From: Alan Jenkins <alan.christopher.jenkins@gmail.com>
To: David Howells <dhowells@redhat.com>, viro@zeniv.linux.org.uk
Cc: linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org,
torvalds@linux-foundation.org, ebiederm@xmission.com,
linux-security-module@vger.kernel.org
Subject: Re: [PATCH 02/10] vfs: syscall: Add move_mount(2) to move mounts around
Date: Wed, 20 Feb 2019 12:32:00 +0000 [thread overview]
Message-ID: <57a6b419-1541-6f97-6810-d0d376580def@gmail.com> (raw)
In-Reply-To: <155059611887.17079.12991580316407924257.stgit@warthog.procyon.org.uk>
On 19/02/2019 17:08, David Howells wrote:
> Add a move_mount() system call that will move a mount from one place to
> another and, in the next commit, allow to attach an unattached mount tree.
>
> The new system call looks like the following:
>
> int move_mount(int from_dfd, const char *from_path,
> int to_dfd, const char *to_path,
> unsigned int flags);
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: linux-api@vger.kernel.org
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
>
> arch/x86/entry/syscalls/syscall_32.tbl | 1
> arch/x86/entry/syscalls/syscall_64.tbl | 1
> fs/namespace.c | 126 ++++++++++++++++++++++++--------
> include/linux/lsm_hooks.h | 6 ++
> include/linux/security.h | 7 ++
> include/linux/syscalls.h | 3 +
> include/uapi/linux/mount.h | 11 +++
> security/security.c | 5 +
> 8 files changed, 129 insertions(+), 31 deletions(-)
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 112d46f26fc3..f10122028a11 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2537,72 +2537,81 @@ static inline int tree_contains_unbindable(struct mount *mnt)
> return 0;
> }
>
> -static int do_move_mount(struct path *path, const char *old_name)
> +static int do_move_mount(struct path *old_path, struct path *new_path)
> {
> - struct path old_path, parent_path;
> + struct path parent_path = {.mnt = NULL, .dentry = NULL};
> struct mount *p;
> struct mount *old;
> struct mountpoint *mp;
> int err;
> - if (!old_name || !*old_name)
> - return -EINVAL;
> - err = kern_path(old_name, LOOKUP_FOLLOW, &old_path);
> - if (err)
> - return err;
>
> - mp = lock_mount(path);
> - err = PTR_ERR(mp);
> + mp = lock_mount(new_path);
> if (IS_ERR(mp))
> - goto out;
> + return PTR_ERR(mp);
>
> - old = real_mount(old_path.mnt);
> - p = real_mount(path->mnt);
> + old = real_mount(old_path->mnt);
> + p = real_mount(new_path->mnt);
>
> err = -EINVAL;
> if (!check_mnt(p) || !check_mnt(old))
> - goto out1;
> + goto out;
>
> - if (old->mnt.mnt_flags & MNT_LOCKED)
> - goto out1;
> + if (!mnt_has_parent(old))
> + goto out;
>
> - err = -EINVAL;
> - if (old_path.dentry != old_path.mnt->mnt_root)
> - goto out1;
> + if (old->mnt.mnt_flags & MNT_LOCKED)
> + goto out;
>
> - if (!mnt_has_parent(old))
> - goto out1;
> + if (old_path->dentry != old_path->mnt->mnt_root)
> + goto out;
>
> - if (d_is_dir(path->dentry) !=
> - d_is_dir(old_path.dentry))
> - goto out1;
> + if (d_is_dir(new_path->dentry) !=
> + d_is_dir(old_path->dentry))
> + goto out;
> /*
> * Don't move a mount residing in a shared parent.
> */
> if (IS_MNT_SHARED(old->mnt_parent))
> - goto out1;
> + goto out;
> /*
> * Don't move a mount tree containing unbindable mounts to a destination
> * mount which is shared.
> */
> if (IS_MNT_SHARED(p) && tree_contains_unbindable(old))
> - goto out1;
> + goto out;
> err = -ELOOP;
> for (; mnt_has_parent(p); p = p->mnt_parent)
> if (p == old)
> - goto out1;
> + goto out;
>
> - err = attach_recursive_mnt(old, real_mount(path->mnt), mp, &parent_path);
> + err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp,
> + &parent_path);
> if (err)
> - goto out1;
> + goto out;
>
> /* if the mount is moved, it should no longer be expire
> * automatically */
> list_del_init(&old->mnt_expire);
> -out1:
> - unlock_mount(mp);
> out:
> + unlock_mount(mp);
> if (!err)
> path_put(&parent_path);
> + return err;
> +}
> +
> +static int do_move_mount_old(struct path *path, const char *old_name)
> +{
> + struct path old_path;
> + int err;
> +
> + if (!old_name || !*old_name)
> + return -EINVAL;
> +
> + err = kern_path(old_name, LOOKUP_FOLLOW, &old_path);
> + if (err)
> + return err;
> +
> + err = do_move_mount(&old_path, path);
> path_put(&old_path);
> return err;
> }
> @@ -3050,7 +3059,7 @@ long do_mount(const char *dev_name, const char __user *dir_name,
> else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
> retval = do_change_type(&path, flags);
> else if (flags & MS_MOVE)
> - retval = do_move_mount(&path, dev_name);
> + retval = do_move_mount_old(&path, dev_name);
> else
> retval = do_new_mount(&path, type_page, sb_flags, mnt_flags,
> dev_name, data_page);
> @@ -3278,6 +3287,61 @@ SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
> return ksys_mount(dev_name, dir_name, type, flags, data);
> }
>
> +/*
> + * Move a mount from one place to another.
