Re: [PATCH 1/2] Handle component_ref to a structre/union field including flexible array member [PR101832]

[Qing Zhao <qing.zhao@oracle.com>]

Thanks, Kees.

If there is no objection, I will update my patches with this. And send the updated patches soon.

Qing

> On Feb 9, 2023, at 11:46 AM, Kees Cook <keescook@chromium.org> wrote:
> 
> On Thu, Feb 09, 2023 at 02:40:57PM +0000, Qing Zhao wrote:
>> So, the major question here is:
>> 
>> in addition to the C99 standard flexible array member [ ], shall we include [0], [1] or even [4] into this extension, and treat the structure with a trailing [0], [1], or [4] embedded into another structure/union still as flexible-sized?
>> 
>> I think that we might need to limit this extension ONLY to C99 standard FAM [ ].  All other [0], [1], or [4] should be excluded from this extension. The reasons are:
>> 
>> 1. The real usages of such GCC extension (embedding structure with FAM into another structure/union), as my understanding, the old glibc’s <_G_config.h> (https://gcc.gnu.org/legacy-ml/gcc-patches/2002-08/msg01149.html), and the bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101832, ONLY involved C99 standard FAM;
>> 
>> 2. Embedding a structure with C99 FAM [] into the end of another structure, and still treat it flexible sized might have more usages, and as discussed with Kees, it might be reasonable to promote this into a  C standard later if needed.
>> 
>> So, based on this consideration, I think I should only document the following as GCC extension:
>> 
>> struct flex  { int length; char data[ ]; };
>> struct out_flex { int m; struct flex flex_data; };
>> 
>> Issue warnings for the following: (when the structure is not at the end)
>> 
>> struct out_flex_mid  {  struct flex flex_data; int m};
>> 
>> 
>> However, for the trailing [0], [1], or [4], when such structure embedded into the end of another structure, We should NOT treat the outer structure as flexible sized. 
>> Logically, we will NOT issue warnings when such structure is not at the end. 
>> 
>> Let me know if you have any comment or suggestions.
> 
> FWIW this all sounds correct to me.
> 
> -- 
> Kees Cook