Re: [edk2-devel] [PATCH 04/10] MdePkg/PiFirmwareFile: fix undefined behavior in FFS_FILE_SIZE

["Laszlo Ersek" <lersek@redhat.com>]

On 04/18/19 10:45, Laszlo Ersek wrote:

> Beyond that, I checked the documentation of "-fstrict-aliasing" in the
> gcc manual at
> <https://gcc.gnu.org/onlinedocs/gcc-8.3.0/gcc/Optimize-Options.html>. It
> provides several examples; one of them is:
> 
>> Similarly, access by taking the address, casting the resulting pointer
>> and dereferencing the result has undefined behavior, even if the cast
>> uses a union type, e.g.:
>>
>> union a_union {
>>   int i;
>>   double d;
>> };
>>
>> int f() {
>>   double d = 3.0;
>>   return ((union a_union *) &d)->i;
>> }
> 
> I think the access in this example is well defined (I seem able to
> deduce it using the effective type rules), and so I consider this a bug
> in the gcc docs. I reached out to someone in the toolchain team at Red
> Hat to confirm or disprove.

I was told to file an upstream GCC BZ about this, in order to reach a
wider circle of experts. In case someone from edk2-devel would like to
monitor that BZ:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90167

Thanks,
Laszlo