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SubjectRe: [PATCH 05/14] VFS: Implement fsopen() to prepare for a mount [ver #6]
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On Fri, 2017-10-06 at 16:49 +0100, David Howells wrote:
> Provide an fsopen() system call that starts the process of preparing to
> mount, 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, int open_flags,
> void *reserved3, void *reserved4,
> void *reserved5);
>
> where open_flags can be 0 or O_CLOEXEC and reserved* should all be NULL for
> the moment.
>
> For example:
>
> mfd = fsopen("ext4", O_CLOEXEC, NULL, NULL, NULL);
> write(mfd, "s /dev/sdb1"); // note I'm ignoring write's length arg
> write(mfd, "o noatime");
> write(mfd, "o acl");
> write(mfd, "o user_attr");
> write(mfd, "o iversion");
> write(mfd, "o ");
> write(mfd, "r /my/container"); // root inside the fs
> write(mfd, "x create"); // create the superblock
> fsmount(mfd, container_fd, "/mnt", AT_NO_FOLLOW);
>
> mfd = fsopen("afs", -1);
> write(mfd, "s %grand.central.org:root.cell");
> write(mfd, "o cell=grand.central.org");
> write(mfd, "r /");
> write(mfd, "x create");
> fsmount(mfd, AT_FDCWD, "/mnt", 0);
>
> 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 write() 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.
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
>
> arch/x86/entry/syscalls/syscall_32.tbl | 1
> arch/x86/entry/syscalls/syscall_64.tbl | 1
> fs/Makefile | 2
> fs/fsopen.c | 273 ++++++++++++++++++++++++++++++++
> include/linux/fs_context.h | 1
> include/linux/syscalls.h | 2
> include/uapi/linux/magic.h | 1
> kernel/sys_ni.c | 3
> 8 files changed, 283 insertions(+), 1 deletion(-)
> create mode 100644 fs/fsopen.c
>
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 448ac2161112..9bf8d4c62f85 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -391,3 +391,4 @@
> 382 i386 pkey_free sys_pkey_free
> 383 i386 statx sys_statx
> 384 i386 arch_prctl sys_arch_prctl compat_sys_arch_prctl
> +385 i386 fsopen sys_fsopen
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index 5aef183e2f85..9b198c5fc412 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -339,6 +339,7 @@
> 330 common pkey_alloc sys_pkey_alloc
> 331 common pkey_free sys_pkey_free
> 332 common statx sys_statx
> +333 common fsopen sys_fsopen
>
> #
> # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/fs/Makefile b/fs/Makefile
> index ffe728cc15e1..c42d1d9351a6 100644
> --- a/fs/Makefile
> +++ b/fs/Makefile
> @@ -12,7 +12,7 @@ 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 \
> stack.o fs_struct.o statfs.o fs_pin.o nsfs.o \
> - fs_context.o
> + fs_context.o fsopen.o
>
> ifeq ($(CONFIG_BLOCK),y)
> obj-y += buffer.o block_dev.o direct-io.o mpage.o
> diff --git a/fs/fsopen.c b/fs/fsopen.c
> new file mode 100644
> index 000000000000..6ca7e1979273
> --- /dev/null
> +++ b/fs/fsopen.c
> @@ -0,0 +1,273 @@
> +/* 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/mount.h>
> +#include <linux/slab.h>
> +#include <linux/uaccess.h>
> +#include <linux/file.h>
> +#include <linux/magic.h>
> +#include <linux/syscalls.h>
> +
> +static struct vfsmount *fs_fs_mnt __read_mostly;
> +
> +static int fs_fs_release(struct inode *inode, struct file *file)
> +{
> + struct fs_context *fc = file->private_data;
> +
> + file->private_data = NULL;
> +
> + put_fs_context(fc);
> + return 0;
> +}
> +
> +/*
> + * Userspace writes configuration data and commands to the fd and we parse it
> + * here. For the moment, we assume a single option or command per write. Each
> + * line written is of the form
> + *
> + * <option_type><space><stuff...>
> + *
> + * d /dev/sda1 -- Device name

nit: I think you mean "s /dev/sda1", according to sample program.

