setns: support user namespaces#13323
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User namespace entries under /proc/[pid]/ns currently render as fake namespace symlinks. They look like the other namespace files, but opening them does not produce an nsfs file that setns(2) can use. Rootless container tools such as buildah and podman rely on that file when they re-enter the pause process user namespace, so the second lifecycle command fails with EINVAL. Make UserNamespace implement vfs.Namespace and give each user namespace an nsfs inode when it is created. /proc/[pid]/ns/user now uses the regular namespace symlink path, so opening it returns a joinable namespace file instead of a fake link target. Setns now accepts CLONE_NEWUSER from both nsfds and pidfds. It follows the Linux restrictions for user namespace joins by rejecting the caller's current user namespace, requiring CAP_SYS_ADMIN in the target user namespace, rejecting multithreaded callers, and rejecting callers with fs state shared outside the thread group. The capability checks for any other namespaces in the same setns call use the credentials the caller would have after joining the user namespace. Add a syscall regression test that creates a child user namespace, opens /proc/<pid>/ns/user, and verifies that setns(CLONE_NEWUSER) succeeds.
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User namespace entries under
/proc/[pid]/nscurrently render as fakenamespace symlinks. They look like the other namespace files, but opening
them does not produce an
nsfsfile thatsetns(2)can use. Rootlesscontainer tools such as
buildahandpodmanrely on that file when theyre-enter the pause process user namespace, so the second lifecycle command
fails with
EINVAL.Make
UserNamespaceimplementvfs.Namespaceand give each user namespacean
nsfsinode when it is created./proc/[pid]/ns/usernow uses theregular namespace symlink path, so opening it returns a joinable namespace
file instead of a fake link target.
Setnsnow acceptsCLONE_NEWUSERfrom bothnsfds andpidfds. Itfollows the Linux restrictions for user namespace joins by rejecting the
caller's current user namespace, requiring
CAP_SYS_ADMINin the targetuser namespace, rejecting multithreaded callers, and rejecting callers with
fsstate shared outside the thread group. The capability checks for anyother namespaces in the same
setnscall use the credentials the callerwould have after joining the user namespace.
Add a syscall regression test that creates a child user namespace, opens
/proc/<pid>/ns/user, and verifies thatsetns(CLONE_NEWUSER)succeeds.Fixes #13314