Re: [PATCH 1/1] net: Fix use of proc_fs

[Jakub Kicinski <kuba@kernel.org>]

On Fri, 11 Dec 2020 18:37:49 +0200 Yonatan Linik wrote:
> proc_fs was used, in af_packet, without a surrounding #ifdef,
> although there is no hard dependency on proc_fs.
> That caused the initialization of the af_packet module to fail
> when CONFIG_PROC_FS=n.
> 
> Specifically, proc_create_net() was used in af_packet.c,
> and when it fails, packet_net_init() returns -ENOMEM.
> It will always fail when the kernel is compiled without proc_fs,
> because, proc_create_net() for example always returns NULL.
> 
> The calling order that starts in af_packet.c is as follows:
> packet_init()
> register_pernet_subsys()
> register_pernet_operations()
> __register_pernet_operations()
> ops_init()
> ops->init() (packet_net_ops.init=packet_net_init())
> proc_create_net()
> 
> It worked in the past because register_pernet_subsys()'s return value
> wasn't checked before this Commit 36096f2f4fa0 ("packet: Fix error path in
> packet_init.").
> It always returned an error, but was not checked before, so everything
> was working even when CONFIG_PROC_FS=n.
> 
> The fix here is simply to add the necessary #ifdef.
> 
> Signed-off-by: Yonatan Linik <yonatanlinik@gmail.com>

Hm, I'm guessing you hit this on a kernel upgrade of a real system?
It seems like all callers to proc_create_net (and friends) interpret
NULL as an error, but only handful is protected by an ifdef.

I checked a few and none of them cares about the proc_dir_entry pointer
that gets returned. Should we perhaps rework the return values of the
function so that we can return success if !CONFIG_PROC_FS without
having to yield a pointer?

Obviously we can apply this fix so we can backport to 5.4 if you need
it. I think the ifdef is fine, since it's what other callers have.

> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 2b33e977a905..031f2b593720 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -4612,9 +4612,11 @@ static int __net_init packet_net_init(struct net *net)
>  	mutex_init(&net->packet.sklist_lock);
>  	INIT_HLIST_HEAD(&net->packet.sklist);
>  
> +#ifdef CONFIG_PROC_FS
>  	if (!proc_create_net("packet", 0, net->proc_net, &packet_seq_ops,
>  			sizeof(struct seq_net_private)))
>  		return -ENOMEM;
> +#endif /* CONFIG_PROC_FS */
>  
>  	return 0;
>  }