netdev - Re: [PATCH] can: Use correct type in sizeof() in nla

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Date:	Fri, 30 Oct 2015 15:53:31 +0100
From:	Oliver Hartkopp <socketcan@...tkopp.net>
To:	Marek Vasut <marex@...x.de>, Marc Kleine-Budde <mkl@...gutronix.de>
CC:	linux-can@...r.kernel.org, Wolfgang Grandegger <wg@...ndegger.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH] can: Use correct type in sizeof() in nla_put()



On 10/30/2015 03:24 PM, Marek Vasut wrote:
> On Friday, October 30, 2015 at 03:17:44 PM, Marc Kleine-Budde wrote:
>> On 10/30/2015 03:01 PM, Marek Vasut wrote:

>>> Are you absolutelly positive this doesn't break kernel ABI please ?
>>>
>>> I am a little worried there, since the size of can_clock and can_ctrlmode
>>> structures differ.
>>
>> struct can_clock is a u32, see [1]
>> struct can_ctrlmode is 2 x u32.
>>
>> in libsocketcan[2] it's accessed like this:
>>> 	memcpy(res,
>>> 	
>>> 		RTA_DATA(can_attr[IFLA_CAN_CLOCK]),
>>> 		sizeof(struct can_clock));
>>
>> I think it should be ok.
> 
> In that case, yes, it's good. Hopefully, noone wrote his own thing.
> 

Fortunately ip from iproute2 does it similary:

https://git.kernel.org/cgit/linux/kernel/git/shemminger/iproute2.git/tree/ip/iplink_can.c#n338


	if (tb[IFLA_CAN_CLOCK]) {
		struct can_clock *clock = RTA_DATA(tb[IFLA_CAN_CLOCK]);

		fprintf(f, "\n	  clock %d", clock->freq);
	}

As the clock is a read-only value kernel->userspace and nla_put creates its
own small ID/length information each time we are REALLY LUCKY that this fix
doesn't break the ABI in this case.

When can_clock would have been greater then can_ctrlmode we really had a
problem ...

Thanks for caching this!

Oliver
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