bybrooklyn/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-04-18 14:53:58 -04:00
Code Issues Packages Projects Releases Wiki Activity

namespace: allow creating empty mount namespaces

Browse Source 
Add support for creating a mount namespace that contains only a copy of
the root mount from the caller's mount namespace, with none of the
child mounts.  This is useful for containers and sandboxes that want to
start with a minimal mount table and populate it from scratch rather
than inheriting and then tearing down the full mount tree.

Two new flags are introduced:

- CLONE_EMPTY_MNTNS for clone3(), using the 64-bit flag space.

- UNSHARE_EMPTY_MNTNS for unshare(), reusing the
  CLONE_PARENT_SETTID bit which has no meaning for unshare.

Both flags imply CLONE_NEWNS.  For the unshare path,
UNSHARE_EMPTY_MNTNS is converted to CLONE_EMPTY_MNTNS in
unshare_nsproxy_namespaces() before it reaches copy_mnt_ns(), so the
mount namespace code only needs to handle a single flag.

In copy_mnt_ns(), when CLONE_EMPTY_MNTNS is set, clone_mnt() is used
instead of copy_tree() to clone only the root mount.  The caller's root
and working directory are both reset to the root dentry of the new
mount.

The cleanup variables are changed from vfsmount pointers with
__free(mntput) to struct path with __free(path_put) because the empty
mount namespace path needs to release both mount and dentry references
when replacing the caller's root and pwd.  In the normal (non-empty)
path only the mount component is set, and dput(NULL) is a no-op so
path_put remains correct there as well.

Link: https://patch.msgid.link/20260306-work-empty-mntns-consolidated-v1-1-6eb30529bbb0@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Christian Brauner
2026-03-06 17:28:37 +01:00
parent 0209e31659
commit 9d4e752a24
4 changed files with 95 additions and 37 deletions
Download Patch File Download Diff File Expand all files Collapse all files

87
fs/namespace.c
View File

@@ -4233,8 +4233,8 @@ struct mnt_namespace *copy_mnt_ns(u64 flags, struct mnt_namespace *ns,
struct user_namespace *user_ns, struct fs_struct *new_fs)
{
struct mnt_namespace *new_ns;
struct vfsmount *rootmnt __free(mntput) = NULL;
struct vfsmount *pwdmnt __free(mntput) = NULL;
struct path old_root __free(path_put) = {};
struct path old_pwd __free(path_put) = {};
struct mount *p, *q;
struct mount *old;
struct mount *new;
@@ -4254,11 +4254,18 @@ struct mnt_namespace *copy_mnt_ns(u64 flags, struct mnt_namespace *ns,
return new_ns;
guard(namespace_excl)();
/* First pass: copy the tree topology */
copy_flags = CL_COPY_UNBINDABLE | CL_EXPIRE;
if (flags & CLONE_EMPTY_MNTNS)
copy_flags = 0;
else
copy_flags = CL_COPY_UNBINDABLE | CL_EXPIRE;
if (user_ns != ns->user_ns)
copy_flags |= CL_SLAVE;
new = copy_tree(old, old->mnt.mnt_root, copy_flags);
if (flags & CLONE_EMPTY_MNTNS)
new = clone_mnt(old, old->mnt.mnt_root, copy_flags);
else
new = copy_tree(old, old->mnt.mnt_root, copy_flags);
if (IS_ERR(new)) {
emptied_ns = new_ns;
return ERR_CAST(new);
@@ -4269,33 +4276,53 @@ struct mnt_namespace *copy_mnt_ns(u64 flags, struct mnt_namespace *ns,
}
new_ns->root = new;
/*
* Second pass: switch the tsk->fs->* elements and mark new vfsmounts
* as belonging to new namespace. We have already acquired a private
* fs_struct, so tsk->fs->lock is not needed.
*/
p = old;
q = new;
while (p) {
mnt_add_to_ns(new_ns, q);
new_ns->nr_mounts++;
if (flags & CLONE_EMPTY_MNTNS) {
/*
* Empty mount namespace: only the root mount exists.
* Reset root and pwd to the cloned mount's root dentry.
*/
if (new_fs) {
if (&p->mnt == new_fs->root.mnt) {
new_fs->root.mnt = mntget(&q->mnt);
rootmnt = &p->mnt;
}
if (&p->mnt == new_fs->pwd.mnt) {
new_fs->pwd.mnt = mntget(&q->mnt);
pwdmnt = &p->mnt;
}
old_root = new_fs->root;
old_pwd = new_fs->pwd;
new_fs->root.mnt = mntget(&new->mnt);
new_fs->root.dentry = dget(new->mnt.mnt_root);
new_fs->pwd.mnt = mntget(&new->mnt);
new_fs->pwd.dentry = dget(new->mnt.mnt_root);
}
mnt_add_to_ns(new_ns, new);
new_ns->nr_mounts++;
} else {
/*
* Full copy: walk old and new trees in parallel, switching
* the tsk->fs->* elements and marking new vfsmounts as
* belonging to new namespace. We have already acquired a
* private fs_struct, so tsk->fs->lock is not needed.
*/
p = old;
q = new;
while (p) {
mnt_add_to_ns(new_ns, q);
new_ns->nr_mounts++;
if (new_fs) {
if (&p->mnt == new_fs->root.mnt) {
old_root.mnt = new_fs->root.mnt;
new_fs->root.mnt = mntget(&q->mnt);
}
if (&p->mnt == new_fs->pwd.mnt) {
old_pwd.mnt = new_fs->pwd.mnt;
new_fs->pwd.mnt = mntget(&q->mnt);
}
}
p = next_mnt(p, old);
q = next_mnt(q, new);
if (!q)
break;
// an mntns binding we'd skipped?
while (p->mnt.mnt_root != q->mnt.mnt_root)
p = next_mnt(skip_mnt_tree(p), old);
}
p = next_mnt(p, old);
q = next_mnt(q, new);
if (!q)
break;
// an mntns binding we'd skipped?
while (p->mnt.mnt_root != q->mnt.mnt_root)
p = next_mnt(skip_mnt_tree(p), old);
}
ns_tree_add_raw(new_ns);
return new_ns;