Re: [PATCH v2] fs/locks: filter OFD locks from /proc/locks for foreign pid namespaces

[Jeff Layton]

On Wed, 2026-09-02 at 20:51 +0800, Tao Cui wrote:
> From: Tao Cui <cuitao@xxxxxxxxxx>
> 
> Since Linux 4.9, /proc/locks only shows locks whose owning process is
> visible in the reader's pid namespace.  The check cannot see the
> owner of an OFD lock, though: locks_translate_pid() always returns
> -1 for FL_OFDLCK, so an OFD lock held outside of that namespace
> bypasses the filter and is shown with its device/inode numbers and
> byte range, e.g.
> 
>     4: OFDLCK ADVISORY  WRITE -1 fd:00:70536961 0 EOF
> 
> This was observed on a Kubernetes node: a pod reading /proc/locks
> listed the OFD write lock of an unrelated host process, while the
> POSIX locks of the very same process were correctly hidden.  The
> device/inode pair identifies which host file is locked, the byte
> range discloses where it is actively written, and the
> appearance/disappearance of entries reflects host task activity,
> contrary to the documented per-pidns visibility of /proc/locks
> (proc_locks(5)).
> 
> OFD locks record the owner tgid in flc_pid, so use it for the
> visibility decision in locks_show(), mirroring what
> locks_translate_pid() does for POSIX locks.  The pid column is still
> reported as -1 for OFD locks; only the filter decision changes.
> Remote locks keep their negative flc_pid and stay visible as before.
> 
> Verified with an OFD and a POSIX write lock held in the initial pid
> namespace while a process in a fresh pid namespace reads /proc/locks:
> the OFD entry is visible without this patch and hidden with it, the
> POSIX entry is hidden in both cases.
> 
> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
> 
> ---
> 
> Changes since v1: keep remote OFD locks (negative flc_pid) visible in
> non-initial pid namespaces, as locks_translate_pid() does for remote
> POSIX locks.
> ---
>  fs/locks.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index 6e4ff7fcec05..4af1385682e4 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -3022,6 +3022,25 @@ static int locks_show(struct seq_file *f, void *v)
>  
>  	cur = hlist_entry(v, struct file_lock_core, flc_link);
>  
> +	/*
> +	 * OFD locks are reported with pid -1, so the filter below cannot see
> +	 * their owner; flc_pid holds the owner tgid, so filter on it.
> +	 * Remote locks keep a negative flc_pid and stay visible as before.
> +	 */
> +	if ((cur->flc_flags & FL_OFDLCK) && cur->flc_pid > 0 &&
> +	    proc_pidns != &init_pid_ns) {
> +		struct pid *pid;
> +		bool visible = false;
> +
> +		rcu_read_lock();
> +		pid = find_pid_ns(cur->flc_pid, &init_pid_ns);
> +		if (pid)
> +			visible = pid_nr_ns(pid, proc_pidns) != 0;
> +		rcu_read_unlock();
> +		if (!visible)
> +			return 0;
> +	}
> +
>  	if (locks_translate_pid(cur, proc_pidns) == 0)
>  		return 0;
>  

First: I think this may be the wrong place to do this. Why not fold
this change into locks_translate_pid()? It seems like we'd have
inconsistent results wrt lock visibility between /proc/locks and
F_GETLK if you do this here.

Thinking about this some more though, I wonder if trying to hide these
locks is the right thing to do. These locks do exist and they do block
you from acquiring a lock. If you go to look at /proc/locks and don't
see them, that seems confusing.

Would we be better off showing all the locks and reporting the pid as a
negative value for ones acquired in foreign namespaces, like we do for
remote locks?
-- 
Jeff Layton <jlayton@xxxxxxxxxx>