Re: ToT glibc build problem with ToT GCC

[Carlos O'Donell <carlos at redhat dot com>]

On 8/29/19 2:36 PM, Martin Sebor wrote:
> On 8/29/19 11:54 AM, Carlos O'Donell wrote:
>> On 8/29/19 1:28 PM, Martin Sebor wrote:
>>> A longer term solution is to replace the array with a suitably
>>> aligned char buffer and write the data into it, e.g., like this:
>>>
>>>    union {
>>>      struct charseq replace[2];
>>>      char buf[sizeof (struct charseq) * 2];
>>>    } u;
>>>
>>>    u.replace[0].nbytes = 1;
>>>    const char s[] = "?";
>>>    memcpy (u.buf + offsetof (struct charseq, bytes), "?", 2);
>>>
>>> This is still iffy but it won't trigger an out-of-bounds warning.
>>> This code should still be flagged for the declaration of an object
>>> with an internal zero-length array, so that will need to be dealt
>>> with when such a warning is introduced (e.g., by suppressing
>>> the warning).
>>
>> Why is this iffy? This is the kind of solution I was thinking of using.
>>
>> The alternative is an entirely new type that has a non-zero-length
>> array that can be used for static object allocation like this, but then
>> you need to cast between them in a safe way?
> 
> It's iffy because of the interior zero-length array.
> 
> GCC accepts zero-length array members anywhere but it doesn't
> always avoid assuming their elements don't overlap other members.
> Some other compilers don't allow zero-length arrays in definitions
> where they're followed by another member or element of a larger
> array.  I'd like to see GCC warn for such uses because of
> the overlapping problem.
> 
> Martin
> 
> PS For example, GCC folds the test below to false.  Clang does
> the same with slightly different test cases.
> 
>   struct charseq replace[2];
> 
>   void f (int i)
>   {
>     int x = *replace[1].name;
> 
>     replace[0].bytes[i] = 0;
> 
>     if (x != *replace[1].name)
>       __builtin_abort ();
>   }

This static allocation is rare, it's the only one.

Is there a viable solution for a static allocation of a structure
that ends in a VLA?

-- 
Cheers,
Carlos.