Date:   Wed, 10 May 2017 14:59:49 -0700
From:   Sargun Dhillon <sargun@...gun.me>
To:     David Howells <dhowells@...hat.com>
Cc:     mszeredi@...hat.com, viro@...iv.linux.org.uk, jlayton@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 07/14] Implement fsopen() to prepare for a mount

On Wed, May 10, 2017 at 9:19 AM, David Howells <dhowells@...hat.com> 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 reserved,
>                          int open_flags);
>
> where reserved should be -1 for the moment (it will be used to pass the
> namespace information in future) and open_flags can be 0 or O_CLOEXEC.
>
> For example:
>
>         mfd = fsopen("ext4", -1, O_CLOEXEC);
>         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
>         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 /");
>         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@...hat.com>
> ---
>
>  arch/x86/entry/syscalls/syscall_32.tbl |    1
>  arch/x86/entry/syscalls/syscall_64.tbl |    1
>  fs/Makefile                            |    2
>  fs/fsopen.c                            |  279 ++++++++++++++++++++++++++++++++
>  include/linux/syscalls.h               |    1
>  include/uapi/linux/magic.h             |    1
>  kernel/sys_ni.c                        |    3
>  7 files changed, 287 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 8f5142525866..b8fcf48b0400 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 \
> -               sb_config.o
> +               sb_config.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..a4e9d5a7ce2b
> --- /dev/null
> +++ b/fs/fsopen.c
> @@ -0,0 +1,279 @@
> +/* Filesystem access-by-fd.
> + *
> + * 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.
> + */
> +
> +#include <linux/sb_config.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 sb_config *sc = file->private_data;
> +
> +       file->private_data = NULL;
> +
> +       put_sb_config(sc);
> +       return 0;
> +}
> +
> +/*
> + * Read any error message back from the fd.  Will be prefixed by "e ".
> + */
> +static ssize_t fs_fs_read(struct file *file, char __user *_buf, size_t len, loff_t *pos)
> +{
> +       struct sb_config *sc = file->private_data;
> +       const char *msg;
> +       size_t mlen;
> +
> +       msg = READ_ONCE(sc->error_msg);
> +       if (!msg)
> +               return -ENODATA;
> +
> +       mlen = strlen(msg);
> +       if (mlen + 2 > len)
> +               return -ETOOSMALL;
> +       if (copy_to_user(_buf, "e ", 2) != 0 ||
> +           copy_to_user(_buf + 2, msg, mlen) != 0)
> +               return -EFAULT;
> +       return mlen + 2;
> +}
> +
> +/*
> + * Userspace writes configuration data to the fd and we parse it here.  For the
> + * moment, we assume a single option per write.  Each line written is of the form
> + *
> + *     <option_type><space><stuff...>
> + *
> + *     d /dev/sda1                             -- Device name
> + *     o noatime                               -- Option without value
> + *     o cell=grand.central.org                -- Option with value
> + *     r /                                     -- Dir within device to mount
> + */
> +static ssize_t fs_fs_write(struct file *file,
> +                          const char __user *_buf, size_t len, loff_t *pos)
> +{
> +       struct sb_config *sc = 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':
> +               break;
> +       default:
> +               return sb_cfg_inval(sc, "VFS: Unsupported write spec");
> +       }
> +       if (opt[1] != ' ')
> +               return sb_cfg_inval(sc, "VFS: Unsupported write spec");
> +
> +       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;
> +
> +       ret = -EBUSY;
> +       if (sc->mounted)
> +               goto err_unlock;
> +
> +       ret = -EINVAL;
> +       switch (opt[0]) {
> +       case 's':
> +               if (sc->device)
> +                       goto err_unlock;
> +               sc->device = data;
> +               data = NULL;
> +               break;
> +
> +       case 'o':
> +               ret = vfs_parse_mount_option(sc, data);
> +               if (ret < 0)
> +                       goto err_unlock;
> +               break;
> +
> +       default:
> +               goto err_unlock;
> +       }
> +
> +       ret = len;
> +err_unlock:
> +       inode_unlock(inode);
> +err_free:
> +       kfree(data);
> +       return ret;
> +}
> +
> +const struct file_operations fs_fs_fops = {
> +       .read           = fs_fs_read,
> +       .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 sb_config *sc)
> +{
> +       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 = sc;
> +       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_DEFINE3(fsopen, const char __user *, _fs_name, int, reserved,
> +               unsigned int, flags)
> +{
> +       struct sb_config *sc;
> +       struct file *file;
> +       const char *fs_name;
> +       int fd, ret;
> +
> +       if (flags & ~O_CLOEXEC || reserved != -1)
> +               return -EINVAL;
> +
> +       fs_name = strndup_user(_fs_name, PAGE_SIZE);
> +       if (IS_ERR(fs_name))
> +               return PTR_ERR(fs_name);
> +
> +       sc = vfs_new_sb_config(fs_name);
> +       kfree(fs_name);
> +       if (IS_ERR(sc))
> +               return PTR_ERR(sc);
> +
> +       ret = -ENOTSUPP;
> +       if (!sc->ops)
> +               goto err_sc;
> +
> +       file = create_fs_file(sc);
> +       if (IS_ERR(file)) {
> +               ret = PTR_ERR(file);
> +               goto err_sc;
> +       }
> +
> +       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_sc:
> +       put_sb_config(sc);
> +       return ret;
> +}
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 980c3c9b06f8..91ec8802ad5d 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -905,5 +905,6 @@ 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, int containerfd, unsigned int flags);
>
>  #endif
> diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
> index e230af2e6855..88ae83492f7c 100644
> --- a/include/uapi/linux/magic.h
> +++ b/include/uapi/linux/magic.h
> @@ -84,5 +84,6 @@
>  #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);
>

Instead of string based configuration, does it perhaps make sense to
pass in structured mount data? Something like:

enum mount_command_id {
    MOUNT_OPTION_STR,
    MOUNT_SET_USER_NS
};

struct mount_attr {
   __u64 command_id;
   union {
       char option_str[4095];
       char mount_source[PATH_MAX];
       struct {
           __u32 user_ns_fd
       }
   }
}

It seems a lot less error prone to me.

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