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Subject[PATCH] mnt: allow to add a mount into an existing group
Date
Now a shared group can be only inherited from a source mount.
This patch adds an ability to add a mount into an existing shared
group.

mount(source, target, NULL, MS_SET_GROUP, NULL)

mount() with the MS_SET_GROUP flag adds the "target" mount into a group
of the "source" mount. The calling process has to have the CAP_SYS_ADMIN
capability in namespaces of these mounts. The source and the target
mounts have to have the same super block.

This new functionality together with "mnt: Tuck mounts under others
instead of creating shadow/side mounts." allows CRIU to dump and restore
any set of mount namespaces.

Currently we have a lot of issues about dumping and restoring mount
namespaces. The bigest problem is that we can't construct mount trees
directly due to several reasons:
* groups can't be set, they can be only inherited
* file systems has to be mounted from the specified user namespaces
* the mount() syscall doesn't just create one mount -- the mount is
also propagated to all members of a parent group
* umount() doesn't detach mounts from all members of a group
(mounts with children are not umounted)
* mounts are propagated underneath of existing mounts
* mount() doesn't allow to make bind-mounts between two namespaces
* processes can have opened file descriptors to overmounted files

All these operations are non-trivial, making the task of restoring
a mount namespace practically unsolvable for reasonable time. The
proposed change allows to restore a mount namespace in a direct
manner, without any super complex logic.

Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrei Vagin <avagin@openvz.org>
---
fs/namespace.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++
include/uapi/linux/fs.h | 1 +
2 files changed, 54 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index b5b1259..df52fd4 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2301,6 +2301,57 @@ static inline int tree_contains_unbindable(struct mount *mnt)
return 0;
}

+static int do_set_group(struct path *path, const char *sibling_name)
+{
+ struct mount *sibling, *mnt;
+ struct path sibling_path;
+ int err;
+
+ if (!sibling_name || !*sibling_name)
+ return -EINVAL;
+
+ err = kern_path(sibling_name, LOOKUP_FOLLOW, &sibling_path);
+ if (err)
+ return err;
+
+ sibling = real_mount(sibling_path.mnt);
+ mnt = real_mount(path->mnt);
+
+ namespace_lock();
+
+ err = -EPERM;
+ if (!sibling->mnt_ns ||
+ !ns_capable(sibling->mnt_ns->user_ns, CAP_SYS_ADMIN))
+ goto out_unlock;
+
+ err = -EINVAL;
+ if (sibling->mnt.mnt_sb != mnt->mnt.mnt_sb)
+ goto out_unlock;
+
+ if (IS_MNT_SHARED(mnt) || IS_MNT_SLAVE(mnt))
+ goto out_unlock;
+
+ if (IS_MNT_SLAVE(sibling)) {
+ struct mount *m = sibling->mnt_master;
+
+ list_add(&mnt->mnt_slave, &m->mnt_slave_list);
+ mnt->mnt_master = m;
+ }
+
+ if (IS_MNT_SHARED(sibling)) {
+ mnt->mnt_group_id = sibling->mnt_group_id;
+ list_add(&mnt->mnt_share, &sibling->mnt_share);
+ set_mnt_shared(mnt);
+ }
+
+ err = 0;
+out_unlock:
+ namespace_unlock();
+
+ path_put(&sibling_path);
+ return err;
+}
+
static int do_move_mount(struct path *path, const char *old_name)
{
struct path old_path, parent_path;
@@ -2779,6 +2830,8 @@ long do_mount(const char *dev_name, const char __user *dir_name,
retval = do_change_type(&path, flags);
else if (flags & MS_MOVE)
retval = do_move_mount(&path, dev_name);
+ else if (flags & MS_SET_GROUP)
+ retval = do_set_group(&path, dev_name);
else
retval = do_new_mount(&path, type_page, flags, mnt_flags,
dev_name, data_page);
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 36da93f..6e6e37d 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -130,6 +130,7 @@ struct inodes_stat_t {
#define MS_I_VERSION (1<<23) /* Update inode I_version field */
#define MS_STRICTATIME (1<<24) /* Always perform atime updates */
#define MS_LAZYTIME (1<<25) /* Update the on-disk [acm]times lazily */
+#define MS_SET_GROUP (1<<26) /* Add a mount into a shared group */

/* These sb flags are internal to the kernel */
#define MS_NOREMOTELOCK (1<<27)
--
2.7.4
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