linux-kernel - Re: [PATCH v2 24/33] user: support ns lookup

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Message-ID: <bh6wllwygal6hfdjbv3amgok2yxzjgmemyvzriqf2wos6b3plp@tvhvgz47mll3>
Date: Mon, 15 Sep 2025 14:11:55 +0200
From: Jan Kara <jack@...e.cz>
To: Christian Brauner <brauner@...nel.org>
Cc: Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>, 
	linux-fsdevel@...r.kernel.org, Josef Bacik <josef@...icpanda.com>, 
	Jeff Layton <jlayton@...nel.org>, Mike Yuan <me@...dnzj.com>, 
	Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>, Lennart Poettering <mzxreary@...inter.de>, 
	Daan De Meyer <daan.j.demeyer@...il.com>, Aleksa Sarai <cyphar@...har.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Koutný <mkoutny@...e.com>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Chuck Lever <chuck.lever@...cle.com>, linux-nfs@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH v2 24/33] user: support ns lookup

On Fri 12-09-25 13:52:47, Christian Brauner wrote:
> Support the generic ns lookup infrastructure to support file handles for
> namespaces.
> 
> Signed-off-by: Christian Brauner <brauner@...nel.org>
...
> @@ -200,6 +202,7 @@ static void free_user_ns(struct work_struct *work)
>  	do {
>  		struct ucounts *ucounts = ns->ucounts;
>  		parent = ns->parent;
> +		ns_tree_remove(ns);
>  		if (ns->gid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
>  			kfree(ns->gid_map.forward);
>  			kfree(ns->gid_map.reverse);
> @@ -218,7 +221,8 @@ static void free_user_ns(struct work_struct *work)
>  		retire_userns_sysctls(ns);
>  		key_free_user_ns(ns);
>  		ns_free_inum(&ns->ns);
> -		kmem_cache_free(user_ns_cachep, ns);
> +		/* Concurrent nstree traversal depends on a grace period. */
> +		kfree_rcu(ns, ns.ns_rcu);

So this is correct for now but it's a bit of a landmine. A lot of stuff
that ns references is kfreed before the RCU expires. Thus if you lookup ns
using id, then even if you're under RCU protection you have to be very
careful about what you can and cannot dereference. IMHO this deserves a
careful documentation at least or, preferably, split free_user_ns() into
pre and post-RCU period parts...

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR