Fortify_source and stack-protector-strong

[Reinoud Koornstra]

One more question, for FORTIFY_SOURCE=2, can I use either -O1 or -O2
optimization flags?
I noticed it doesn't like -Os. Thanks,

Reinoud.

On Wed, Mar 2, 2022, 12:45 PM Reinoud Koornstra <reinoudkoornstra@gmail.com>
wrote:

> Thanks for the explanation!
> Better to use =2 then.
> But Strack protector and FORTIFY can be active as compiler arguments at
> the same time as I understand. Just the binary size will grow. Thanks,
>
> Reinoud.
>
> On Wed, Mar 2, 2022, 12:23 PM Xi Ruoyao <xry111@mengyan1223.wang> wrote:
>
>> On Wed, 2022-03-02 at 12:05 -0800, Reinoud Koornstra wrote:
>> > Hi Xi,
>> >
>> > Thanks for your reply.
>> > Then what is the difference between -D_FORTIFY_SOURCE=1 and
>> -D_FORTIFY_SOURCE=2 exactly?
>>
>> -D_FORTIFY_SOURCE=1 uses __builtin_object_size(..., 0) as the buffer
>> size, but -D_FORTIFY_SOURCE=2 uses __builtin_object_size(..., 1).  Read
>> https://gcc.gnu.org/onlinedocs/gcc/Object-Size-Checking.html for the
>> details.
>>
>> One case is:
>>
>> struct frame
>> {
>>   int size;
>>   char buf[0];
>> };
>>
>> union
>> {
>>   struct frame f;
>>   char padding[100 + sizeof(struct frame)];
>> } u;
>>
>> u.frame.size = strlen(s) + 1;
>> strcpy(u.frame.buf, s);
>>
>> -D_FORTIFY_SOURCE=2 will abort this, but -D_FORTIFY_SOURCE=1 won't.
>> (Yes, I know "char buf[0]" should be changed to a flexible array member
>> "char buf[]" to fix this, but it is just an example.)
>> --
>> Xi Ruoyao <xry111@mengyan1223.wang>
>> School of Aerospace Science and Technology, Xidian University
>>
>