Re: [PATCH] treewide: Replace zero-length arrays with flexible-array member

[Gustavo A. R. Silva]

On 2/11/20 14:12, Kees Cook wrote:
> On Tue, Feb 11, 2020 at 01:54:22PM -0600, Gustavo A. R. Silva wrote:
>>
>>
>> On 2/11/20 13:38, Greg KH wrote:
>>> On Tue, Feb 11, 2020 at 11:32:04AM -0800, Kees Cook wrote:
>>>> On Tue, Feb 11, 2020 at 01:20:36PM -0600, Gustavo A. R. Silva wrote:
>>>>>
>>>>>
>>>>> On 2/11/20 12:32, Greg KH wrote:
>>>>>> On Tue, Feb 11, 2020 at 11:41:26AM -0600, Gustavo A. R. Silva wrote:
>>>>>>> The current codebase makes use of the zero-length array language
>>>>>>> extension to the C90 standard, but the preferred mechanism to declare
>>>>>>> variable-length types such as these ones is a flexible array member[1][2],
>>>>>>> introduced in C99:
>>>>>>>
>>>>>>> struct foo {
>>>>>>>         int stuff;
>>>>>>>         struct boo array[];
>>>>>>> };
>>>>>>>
>>>>>>> By making use of the mechanism above, we will get a compiler warning
>>>>>>> in case the flexible array does not occur last in the structure, which
>>>>>>> will help us prevent some kind of undefined behavior bugs from being
>>>>>>> unadvertenly introduced[3] to the codebase from now on.
>>>>>>>
>>>>>>> All these instances of code were found with the help of the following
>>>>>>> Coccinelle script:
>>>>>>>
>>>>>>> @@
>>>>>>> identifier S, member, array;
>>>>>>> type T1, T2;
>>>>>>> @@
>>>>>>>
>>>>>>> struct S {
>>>>>>>   ...
>>>>>>>   T1 member;
>>>>>>>   T2 array[
>>>>>>> - 0
>>>>>>>   ];
>>>>>>> };
>>>>>>>
>>>>>>> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
>>>>>>> [2] https://github.com/KSPP/linux/issues/21
>>>>>>> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>>>>>>>
>>>>>>> NOTE: I'll carry this in my -next tree for the v5.6 merge window.
>>>>>>
>>>>>> Why not carve this up into per-subsystem patches so that we can apply
>>>>>> them to our 5.7-rc1 trees and then you submit the "remaining" that don't
>>>>>> somehow get merged at that timeframe for 5.7-rc2?
>>>>>>
>>>>>
>>>>> Yep, sounds good. I'll do that.
>>>>
>>>> FWIW, I'd just like to point out that since this is a mechanical change
>>>> with no code generation differences (unlike the pre-C90 1-byte array
>>>> conversions), it's a way better use of everyone's time to just splat
>>>> this in all at once.
>>>>
>>>> That said, it looks like Gustavo is up for it, but I'd like us to
>>>> generally consider these kinds of mechanical changes as being easier to
>>>> manage in a single patch. (Though getting Acks tends to be a bit
>>>> harder...)
>>>
>>> Hey, if this is such a mechanical patch, let's get it to Linus now,
>>> what's preventing that from being merged now?
> 
> Now would be a good time, yes. (Linus has wanted Acks for such things
> sometimes, but those were more "risky" changes...)
> 
>> Well, the only thing is that this has never been in linux-next.
> 
> Hmm. Was it in one of your 0day-tested trees?
> 

It was in my tree for quite a while, but it was never 0day-tested.
Just recently, the 0day guys started testing my _new_ branches,
regularly.

Today, I updated my -next branch to v5.6-rc1 and added the
treewide patch. So, I expect it to be 0day-tested in a couple
of days.

--
Gustavo