linux-kernel - Re: [syzbot] [can?] KCSAN: data-race in can_send / can

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Message-ID: <b7cb5a2f-29a8-4174-8b0e-c9cbf7349531@hartkopp.net>
Date: Mon, 10 Mar 2025 15:36:28 +0100
From: Oliver Hartkopp <socketcan@...tkopp.net>
To: Vincent Mailhol <mailhol.vincent@...adoo.fr>, mkl@...gutronix.de
Cc: syzbot <syzbot+78ce4489b812515d5e4d@...kaller.appspotmail.com>,
 linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
 linux-can@...r.kernel.org
Subject: Re: [syzbot] [can?] KCSAN: data-race in can_send / can_send (5)

Hi Vincent, Marc,

I sent a patch to be reviewed:
https://lore.kernel.org/linux-can/20250310143353.3242-1-socketcan@hartkopp.net/T/#u

I've also tested this patch without any new issues.

Best regards,
Oliver

On 10.03.25 10:55, Vincent Mailhol wrote:
> On Mon. 10 Mar 2025 at 18:46, Oliver Hartkopp <socketcan@...tkopp.net> wrote:
>> On 10.03.25 10:29, Vincent Mailhol wrote:
>>> On Mon. 10 Mar 2025 at 03:59, Oliver Hartkopp <socketcan@...tkopp.net> wrote:
> 
> (...)
> 
>>>> Isn't there some lock-less per-cpu safe statistic handling within netdev
>>>> we might pick for our use-case?
>>>
>>> I see two solutions. Either we use lock_sock(skb->sk) and
>>> release_sock(skb->sk) or we can change the types of
>>> can_pkg_stats->tx_frames and can_pkg_stats->tx_frames_delta from long
>>> to atomic_long_t.
>>>
>>> The atomic_long_t is the closest solution to a lock-less. But my
>>> preference goes to the lock_sock() which looks more natural in this
>>> context. And look_sock() is just a spinlock which under the hood is
>>> also an atomic, so no big penalty either.
>>
>> When we get skbs from the netdevice (and not from user space), we do not
>> have a valid sk value. It is set to zero.
>>
>> See:
>> https://elixir.bootlin.com/linux/v6.13.6/source/net/can/raw.c#L203
>>
>> And those skbs can also be forwarded by can-gw using can_send().
>>
>> Therefore there is no lock_sock() without a valid sk ;-)
>>
>> When 'atomic_long_t' would also fix this simple statistics handling, we
>> should use that.
> 
> I see, Thanks for the explanation. Then atomic_long_t seems the best
> (and easiest).