Re: [PATCH 0/2] topic: meson: add more compiler hardening flags

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Oct 09, 2023 at 09:21:01AM +0200, Thomas Huth wrote:
> On 05/10/2023 19.38, Daniel P. Berrangé wrote:
> ...
> > 
> > I also tested enabling -ftrapv, to change signed integer
> > overflow from wrapping, to trapping instead. This exposed a
> > bug in the string-input-visitor which overflows when parsing
> > ranges, and exposed the test-int128 code as (harmlessly)
> > overflowing during its testing. Both can be fixed, but I'm
> > not entirely sure whether -ftrapv is viable or not. I was
> > wondering about TCG and whether it has a need to intentionally
> > allow integer overflow for any of its instruction emulation
> > requirements ?
> I'm not an expert when it comes to this question, but as far as I
> understood, we are using -fwrapv (with "w", not "t") on purpose, see
> meson.build:
> 
> # We use -fwrapv to tell the compiler that we require a C dialect where
> # left shift of signed integers is well defined and has the expected
> # 2s-complement style results. (Both clang and gcc agree that it
> # provides these semantics.)
> 
> And according to the man-page of gcc:
> 
>  The options -ftrapv and -fwrapv override each other,
>  so using -ftrapv -fwrapv on the command-line results
>  in -fwrapv being effective.
> 
> If I got that right, this means you cannot use -ftrapv with QEMU.

Opps, I didn't notice we had -fwrapv in our flags, that is clearly
mutually exclusive with -ftrapv, so nothing further to do here.

With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|