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change the locking order for namespace_sem
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Have it nested inside ->i_mutex.  Instead of using follow_down()
under namespace_sem, followed by grabbing i_mutex and checking that
mountpoint to be is not dead, do the following:
	grab i_mutex
	check that it's not dead
	grab namespace_sem
	see if anything is mounted there
	if not, we've won
	otherwise
		drop locks
		put_path on what we had
		replace with what's mounted
		retry everything with new mountpoint to be

New helper (lock_mount()) does that.  do_add_mount(), do_move_mount(),
do_loopback() and pivot_root() switched to it; in case of the last
two that eliminates a race we used to have - original code didn't
do follow_down().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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Al Viro committed Mar 18, 2011
1 parent 27cb157 commit b12cea9
Showing 1 changed file with 73 additions and 60 deletions.
133 changes: 73 additions & 60 deletions fs/namespace.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1663,26 +1663,46 @@ static int attach_recursive_mnt(struct vfsmount *source_mnt,
return err;
}

static int lock_mount(struct path *path)
{
struct vfsmount *mnt;
retry:
mutex_lock(&path->dentry->d_inode->i_mutex);
if (unlikely(cant_mount(path->dentry))) {
mutex_unlock(&path->dentry->d_inode->i_mutex);
return -ENOENT;
}
down_write(&namespace_sem);
mnt = lookup_mnt(path);
if (likely(!mnt))
return 0;
up_write(&namespace_sem);
mutex_unlock(&path->dentry->d_inode->i_mutex);
path_put(path);
path->mnt = mnt;
path->dentry = dget(mnt->mnt_root);
goto retry;
}

static void unlock_mount(struct path *path)
{
up_write(&namespace_sem);
mutex_unlock(&path->dentry->d_inode->i_mutex);
}

static int graft_tree(struct vfsmount *mnt, struct path *path)
{
int err;
if (mnt->mnt_sb->s_flags & MS_NOUSER)
return -EINVAL;

if (S_ISDIR(path->dentry->d_inode->i_mode) !=
S_ISDIR(mnt->mnt_root->d_inode->i_mode))
return -ENOTDIR;

err = -ENOENT;
mutex_lock(&path->dentry->d_inode->i_mutex);
if (cant_mount(path->dentry))
goto out_unlock;
if (d_unlinked(path->dentry))
return -ENOENT;

if (!d_unlinked(path->dentry))
err = attach_recursive_mnt(mnt, path, NULL);
out_unlock:
mutex_unlock(&path->dentry->d_inode->i_mutex);
return err;
return attach_recursive_mnt(mnt, path, NULL);
}

/*
Expand Down Expand Up @@ -1745,6 +1765,7 @@ static int do_change_type(struct path *path, int flag)
static int do_loopback(struct path *path, char *old_name,
int recurse)
{
LIST_HEAD(umount_list);
struct path old_path;
struct vfsmount *mnt = NULL;
int err = mount_is_safe(path);
Expand All @@ -1756,13 +1777,16 @@ static int do_loopback(struct path *path, char *old_name,
if (err)
return err;

down_write(&namespace_sem);
err = lock_mount(path);
if (err)
goto out;

err = -EINVAL;
if (IS_MNT_UNBINDABLE(old_path.mnt))
goto out;
goto out2;

if (!check_mnt(path->mnt) || !check_mnt(old_path.mnt))
goto out;
goto out2;

err = -ENOMEM;
if (recurse)
Expand All @@ -1771,20 +1795,18 @@ static int do_loopback(struct path *path, char *old_name,
mnt = clone_mnt(old_path.mnt, old_path.dentry, 0);

if (!mnt)
goto out;
goto out2;

err = graft_tree(mnt, path);
if (err) {
LIST_HEAD(umount_list);

br_write_lock(vfsmount_lock);
umount_tree(mnt, 0, &umount_list);
br_write_unlock(vfsmount_lock);
release_mounts(&umount_list);
}

out2:
unlock_mount(path);
release_mounts(&umount_list);
out:
up_write(&namespace_sem);
path_put(&old_path);
return err;
}
Expand Down Expand Up @@ -1873,18 +1895,12 @@ static int do_move_mount(struct path *path, char *old_name)
if (err)
return err;

down_write(&namespace_sem);
err = follow_down(path, true);
err = lock_mount(path);
if (err < 0)
goto out;

err = -EINVAL;
if (!check_mnt(path->mnt) || !check_mnt(old_path.mnt))
goto out;

