setns: support user namespaces#13359
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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 `nsfd`s and `pidfd`s. 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. Fixes #13314 COPYBARA_INTEGRATE_REVIEW=#13323 from shayonj:issue-13314-userns-setns 8060b5f PiperOrigin-RevId: 926855389
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setns: support user namespaces
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
FUTURE_COPYBARA_INTEGRATE_REVIEW=#13323 from shayonj:issue-13314-userns-setns 8060b5f