ns: support file handles
For a while now we have supported file handles for pidfds. This has
proven to be very useful.
Extend the concept to cover namespaces as well. After this patchset it
is possible to encode and decode namespace file handles using the
commong name_to_handle_at() and open_by_handle_at() apis.
Namespaces file descriptors can already be derived from pidfds which
means they aren't subject to overmount protection bugs. IOW, it's
irrelevant if the caller would not have access to an appropriate
/proc/<pid>/ns/ directory as they could always just derive the namespace
based on a pidfd already.
It has the same advantage as pidfds. It's possible to reliably and for
the lifetime of the system refer to a namespace without pinning any
resources and to compare them.
Permission checking is kept simple. If the caller is located in the
namespace the file handle refers to they are able to open it otherwise
they must hold privilege over the owning namespace of the relevant
namespace.
Both the network namespace and the mount namespace already have an
associated cookie that isn't recycled and is fully exposed to userspace.
Move this into ns_common and use the same id space for all namespaces so
they can trivially and reliably be compared.
There's more coming based on the iterator infrastructure but the series
is large enough and focuses on file handles.
Extensive selftests included.
To: Jan Kara <jack@suse.cz>
To: Amir Goldstein <amir73il@gmail.com>
To: linux-fsdevel@vger.kernel.org
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Mike Yuan <me@yhndnzj.com>
Cc: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Cc: Lennart Poettering <mzxreary@0pointer.de>
Cc: Daan De Meyer <daan.j.demeyer@gmail.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: linux-nfs@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: linux-block@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: cgroups@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
Changes in v3:
- EDITME: describe what is new in this series revision.
- EDITME: use bulletpoints and terse descriptions.
- Link to v2: https://lore.kernel.org/20250912-work-namespace-v2-0-1a247645cef5@kernel.org
Changes in v2:
- Address various review comments.
- Use a common NS_GET_ID ioctl() instead of individual ioctls.
- Link to v1: https://lore.kernel.org/20250910-work-namespace-v1-0-4dd56e7359d8@kernel.org
--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
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"change-id": "20250905-work-namespace-c68826dda0d4",
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