err = -ENOENT;
mutex_lock(&path->dentry->d_inode->i_mutex);
if (cant_mount(path->dentry))
goto out1;

if (d_unlinked(path->dentry))
Expand Down Expand Up @@ -1926,9 +1942,8 @@ static int do_move_mount(struct path *path, char *old_name)
* automatically */
list_del_init(&old_path.mnt->mnt_expire);
out1:
mutex_unlock(&path->dentry->d_inode->i_mutex);
unlock_mount(path);
out:
up_write(&namespace_sem);
if (!err)
path_put(&parent_path);
path_put(&old_path);
Expand Down Expand Up @@ -1983,11 +1998,9 @@ static int do_add_mount(struct vfsmount *newmnt, struct path *path, int mnt_flag

mnt_flags &= ~(MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL);

down_write(&namespace_sem);
/* Something was mounted here while we slept */
err = follow_down(path, true);
if (err < 0)
goto unlock;
err = lock_mount(path);
if (err)
return err;

err = -EINVAL;
if (!(mnt_flags & MNT_SHRINKABLE) && !check_mnt(path->mnt))
Expand All @@ -2007,7 +2020,7 @@ static int do_add_mount(struct vfsmount *newmnt, struct path *path, int mnt_flag
err = graft_tree(newmnt, path);

unlock:
up_write(&namespace_sem);
unlock_mount(path);
return err;
}

Expand Down Expand Up @@ -2575,55 +2588,53 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
goto out1;

error = security_sb_pivotroot(&old, &new);
if (error) {
path_put(&old);
goto out1;
}
if (error)
goto out2;

get_fs_root(current->fs, &root);
down_write(&namespace_sem);
mutex_lock(&old.dentry->d_inode->i_mutex);
error = lock_mount(&old);
if (error)
goto out3;

error = -EINVAL;
if (IS_MNT_SHARED(old.mnt) ||
IS_MNT_SHARED(new.mnt->mnt_parent) ||
IS_MNT_SHARED(root.mnt->mnt_parent))
goto out2;
goto out4;
if (!check_mnt(root.mnt) || !check_mnt(new.mnt))
goto out2;
goto out4;
error = -ENOENT;
if (cant_mount(old.dentry))
goto out2;
if (d_unlinked(new.dentry))
goto out2;
goto out4;
if (d_unlinked(old.dentry))
goto out2;
goto out4;
error = -EBUSY;
if (new.mnt == root.mnt ||
old.mnt == root.mnt)
goto out2; /* loop, on the same file system */
goto out4; /* loop, on the same file system */
error = -EINVAL;
if (root.mnt->mnt_root != root.dentry)
goto out2; /* not a mountpoint */
goto out4; /* not a mountpoint */
if (root.mnt->mnt_parent == root.mnt)
goto out2; /* not attached */
goto out4; /* not attached */
if (new.mnt->mnt_root != new.dentry)
goto out2; /* not a mountpoint */
goto out4; /* not a mountpoint */
if (new.mnt->mnt_parent == new.mnt)
goto out2; /* not attached */
goto out4; /* not attached */
/* make sure we can reach put_old from new_root */
tmp = old.mnt;
if (tmp != new.mnt) {
for (;;) {
if (tmp->mnt_parent == tmp)
goto out2; /* already mounted on put_old */
goto out4; /* already mounted on put_old */
if (tmp->mnt_parent == new.mnt)
break;
tmp = tmp->mnt_parent;
}
if (!is_subdir(tmp->mnt_mountpoint, new.dentry))
goto out2;
goto out4;
} else if (!is_subdir(old.dentry, new.dentry))
goto out2;
goto out4;
br_write_lock(vfsmount_lock);
detach_mnt(new.mnt, &parent_path);
detach_mnt(root.mnt, &root_parent);
Expand All @@ -2634,14 +2645,16 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
touch_mnt_namespace(current->nsproxy->mnt_ns);
br_write_unlock(vfsmount_lock);
chroot_fs_refs(&root, &new);

error = 0;
path_put(&root_parent);
path_put(&parent_path);
out2:
mutex_unlock(&old.dentry->d_inode->i_mutex);
up_write(&namespace_sem);
out4:
unlock_mount(&old);
if (!error) {
path_put(&root_parent);
path_put(&parent_path);
}
out3:
path_put(&root);
out2:
path_put(&old);
out1:
path_put(&new);
Expand Down

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