netdev - Re: [PATCH net 1/4] bonding: fix send_peer

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Date:   Thu, 20 Apr 2023 22:13:17 -0700
From:   Jay Vosburgh <jay.vosburgh@...onical.com>
To:     Hangbin Liu <liuhangbin@...il.com>
cc:     Jakub Kicinski <kuba@...nel.org>,
        kernel test robot <lkp@...el.com>, netdev@...r.kernel.org,
        oe-kbuild-all@...ts.linux.dev,
        "David S . Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Liang Li <liali@...hat.com>, Vincent Bernat <vincent@...nat.ch>
Subject: Re: [PATCH net 1/4] bonding: fix send_peer_notif overflow

Hangbin Liu <liuhangbin@...il.com> wrote:

>On Thu, Apr 20, 2023 at 04:21:39PM -0700, Jakub Kicinski wrote:
>> On Thu, 20 Apr 2023 08:59:40 -0700 Jay Vosburgh wrote:
>> > >All errors (new ones prefixed by >>, old ones prefixed by <<):
>> > >  
>> > >>> ERROR: modpost: "__umoddi3" [drivers/net/bonding/bonding.ko] undefined!  
>> > 
>> > 	I assume this is related to send_peer_notif now being u64 in the
>> > modulus at:
>> > 
>> > static bool bond_should_notify_peers(struct bonding *bond)
>> > {
>> > [...]
>> >         if (!slave || !bond->send_peer_notif ||
>> >             bond->send_peer_notif %
>> >             max(1, bond->params.peer_notif_delay) != 0 ||
>> > 
>> > 	but I'm unsure if this is a real coding error, or some issue
>> > with the parisc arch specifically?
>> 
>> Coding error, I think. 
>> An appropriate helper from linux/math64.h should be used.
>
>It looks define send_peer_notif to u64 is a bit too large, which introduce
>complex conversion for 32bit arch.
>
>For the remainder operation,
>bond->send_peer_notif % max(1, bond->params.peer_notif_delay). u32 % u32 look OK.
>
>But for multiplication operation,
>bond->send_peer_notif = bond->params.num_peer_notif * max(1, bond->params.peer_notif_delay);
>It's u8 * u32. How about let's limit the peer_notif_delay to less than max(u32 / u8),
>then we can just use u32 for send_peer_notif. Is there any realistic meaning
>to set peer_notif_delay to max(u32)? I don't think so.
>
>Jay, what do you think?

	I'm fine to limit the peerf_notif_delay range and then use a
smaller type.

	num_peer_notif is already limited to 255; I'm going to suggest a
limit to the delay of 300 seconds.  That seems like an absurdly long
time for this; I didn't do any kind of science to come up with that
number.

	As peer_notif_delay is stored in units of miimon intervals, that
gives a worst case peer_notif_delay value of 300000 if miimon is 1, and
255 * 300000 fits easily in a u32 for send_peer_notif.

	-J

---
	-Jay Vosburgh, jay.vosburgh@...onical.com