> + * o noatime -- Option without value
> + * o cell=grand.central.org -- Option with value
> + * r / -- Dir within device to mount
> + * x create -- Create a superblock
> + */
> +static ssize_t fs_fs_write(struct file *file,
> + const char __user *_buf, size_t len, loff_t *pos)
> +{
> + struct fs_context *fc = file->private_data;
> + struct inode *inode = file_inode(file);
> + char opt[2], *data;
> + ssize_t ret;
> +
> + if (len < 3 || len > 4095)
> + return -EINVAL;
> +
> + if (copy_from_user(opt, _buf, 2) != 0)
> + return -EFAULT;
> + switch (opt[0]) {
> + case 's':
> + case 'o':
> + case 'x':
> + break;
> + default:
> + goto err_bad_cmd;
> + }
> + if (opt[1] != ' ')
> + goto err_bad_cmd;
> +
> + data = memdup_user_nul(_buf + 2, len - 2);
> + if (IS_ERR(data))
> + return PTR_ERR(data);
> +
> + /* From this point onwards we need to lock the fd against someone
> + * trying to mount it.
> + */
> + ret = inode_lock_killable(inode);
> + if (ret < 0)
> + goto err_free;
> +

^^^
Should that be interruptible instead of killable? Allowing someone to ^c
a stuck mount program without a SIGKILL seems reasonable.

As a general design goal, it'd be nice to really try to keep as much of
this as responsive to signals as possible. Mounting and unmounting are
often something that can easily end up stuck.

