Re: [PATCH v2] fs: select: fix information leak to userspace

[Boaz Harrosh <bharrosh@panasas.com>]

On 11/15/2010 09:12 PM, Eric Dumazet wrote:
> Le dimanche 14 novembre 2010 à 18:06 -0800, Andrew Morton a écrit : 
>> On Sun, 14 Nov 2010 12:25:33 +0300 Vasiliy Kulikov <segoon@openwall.com> wrote:
>>>  
>>>  	if (timeval) {
>>> -		rtv.tv_sec = rts.tv_sec;
>>> -		rtv.tv_usec = rts.tv_nsec / NSEC_PER_USEC;
>>> +		struct timeval rtv = {
>>> +			.tv_sec = rts.tv_sec,
>>> +			.tv_usec = rts.tv_nsec / NSEC_PER_USEC
>>> +		};
>>>  
>>>  		if (!copy_to_user(p, &rtv, sizeof(rtv)))
>>>  			return ret;
>>
>> Please check the assembly code - this will still leave four bytes of
>> uninitalised stack data in 'rtv', surely.
> 
> Thats a good question.
> 
> In my understanding, gcc should initialize all holes (and other not
> mentioned fields) with 0, even for automatic storage [C99 only mandates
> this on static storage]
> 
> I tested on x86_64 and this is the case, but could not find a definitive
> answer in gcc documentation.
> 
> This kind of construct is widely used in networking tree.
> 
> Maybe we should ask to gcc experts if this behavior is guaranteed by
> gcc, or if we must review our code ;(
> 
> CC Jakub
> 
> Thanks !
> 

This is what I thought too. If it is not there are tones of bugs I wrote
of code that relays on this behaviour.

It would be interesting to know for sure

Thanks
Boaz