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Subject: Re: [Linux-kernel-mentees] [PATCH net] rds: Prevent kernel-infoleak
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To:     Jason Gunthorpe <jgg@ziepe.ca>, Joe Perches <joe@perches.com>
Cc:     Leon Romanovsky <leon@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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        Jakub Kicinski <kuba@kernel.org>,
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From:   Jack Leadford <leadford.jack@gmail.com>
Message-ID: <ff066616-3bb8-b6c8-d329-7de5ab8ee982@gmail.com>
Date:   Sat, 8 Aug 2020 15:57:33 -0700
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Hello!

Thanks to Jason for getting this conversation back on track.

Yes: in general, {} or a partial initializer /will/ zero padding bits.

However, there is a bug in some versions of GCC where {} will /not/ zero
padding bits; actually, Jason's test program in this mail 
https://lore.kernel.org/lkml/20200731143604.GF24045@ziepe.ca/
has the right ingredients to trigger the bug, but the GCC
versions used are outside of the bug window. :)

For more details on these cases and more (including said GCC bug), see 
my paper at:

https://www.nccgroup.com/us/about-us/newsroom-and-events/blog/2019/october/padding-the-struct-how-a-compiler-optimization-can-disclose-stack-memory/

Hopefully this paper can serve as a helpful reference when these cases 
are encountered in the kernel.

Thank you.

Jack Leadford

On 8/3/20 4:06 PM, Jason Gunthorpe wrote:
> On Sun, Aug 02, 2020 at 03:45:40PM -0700, Joe Perches wrote:
>> On Sun, 2020-08-02 at 19:28 -0300, Jason Gunthorpe wrote:
>>> On Sun, Aug 02, 2020 at 03:23:58PM -0700, Joe Perches wrote:
>>>> On Sun, 2020-08-02 at 19:10 -0300, Jason Gunthorpe wrote:
>>>>> On Sat, Aug 01, 2020 at 08:38:33AM +0300, Leon Romanovsky wrote:
>>>>>
>>>>>> I'm using {} instead of {0} because of this GCC bug.
>>>>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119
>>>>>
>>>>> This is why the {} extension exists..
>>>>
>>>> There is no guarantee that the gcc struct initialization {}
>>>> extension also zeros padding.
>>>
>>> We just went over this. Yes there is, C11 requires it.
>>
>> c11 is not c90.  The kernel uses c90.
> 
> The kernel already relies on a lot of C11/C99 features and
> behaviors. For instance Linus just bumped the minimum compiler version
> so that C11's _Generic is usable.
> 
> Why do you think this particular part of C11 shouldn't be relied on?
> 
> Jason
> 
>