> + *
> + * Note the flags value is a combination of MOVE_MOUNT_* flags.
> + */
> +SYSCALL_DEFINE5(move_mount,
> + int, from_dfd, const char *, from_pathname,
> + int, to_dfd, const char *, to_pathname,
> + unsigned int, flags)
> +{
> + struct path from_path, to_path;
> + unsigned int lflags;
> + int ret = 0;
> +
> + if (!may_mount())
> + return -EPERM;
> +
> + if (flags & ~MOVE_MOUNT__MASK)
> + return -EINVAL;
> +
> + /* If someone gives a pathname, they aren't permitted to move
> + * from an fd that requires unmount as we can't get at the flag
> + * to clear it afterwards.
> + */
Comment is incorrect.
* FMODE_NEED_UNMOUNT is never cleared.
* Technically I don't see anything preventing them giving a pathname,
but it needs to be "." or equivalent. Otherwise it will fail the
"!attached" check in the next patch.
* The only argument I remember for preventing this, was that it might
confuse users (not the kernel). If you are allowed to move from a
sub-mount, then in certain programming styles - like my shell script
test cases - you might accidentally close the original file too early.
Then you won't be able to do move_mount() from the tree, because the
tree was unmounted ("dissolved") when you closed it.
I think the description in the previous patch, for open_tree(), makes it
clear though. "The detached tree will be dissolved on the final close of
obtained file".
If there is a good reason, I expect we can simply remove the "!attached"
part of the check. If the constraint is generating more confusion than
the added flexibility, I think that would be a good reason :-).
> + lflags = 0;
> + if (flags & MOVE_MOUNT_F_SYMLINKS) lflags |= LOOKUP_FOLLOW;
> + if (flags & MOVE_MOUNT_F_AUTOMOUNTS) lflags |= LOOKUP_AUTOMOUNT;
> + if (flags & MOVE_MOUNT_F_EMPTY_PATH) lflags |= LOOKUP_EMPTY;
> +
> + ret = user_path_at(from_dfd, from_pathname, lflags, &from_path);
> + if (ret < 0)
> + return ret;
> +
> + lflags = 0;
> + if (flags & MOVE_MOUNT_T_SYMLINKS) lflags |= LOOKUP_FOLLOW;
> + if (flags & MOVE_MOUNT_T_AUTOMOUNTS) lflags |= LOOKUP_AUTOMOUNT;
> + if (flags & MOVE_MOUNT_T_EMPTY_PATH) lflags |= LOOKUP_EMPTY;
> +
> + ret = user_path_at(to_dfd, to_pathname, lflags, &to_path);
> + if (ret < 0)
> + goto out_from;
> +
> + ret = security_move_mount(&from_path, &to_path);
> + if (ret < 0)
> + goto out_to;
> +
> + ret = do_move_mount(&from_path, &to_path);
> +
> +out_to:
> + path_put(&to_path);
> +out_from:
> + path_put(&from_path);
> + return ret;
> +}
> +
> /*
> * Return true if path is reachable from root
> *
>
next prev parent reply other threads:[~2019-02-20 12:32 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-19 17:08 [PATCH 00/10] VFS: Provide new mount UAPI David Howells
2019-02-19 17:08 ` [PATCH 01/10] vfs: syscall: Add open_tree(2) to reference or clone a mount David Howells
2019-02-19 17:08 ` [PATCH 02/10] vfs: syscall: Add move_mount(2) to move mounts around David Howells
2019-02-20 12:32 ` Alan Jenkins [this message]
2019-02-20 12:41 ` Alan Jenkins
2019-02-20 16:23 ` Jann Horn
2019-07-08 12:02 ` Tetsuo Handa
2019-07-08 13:18 ` Al Viro
2019-07-08 17:12 ` Eric W. Biederman
2019-07-08 18:01 ` Al Viro
2019-07-08 18:13 ` Al Viro
2019-07-08 20:21 ` Al Viro
2019-07-09 0:13 ` Eric W. Biederman
2019-07-09 10:51 ` Tetsuo Handa
2019-07-22 10:12 ` Tetsuo Handa
2019-07-23 4:16 ` John Johansen
2019-07-23 13:45 ` Tetsuo Handa
2019-08-06 10:43 ` Tetsuo Handa
2019-08-22 3:51 ` [RFC][PATCH] fix d_absolute_path() interplay with fsmount() Al Viro
2019-08-30 10:11 ` Tetsuo Handa
2019-07-23 21:45 ` [PATCH 02/10] vfs: syscall: Add move_mount(2) to move mounts around James Morris
2019-07-23 23:30 ` Al Viro
2019-02-19 17:08 ` [PATCH 03/10] teach move_mount(2) to work with OPEN_TREE_CLONE David Howells
2019-02-20 18:59 ` Alan Jenkins
2019-02-26 17:45 ` Alan Jenkins
2019-02-19 17:08 ` [PATCH 04/10] Make anon_inodes unconditional David Howells
2019-02-19 17:09 ` [PATCH 05/10] vfs: syscall: Add fsopen() to prepare for superblock creation David Howells
2019-02-19 17:09 ` [PATCH 06/10] vfs: Implement logging through fs_context David Howells
2019-02-19 17:09 ` [PATCH 07/10] vfs: syscall: Add fsconfig() for configuring and managing a context David Howells
2019-02-19 17:09 ` [PATCH 08/10] vfs: syscall: Add fsmount() to create a mount for a superblock David Howells
2019-02-19 17:09 ` [PATCH 09/10] vfs: syscall: Add fspick() to select a superblock for reconfiguration David Howells
2019-02-19 17:09 ` [PATCH 10/10] vfs: Add a sample program for the new mount API David Howells
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