On 8/4/25 10:45 PM, Aleksa Sarai wrote:

/proc has historically had very opaque semantics about PID namespaces,
which is a little unfortunate for container runtimes and other programs
that deal with switching namespaces very often. One common issue is that
of converting between PIDs in the process's namespace and PIDs in the
namespace of /proc.

In principle, it is possible to do this today by opening a pidfd with
pidfd_open(2) and then looking at /proc/self/fdinfo/$n (which will
contain a PID value translated to the pid namespace associated with that
procfs superblock). However, allocating a new file for each PID to be
converted is less than ideal for programs that may need to scan procfs,
and it is generally useful for userspace to be able to finally get this
information from procfs.

So, add a new API to get the pid namespace of a procfs instance, in the
form of an ioctl(2) you can call on the root directory of said procfs.
The returned file descriptor will have O_CLOEXEC set. This acts as a
sister feature to the new "pidns" mount option, finally allowing
userspace full control of the pid namespaces associated with procfs
instances.

The permission model for this is a bit looser than that of the "pidns"
mount option (and also setns(2)) because /proc/1/ns/pid provides the
same information, so as long as you have access to that magic-link (or
something equivalently reasonable such as being in an ancestor pid
namespace) it makes sense to allow userspace to grab a handle. Ideally
we would check for ptrace-read access against all processes in the pidns
(which is very likely to be true for at least one process, as
SUID_DUMP_DISABLE is cleared on exec(2) and is rarely set by most
programs), but this would obviously not scale.

setns(2) will still have their own permission checks, so being able to
open a pidns handle doesn't really provide too many other capabilities.

Signed-off-by: Aleksa Sarai <redacted>
---
 Documentation/filesystems/proc.rst |  4 +++
 fs/proc/root.c                     | 68 ++++++++++++++++++++++++++++++++++++--
 include/uapi/linux/fs.h            |  4 +++
 3 files changed, 74 insertions(+), 2 deletions(-)

Since the _IO() nr here is 32, Documentation/userspace-api/ioctl/ioctl-number.rst
should be updated like:

-'f'   00-0F  linux/fs.h                                                conflict!
+'f'   00-1F  linux/fs.h                                                conflict!

(17 is already used for PROCFS_IOCTL_MAGIC somewhere else, so that probably should
have update the Doc/rst file.)

+
 /* Pagemap ioctl */
 #define PAGEMAP_SCAN	_IOWR(PROCFS_IOCTL_MAGIC, 16, struct pm_scan_arg)
 

Thanks.
-- 
~Randy