Re: [PATCH 2/2] Add a new warning option -Wstrict-flex-arrays.

[Richard Biener <rguenther@suse.de>]

On Mon, 21 Nov 2022, Qing Zhao wrote:

> 
> 
> > On Nov 18, 2022, at 11:31 AM, Kees Cook <keescook@chromium.org> wrote:
> > 
> > On Fri, Nov 18, 2022 at 03:19:07PM +0000, Qing Zhao wrote:
> >> Hi, Richard,
> >> 
> >> Honestly, it?s very hard for me to decide what?s the best way to handle the interaction 
> >> between -fstrict-flex-array=M and -Warray-bounds=N. 
> >> 
> >> Ideally,  -fstrict-flex-array=M should completely control the behavior of -Warray-bounds.
> >> If possible, I prefer this solution.
> >> 
> >> However, -Warray-bounds is included in -Wall, and has been used extensively for a long time.
> >> It?s not safe to change its default behavior. 
> > 
> > I prefer that -fstrict-flex-arrays controls -Warray-bounds. That
> > it is in -Wall is _good_ for this reason. :) No one is going to add
> > -fstrict-flex-arrays (at any level) without understanding what it does
> > and wanting those effects on -Warray-bounds.
> 
> 
> The major difficulties to let -fstrict-flex-arrays controlling -Warray-bounds was discussed in the following threads:
> 
> https://gcc.gnu.org/pipermail/gcc-patches/2022-October/604133.html
> 
> Please take a look at the discussion and let me know your opinion.

My opinion is now, after re-considering and with seeing your new 
patch, that -Warray-bounds=2 should be changed to only add
"the intermediate results of pointer arithmetic that may yield out of 
bounds values" and that what it considers a flex array should now
be controlled by -fstrict-flex-arrays only.

That is, I think, the only thing that's not confusing to users even
if that implies a change from previous behavior that we should
document by clarifying the -Warray-bounds documentation as well as
by adding an entry to the Caveats section of gcc-13/changes.html

That also means that =2 will get _less_ warnings with GCC 13 when
the user doesn't use -fstrict-flex-arrays as well.

Richard.

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)