Optimization auto-setting _FORTIFY_SOURCE ??

Optimization auto-setting _FORTIFY_SOURCE ??

[Patrick Herbst]

Using GCC 9.4, I've noticed setting -O1 will set
_FORTIFY_SOURCE=2

I'm able to see this by running
gcc -c -E -dM -O1 - < /dev/null | grep FORTIFY_SOURCE

But running the same command without -O1 doesn't set it.

Is this documented?  is this expected behavior?  I could not find any
reference to how optimization leads to setting this macro.

Any tips on where I can look, or what i'm missing to see this connection?

Thanks!

Re: Optimization auto-setting _FORTIFY_SOURCE ??

[Xi Ruoyao]

On Sun, 2023-01-29 at 18:05 -0500, Patrick Herbst via Gcc-help wrote:
> Using GCC 9.4, I've noticed setting -O1 will set
> _FORTIFY_SOURCE=2
> 
> I'm able to see this by running
> gcc -c -E -dM -O1 - < /dev/null | grep FORTIFY_SOURCE
> 
> But running the same command without -O1 doesn't set it.
> 
> Is this documented?  is this expected behavior?  I could not find any
> reference to how optimization leads to setting this macro.
> 
> Any tips on where I can look, or what i'm missing to see this
> connection?

It's because your distro hacked GCC code to define _FORTIFY_SOURCE when
optimization is enabled.  Vanilla GCC does not have this behavior.


-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University

Re: Optimization auto-setting _FORTIFY_SOURCE ??

[LIU Hao]

[-- Attachment #1.1: Type: text/plain, Size: 1230 bytes --]

在 2023/1/30 12:23, Xi Ruoyao via Gcc-help 写道:
> On Sun, 2023-01-29 at 18:05 -0500, Patrick Herbst via Gcc-help wrote:
>> Using GCC 9.4, I've noticed setting -O1 will set
>> _FORTIFY_SOURCE=2
>>
>> I'm able to see this by running
>> gcc -c -E -dM -O1 - < /dev/null | grep FORTIFY_SOURCE
>>
>> But running the same command without -O1 doesn't set it.
>>
>> Is this documented?  is this expected behavior?  I could not find any
>> reference to how optimization leads to setting this macro.
>>
>> Any tips on where I can look, or what i'm missing to see this
>> connection?
> 
> It's because your distro hacked GCC code to define _FORTIFY_SOURCE when
> optimization is enabled.  Vanilla GCC does not have this behavior.
> 
> 

For the record: Debian and Ubuntu are known distros that have `_FORTIFY_SOURCE` set when 
optimization is enabled. The relevant patch is here

    https://sources.debian.org/src/gcc-8/8.3.0-2/debian/patches/gcc-default-fortify-source.diff/

however the hunk against 'invoke.texi' is incorrect: It says `_FORTIFY_SOURCE` is enabled if 
optimization level 'is set to 2 or higher', but it is actually defined for any optimization level > 0.

-- 
Best regards,
LIU Hao


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 840 bytes --]