Re: [PATCH 00/14] xattr: rework simple xattrs and support user.* xattrs on sockets

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Feb 16, 2026 at 02:31:56PM +0100, Christian Brauner wrote:
> Hey,
> 
> This reworks the simple_xattr infrastructure and adds support for
> user.* extended attributes on sockets.
> 
> The simple_xattr subsystem currently uses an rbtree protected by a
> reader-writer spinlock. This series replaces the rbtree with an
> rhashtable giving O(1) average-case lookup with RCU-based lockless
> reads. This sped up concurrent access patterns on tmpfs quite a bit and
> it's an overall easy enough conversion to do and gets rid or rwlock_t.
> 
> The conversion is done incrementally: a new rhashtable path is added
> alongside the existing rbtree, consumers are migrated one at a time
> (shmem, kernfs, pidfs), and then the rbtree code is removed. All three
> consumers switch from embedded structs to pointer-based lazy allocation
> so the rhashtable overhead is only paid for inodes that actually use
> xattrs.

Patches 1-6 look ok to me, at least in the sense that nothing stood out
to me as obviously wrong, so
Acked-by: "Darrick J. Wong" <djwong@kernel.org>

> With this infrastructure in place the series adds support for user.*
> xattrs on sockets. Path-based AF_UNIX sockets inherit xattr support
> from the underlying filesystem (e.g. tmpfs) but sockets in sockfs -
> that is everything created via socket() including abstract namespace
> AF_UNIX sockets - had no xattr support at all.
> 
> The xattr_permission() checks are reworked to allow user.* xattrs on
> S_IFSOCK inodes. Sockfs sockets get per-inode limits of 128 xattrs and
> 128KB total value size matching the limits already in use for kernfs.
> 
> The practical motivation comes from several directions. systemd and
> GNOME are expanding their use of Varlink as an IPC mechanism. For D-Bus
> there are tools like dbus-monitor that can observe IPC traffic across
> the system but this only works because D-Bus has a central broker. For
> Varlink there is no broker and there is currently no way to identify

Hum.  I suppose there's never going to be a central varlink broker, is
there?  That doesn't sound great for discoverability, unless the plan is
to try to concentrate them in (say) /run/varlink?  But even then, could
you have N services that share the same otherwise private tmpfs in order
to talk to each other via a varlink socket?  I suppose in that case, the
N services probably don't care/want others to discover their socket.

> which sockets speak Varlink. With user.* xattrs on sockets a service
> can label its socket with the IPC protocol it speaks (e.g.,
> user.varlink=1) and an eBPF program can then selectively capture

Who gets to set xattrs?  Can a malicious varlink socket user who has
connect() abilities also delete user.varlink to mess with everyone who
comes afterwards?

--D

> traffic on those sockets. Enumerating bound sockets via netlink combined
> with these xattr labels gives a way to discover all Varlink IPC
> entrypoints for debugging and introspection.
> 
> Similarly, systemd-journald wants to use xattrs on the /dev/log socket
> for protocol negotiation to indicate whether RFC 5424 structured syslog
> is supported or whether only the legacy RFC 3164 format should be used.
> 
> In containers these labels are particularly useful as high-privilege or
> more complicated solutions for socket identification aren't available.
> 
> The series comes with comprehensive selftests covering path-based
> AF_UNIX sockets, sockfs socket operations, per-inode limit enforcement,
> and xattr operations across multiple address families (AF_INET,
> AF_INET6, AF_NETLINK, AF_PACKET).
> 
> Christian
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
> Christian Brauner (14):
>       xattr: add rcu_head and rhash_head to struct simple_xattr
>       xattr: add rhashtable-based simple_xattr infrastructure
>       shmem: adapt to rhashtable-based simple_xattrs with lazy allocation
>       kernfs: adapt to rhashtable-based simple_xattrs with lazy allocation
>       pidfs: adapt to rhashtable-based simple_xattrs
>       xattr: remove rbtree-based simple_xattr infrastructure
>       xattr: add xattr_permission_error()
>       xattr: switch xattr_permission() to switch statement
>       xattr: move user limits for xattrs to generic infra
>       xattr,net: support limited amount of extended attributes on sockfs sockets
>       xattr: support extended attributes on sockets
>       selftests/xattr: path-based AF_UNIX socket xattr tests
>       selftests/xattr: sockfs socket xattr tests
>       selftests/xattr: test xattrs on various socket families
> 
>  fs/kernfs/dir.c                                    |  15 +-
>  fs/kernfs/inode.c                                  |  99 +----
>  fs/kernfs/kernfs-internal.h                        |   5 +-
>  fs/pidfs.c                                         |  65 +--
>  fs/xattr.c                                         | 423 +++++++++++++------
>  include/linux/kernfs.h                             |   2 -
>  include/linux/shmem_fs.h                           |   2 +-
>  include/linux/xattr.h                              |  47 ++-
>  mm/shmem.c                                         |  46 +-
>  net/socket.c                                       | 119 ++++--
>  .../testing/selftests/filesystems/xattr/.gitignore |   3 +
>  tools/testing/selftests/filesystems/xattr/Makefile |   6 +
>  .../filesystems/xattr/xattr_socket_test.c          | 470 +++++++++++++++++++++
>  .../filesystems/xattr/xattr_socket_types_test.c    | 177 ++++++++
>  .../filesystems/xattr/xattr_sockfs_test.c          | 363 ++++++++++++++++
>  15 files changed, 1547 insertions(+), 295 deletions(-)
> ---
> base-commit: 72c395024dac5e215136cbff793455f065603b06
> change-id: 20260211-work-xattr-socket-c85f4d3b8847
> 
>