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

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

Hi, Joseph,


> On Feb 6, 2023, at 6:14 PM, Joseph Myers <joseph@codesourcery.com> wrote:
> 
> On Mon, 6 Feb 2023, Qing Zhao via Gcc-patches wrote:
> 
>> In GCC14:
>> 
>> 1. Include this new warning -Wgnu-varaible-sized-type-not-at-end to -Wall
>> 2. Deprecate this extension from GCC. (Or delay this to next release?).
> 
> Any deprecation, or inclusion in -Wall, would best come with evidence 
> about the prevalance of use (possibly unintentional, probably undesirable) 
> of these extensions.  For example, maybe someone could do a distribution 
> rebuild with a patch to enable these warnings and report the results?
Yes, before we deprecate this extension, it’s better to make sure all such
 misuses are updated already.
> 
> Various misuses of flexible array members are only pedwarns-if-pedantic 
> because of such uses - and while the original motivating case 
> <https://gcc.gnu.org/legacy-ml/gcc-patches/2002-08/msg01149.html>

Just checked this patch (which has been in GCC source tree already), the routine flexible_array_type_p 
+/* Determine whether TYPE is a structure with a flexible array member,
+   or a union containing such a structure (possibly recursively).  */
+
+static bool
+flexible_array_type_p (type)

Did not include the following cases:

1.  Structure with flexible array member embedded into other structures recursively, for example:

struct A {
  int n;
  char data[];
};

struct B {
  int m;
  struct A a;
};

struct C {
  int q;
  struct B b;
};

In the above, “struct C” will not be caught by this routine.

Shall “struct C” be included?

2. Only C99 standard flexible array member be included, [0] and [1] are not included, for example:

struct A {
  int n;
  char data[0];
};

struct B {
  int m;
  struct A a;
};

struct C {
  int q;
  struct B b;
};

In the above, “struct B” and “struct C” will not be caught by this routine.

Shall “the above struct B” and “struct C” be included per -fstrict-flex-arrays?

Could you please take a look at my latest patch:

https://gcc.gnu.org/pipermail/gcc-patches/2023-February/611445.html

To see whether the new bit “TYPE_INCLUDE_FLEXARRAY” covers the above “flexible_array_type_p”? Then we can merge them together?


> was 
> _G_config.h, which has since been fixed (though existing installed headers 
> from old glibc would need fixincluding, at least if it becomes an error), 
> it's very plausible there are uses of these extensions elsewhere.
If this is the case, we should definitely deprecate this extension.

Thanks.

Qing
> 
> -- 
> Joseph S. Myers
> joseph@codesourcery.com