On Apr 30, 2024, at 17:55, Kees Cook <keescook@chromium.org> wrote:

On Tue, Apr 30, 2024 at 05:51:20PM -0400, Jason Merrill wrote:
On 4/30/24 14:45, Qing Zhao wrote:


On Apr 30, 2024, at 15:27, Jason Merrill <jason@redhat.com> wrote:

On 4/30/24 07:58, Qing Zhao wrote:
The request for GCC to accept that the C99 flexible array member can be
in a union or alone in a structure has been made a long time ago
around 2012
for supporting several practical cases including glibc.
A GCC PR has been opened for such request at that time:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548
However, this PR was closed as WONTFIX around 2015 due to the
following reason:
"there is an existing extension that makes the requested
functionality possible"
i.e GCC fully supported that the zero-length array can be in a
union or alone
in a structure for a long time. (though I didn't see any
official documentation
on such extension)
It's reasonable to close PR53548 at that time since zero-length
array extension
can be used for such purpose.
However, since GCC13, in order to improve the C/C++ security, we
introduced
-fstrict-flex-arrays=n to gradually eliminate the "fake flexible array"
usages from C/C++ source code. As a result, zero-length arrays eventually
will be replaced by C99 flexiable array member completely.
Therefore, GCC needs to explicitly allow such extensions directly for C99
flexible arrays, since flexable array member in unions or alone
in structs
are common code patterns in active use by the Linux kernel (and
other projects).
For example, these do not error by default with GCC:
union one {
  int a;
  int b[0];
};
union two {
  int a;
  struct {
    struct { } __empty;
    int b[];
  };
};
But these do:
union three {
  int a;
  int b[];
};
struct four {
  int b[];
}
Clang has supported such extensions since March, 2024
https://github.com/llvm/llvm-project/pull/84428
GCC should also support such extensions. This will allow for
a seamless transition for code bases away from zero-length arrays without
losing existing code patterns.
gcc/ChangeLog:
* doc/extend.texi: Add documentation for Flexible Array Members in
Unions and Flexible Array Members alone in Structures.
---
 gcc/doc/extend.texi | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 7b54a241a7bf..cba98c8aadd7 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -42,6 +42,8 @@ extensions, accepted by GCC in C90 mode and in C++.
 * Named Address Spaces::Named address spaces.
 * Zero Length::         Zero-length arrays.
 * Empty Structures::    Structures with no members.
+* Flexible Array Members in Unions::  Unions with Flexible
Array Members.
+* Flexible Array Members alone in Structures::  Structures with
only Flexible Array Members.
 * Variable Length::     Arrays whose length is computed at run time.
 * Variadic Macros::     Macros with a variable number of arguments.
 * Escaped Newlines::    Slightly looser rules for escaped newlines.
@@ -1873,6 +1875,38 @@ The structure has size zero.  In C++,
empty structures are part
 of the language.  G++ treats empty structures as if they had a single
 member of type @code{char}.
 +@node Flexible Array Members in Unions
+@section Unions with Flexible Array Members
+@cindex unions with flexible array members
+@cindex unions with FAMs
+
+GCC permits a C99 flexible array member (FAM) to be in a union:
+
+@smallexample
+union with_fam @{
+  int a;
+  int b[];
+@};
+@end smallexample
+
+If all the members of a union are flexible array member, the size of

It’s for the following case:

union with_fam_3 {
  char a[];
  int b[];
}

And also include:  (the only member of a union is a flexible array
member as you mentioned below)

union with_fam_1 {
  char a[];
}

So, I think the original sentence:

“If all the members of a union are flexible array member, the size of”

Should be better than the below:

"If the only member of a union is a flexible array member”

Makes sense, but then it should be "members" both times rather than
"members" and then "member".

"If every member of a union is a flexible array, the size ..." ?

Yes, I have updated the doc as this. -:).

Qing

--
Kees Cook