netdev - Re: [PATCH 1/1] net: Fix use of proc

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Date:   Sat, 12 Dec 2020 23:39:20 +0200
From:   Yonatan Linik <yonatanlinik@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Willem de Bruijn <willemb@...gle.com>,
        john.ogness@...utronix.de, Arnd Bergmann <arnd@...db.de>,
        Mao Wenan <maowenan@...wei.com>,
        Colin Ian King <colin.king@...onical.com>,
        orcohen@...oaltonetworks.com, Networking <netdev@...r.kernel.org>,
        linux-kernel@...r.kernel.org, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH 1/1] net: Fix use of proc_fs

On Sat, Dec 12, 2020 at 9:48 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> 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@...il.com>
>
> Hm, I'm guessing you hit this on a kernel upgrade of a real system?

Yeah, suddenly using socket with AF_PACKET didn't work,
so I checked what happened.

> 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 guess where there is no ifdef,
there should be a hard dependency on procfs,
using depends on in the Kconfig.
Maybe that's not the case everywhere it should be.

>
> 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?

Sometimes the pointer returned is used,
for example in drivers/acpi/button.c.
Are you suggesting returning a bool while
having the pointer as an out parameter?
Because that would still be problematic where the pointer is used.

>
> 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.
>

It would be great to apply this where the problem exists,
I believe this applies to other versions as well.

-- 
Yonatan Linik