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

["Américo Wang" <xiyou.wangcong@gmail.com>]

On Wed, Nov 24, 2010 at 11:46:33AM +0000, Pádraig Brady wrote:
>On 24/11/10 11:05, Américo Wang wrote:
>> On Wed, Nov 24, 2010 at 10:44:50AM +0000, Pádraig Brady wrote:
>>> On 23/11/10 18:02, Andreas Dilger wrote:
>>>> On 2010-11-23, at 07:45, walter harms wrote:
>>>>> Maybe we can convince the gcc people to make 0 padding default. That will not solve the problems for other compilers but when they claim "works like gcc" we can press then to support this also. I can imagine that this will close some other subtle leaks also.
>>>>
>>>> It makes the most sense to tackle this at the GCC level, since the added overhead of doing memset(0) on the whole struct may be non-trivial for commonly-used and/or large structures.  Since GCC is already explicitly zeroing the _used_ fields in the struct, it can much more easily determine whether there is padding in the structure, and zero those few bytes as needed.
>>>
>>> Zero padding structs is part of C90. Details here:
>>> http://www.pixelbeat.org/programming/gcc/auto_init.html
>> 
>> Nope.
>> 
>>>
>>> gcc doesn't zero pad when _all_ elements are specified.
>>>
>> 
>> That is what gcc does, not what C standard specifies.
>
>Looks like gcc is following the standard exactly.
>
>C90 - 6.5.7
>C99 - 6.7.8
>
>  If there are fewer initializers in a brace-enclosed list than
>  there are elements or members of an aggregate ... the remainder
>  of the aggregate shall be initialized implicitly the same as
>  objects that have static storage duration.
>

Depends on if "the remainder of the aggregate" includes padding bytes
or not.