget    			__ia32_sys_shmget

arch/x86/entry/syscalls/syscall_64.tbl

+1 −0
Original line number Diff line number Diff line
@@ -345,6 +345,7 @@ 
334	common	rseq			__x64_sys_rseq

335	common	open_tree		__x64_sys_open_tree

336	common	move_mount		__x64_sys_move_mount

337	common	fsopen			__x64_sys_fsopen

# don't use numbers 387 through 423, add new calls after the last

# 'common' entry

424	common	pidfd_send_signal	__x64_sys_pidfd_send_signal

fs/Makefile

+1 −1
Original line number Diff line number Diff line
@@ -13,7 +13,7 @@ obj-y := open.o read_write.o file_table.o super.o \ 
		seq_file.o xattr.o libfs.o fs-writeback.o \

		pnode.o splice.o sync.o utimes.o d_path.o \

		stack.o fs_struct.o statfs.o fs_pin.o nsfs.o \

		fs_types.o fs_context.o fs_parser.o

		fs_types.o fs_context.o fs_parser.o fsopen.o



ifeq ($(CONFIG_BLOCK),y)

obj-y +=	buffer.o block_dev.o direct-io.o mpage.o

fs/fs_context.c

+4 −0
Original line number Diff line number Diff line
@@ -271,6 +271,8 @@ static struct fs_context *alloc_fs_context(struct file_system_type *fs_type, 
	fc->cred	= get_current_cred();

	fc->net_ns	= get_net(current->nsproxy->net_ns);



	mutex_init(&fc->uapi_mutex);



	switch (purpose) {

	case FS_CONTEXT_FOR_MOUNT:

		fc->user_ns = get_user_ns(fc->cred->user_ns);
@@ -353,6 +355,8 @@ struct fs_context *vfs_dup_fs_context(struct fs_context *src_fc) 
	if (!fc)

		return ERR_PTR(-ENOMEM);



	mutex_init(&fc->uapi_mutex);



	fc->fs_private	= NULL;

	fc->s_fs_info	= NULL;

	fc->source	= NULL;

fs/fsopen.c

0 → 100644
+88 −0
Original line number Diff line number Diff line 
/* Filesystem access-by-fd.

 *

 * Copyright (C) 2017 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.

 */



#include <linux/fs_context.h>

#include <linux/slab.h>

#include <linux/uaccess.h>

#include <linux/syscalls.h>

#include <linux/security.h>

#include <linux/anon_inodes.h>

#include <linux/namei.h>

#include <linux/file.h>

#include <uapi/linux/mount.h>

#include "mount.h"



static int fscontext_release(struct inode *inode, struct file *file)

{

	struct fs_context *fc = file->private_data;



	if (fc) {

		file->private_data = NULL;

		put_fs_context(fc);

	}

	return 0;

}



const struct file_operations fscontext_fops = {

	.release	= fscontext_release,

	.llseek		= no_llseek,

};



/*

 * Attach a filesystem context to a file and an fd.

 */

static int fscontext_create_fd(struct fs_context *fc, unsigned int o_flags)

{

	int fd;



	fd = anon_inode_getfd("fscontext", &fscontext_fops, fc,

			      O_RDWR | o_flags);

	if (fd < 0)

		put_fs_context(fc);

	return fd;

}



/*

 * Open a filesystem by name so that it can be configured for mounting.

 *

 * We are allowed to specify a container in which the filesystem will be

 * opened, thereby indicating which namespaces will be used (notably, which

 * network namespace will be used for network filesystems).

 */

SYSCALL_DEFINE2(fsopen, const char __user *, _fs_name, unsigned int, flags)

{

	struct file_system_type *fs_type;

	struct fs_context *fc;

	const char *fs_name;



	if (!ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN))

		return -EPERM;



	if (flags & ~FSOPEN_CLOEXEC)

		return -EINVAL;



	fs_name = strndup_user(_fs_name, PAGE_SIZE);

	if (IS_ERR(fs_name))

		return PTR_ERR(fs_name);



	fs_type = get_fs_type(fs_name);

	kfree(fs_name);

	if (!fs_type)

		return -ENODEV;



	fc = fs_context_for_mount(fs_type, 0);

	put_filesystem(fs_type);

	if (IS_ERR(fc))

		return PTR_ERR(fc);



	fc->phase = FS_CONTEXT_CREATE_PARAMS;

	return fscontext_create_fd(fc, flags & FSOPEN_CLOEXEC ? O_CLOEXEC : 0);

}