> + ret = -EINVAL;
> + switch (opt[0]) {
> + case 's':
> + ret = vfs_set_fs_source(fc, data, len - 2);
> + if (ret < 0)
> + goto err_unlock;
> + data = NULL;
> + break;
> +
> + case 'o':
> + ret = vfs_parse_mount_option(fc, data);
> + if (ret < 0)
> + goto err_unlock;
> + break;
> +
> + case 'x':
> + if (strcmp(data, "create") == 0) {
> + ret = vfs_get_tree(fc);
> + } else {
> + ret = -EOPNOTSUPP;
> + }
> + if (ret < 0)
> + goto err_unlock;
> + break;
> +
> + default:
> + goto err_unlock;
> + }
> +
> + ret = len;
> +err_unlock:
> + inode_unlock(inode);
> +err_free:
> + kfree(data);
> + return ret;
> +err_bad_cmd:
> + return -EINVAL;
> +}
> +
> +const struct file_operations fs_fs_fops = {
> + .write = fs_fs_write,
> + .release = fs_fs_release,
> + .llseek = no_llseek,
> +};
> +
> +/*
> + * Indicate the name we want to display the filesystem file as.
> + */
> +static char *fs_fs_dname(struct dentry *dentry, char *buffer, int buflen)
> +{
> + return dynamic_dname(dentry, buffer, buflen, "fs:[%lu]",
> + d_inode(dentry)->i_ino);
> +}
> +
> +static const struct dentry_operations fs_fs_dentry_operations = {
> + .d_dname = fs_fs_dname,
> +};
> +
> +/*
> + * Create a file that can be used to configure a new mount.
> + */
> +static struct file *create_fs_file(struct fs_context *fc)
> +{
> + struct inode *inode;
> + struct file *f;
> + struct path path;
> + int ret;
> +
> + inode = alloc_anon_inode(fs_fs_mnt->mnt_sb);
> + if (!inode)
> + return ERR_PTR(-ENFILE);
> + inode->i_fop = &fs_fs_fops;
> +
> + ret = -ENOMEM;
> + path.dentry = d_alloc_pseudo(fs_fs_mnt->mnt_sb, &empty_name);
> + if (!path.dentry)
> + goto err_inode;
> + path.mnt = mntget(fs_fs_mnt);
> +
> + d_instantiate(path.dentry, inode);
> +
> + f = alloc_file(&path, FMODE_READ | FMODE_WRITE, &fs_fs_fops);
> + if (IS_ERR(f)) {
> + ret = PTR_ERR(f);
> + goto err_file;
> + }
> +
> + f->private_data = fc;
> + return f;
> +
> +err_file:
> + path_put(&path);
> + return ERR_PTR(ret);
> +
> +err_inode:
> + iput(inode);
> + return ERR_PTR(ret);
> +}
> +
> + const struct super_operations fs_fs_ops = {
> + .drop_inode = generic_delete_inode,
> + .destroy_inode = free_inode_nonrcu,
> + .statfs = simple_statfs,
> +};
> +
> +static struct dentry *fs_fs_mount(struct file_system_type *fs_type,
> + int flags, const char *dev_name,
> + void *data)
> +{
> + return mount_pseudo(fs_type, "fs_fs:", &fs_fs_ops,
> + &fs_fs_dentry_operations, FS_FS_MAGIC);
> +}
> +
> +static struct file_system_type fs_fs_type = {
> + .name = "fs_fs",
> + .mount = fs_fs_mount,
> + .kill_sb = kill_anon_super,
> +};
> +
> +static int __init init_fs_fs(void)
> +{
> + int ret;
> +
> + ret = register_filesystem(&fs_fs_type);
> + if (ret < 0)
> + panic("Cannot register fs_fs\n");
> +
> + fs_fs_mnt = kern_mount(&fs_fs_type);
> + if (IS_ERR(fs_fs_mnt))
> + panic("Cannot mount fs_fs: %ld\n", PTR_ERR(fs_fs_mnt));
> + return 0;
> +}
> +
> +fs_initcall(init_fs_fs);
> +
> +/*
> + * 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_DEFINE5(fsopen, const char __user *, _fs_name, unsigned int, flags,
> + void *, reserved3, void *, reserved4, void *, reserved5)
> +{
> + struct file_system_type *fs_type;
> + struct fs_context *fc;
> + struct file *file;
> + const char *fs_name;
> + int fd, ret;
> +
> + if (flags & ~O_CLOEXEC || reserved3 || reserved4 || reserved5)
> + 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 = vfs_new_fs_context(fs_type, NULL, 0, FS_CONTEXT_FOR_USER_MOUNT);
> + put_filesystem(fs_type);
> + if (IS_ERR(fc))
> + return PTR_ERR(fc);
> +
> + ret = -ENOTSUPP;
> + if (!fc->ops)
> + goto err_fc;
> +
> + file = create_fs_file(fc);
> + if (IS_ERR(file)) {
> + ret = PTR_ERR(file);
> + goto err_fc;
> + }
> +
> + ret = get_unused_fd_flags(flags & O_CLOEXEC);
> + if (ret < 0)
> + goto err_file;
> +
> + fd = ret;
> + fd_install(fd, file);
> + return fd;
> +
> +err_file:
> + fput(file);
> + return ret;
> +
> +err_fc:
> + put_fs_context(fc);
> + return ret;
> +}
> diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
> index 8af6ff0e869e..3244b231ede0 100644
> --- a/include/linux/fs_context.h
> +++ b/include/linux/fs_context.h
> @@ -101,4 +101,5 @@ extern int vfs_get_super(struct fs_context *fc,
> int (*fill_super)(struct super_block *sb,
> struct fs_context *fc));
>
> +extern const struct file_operations fs_fs_fops;
> #endif /* _LINUX_FS_CONTEXT_H */
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index a78186d826d7..7cd1b65a4152 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -940,5 +940,7 @@ asmlinkage long sys_pkey_alloc(unsigned long flags, unsigned long init_val);
> asmlinkage long sys_pkey_free(int pkey);
> asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
> unsigned mask, struct statx __user *buffer);
> +asmlinkage long sys_fsopen(const char *fs_name, unsigned int flags,
> + void *reserved3, void *reserved4, void *reserved5);
>
> #endif
> diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
> index e439565df838..722bf42f9564 100644
> --- a/include/uapi/linux/magic.h
> +++ b/include/uapi/linux/magic.h
> @@ -87,5 +87,6 @@
> #define UDF_SUPER_MAGIC 0x15013346
> #define BALLOON_KVM_MAGIC 0x13661366
> #define ZSMALLOC_MAGIC 0x58295829
> +#define FS_FS_MAGIC 0x66736673
>
> #endif /* __LINUX_MAGIC_H__ */
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 8acef8576ce9..de1dc63e7e47 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -258,3 +258,6 @@ cond_syscall(sys_membarrier);
> cond_syscall(sys_pkey_mprotect);
> cond_syscall(sys_pkey_alloc);
> cond_syscall(sys_pkey_free);
> +
> +/* fd-based mount */
> +cond_syscall(sys_fsopen);
>

--
Jeff Layton <jlayton@redhat.com>

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