[V2][PATCH 1/1] Add a new warning option -Wstrict-flex-arrays.

[Qing Zhao]

Hi, Sid,

Thanks a lot for the input.

After more thinking based on your and Kees’ comments, I have the following thought:

1. -fstrict-flex-arrays=level should control both GCC code gen and warnings consistently;
2. We need warnings specifically for -fstrict-flex-arrays=level to report any misuse of flexible 
     array corresponding to the “level” to gradually encourage language standard.

So, based on the above two, I think what I did in this current patch is correct:

1.  We eliminate the control from -Warray-bounds=level on treating flex arrays, 
     now only "-fstrict-flex-arrasy=level" controls how the warning treating the flex arrays.
2.  We add a separate new warning -Wstrict-flex-arrays to report any misuse corresponding to
     the different level of -fstrict-flex-arrays.

Although we can certainly merge these new warnings into -Warray-bounds, however, as Sid mentioned,
-Warray-bounds does issue a lot more warnings than just flexible arrays misuse. I think it’s necessary 
To provide a seperate warning to only issue flexible array misuse.

Let me know if you have any more comments on this.

thanks.

Qing



> On Dec 1, 2022, at 2:45 PM, Siddhesh Poyarekar <siddhesh@gotplt.org> wrote:
> 
> On 2022-12-01 11:42, Kees Cook wrote:
>> On Wed, Nov 30, 2022 at 02:25:56PM +0000, Qing Zhao wrote:
>>> '-Wstrict-flex-arrays'
>>>      Warn about inproper usages of flexible array members according to
>>>      the LEVEL of the 'strict_flex_array (LEVEL)' attribute attached to
>>>      the trailing array field of a structure if it's available,
>>>      otherwise according to the LEVEL of the option
>>>      '-fstrict-flex-arrays=LEVEL'.
>>> 
>>>      This option is effective only when LEVEL is bigger than 0.
>>>      Otherwise, it will be ignored with a warning.
>>> 
>>>      when LEVEL=1, warnings will be issued for a trailing array
>>>      reference of a structure that have 2 or more elements if the
>>>      trailing array is referenced as a flexible array member.
>>> 
>>>      when LEVEL=2, in addition to LEVEL=1, additional warnings will be
>>>      issued for a trailing one-element array reference of a structure if
>>>      the array is referenced as a flexible array member.
>>> 
>>>      when LEVEL=3, in addition to LEVEL=2, additional warnings will be
>>>      issued for a trailing zero-length array reference of a structure if
>>>      the array is referenced as a flexible array member.
>>> 
>>> At the same time, -Warray-bounds is updated:
>> Why is there both -Wstrict-flex-arrays and -Warray-bounds? I thought
>> only the latter was going to exist?
> 
> Oh my understanding of the consensus was to move flex array related diagnosis from -Warray-bounds to -Wstring-flex-arrays as Qing has done. If only the former exists then instead of removing the flex array related statement in the documentation as Richard suggested, we need to enhance it to say that behaviour of -Warray-bounds will depend on -fstrict-flex-arrays.
> 
> -Warray-bounds does diagnosis beyond just flexible arrays, in case that's the confusion.
> 
> Sid