Re: [PATCH] pidfs: protect PIDFD_GET_* ioctls() via ifdef

[Brendan Jackman <jackmanb@google.com>]

On Wed Dec 24, 2025 at 12:00 PM UTC, Christian Brauner wrote:
> We originally protected PIDFD_GET_<ns-type>_NAMESPACE ioctls() through
> ifdefs and recent rework made it possible to drop them. There was an
> oversight though. When the relevant namespace is turned off ns->ops will
> be NULL so even though opening a file descriptor is perfectly legitimate
> it would fail during inode eviction when the file was closed.
>
> The simple fix would be to check ns->ops for NULL and continue allow to
> retrieve namespace fds from pidfds but we don't allow retrieving them
> when the relevant namespace type is turned off. So keep the
> simplification but add the ifdefs back in.
>
> Reported-by: Eric Biggers <ebiggers@kernel.org>
> Link: https://lore.kernel.org/20251222214907.GA189632@quark
> Fixes: a71e4f103aed ("pidfs: simplify PIDFD_GET_<type>_NAMESPACE ioctls")
> Signed-off-by: Christian Brauner <brauner@kernel.org>

Thanks Christian, I was also getting a NULL deref from running
the x86:ldt_gdt_32 kselftest on QEMU under a minimal kconfig (via
task_active_pid_ns()), this fixes it.

Tested-by: Brendan Jackman <jackmanb@kernel.org>