qt4 broken on i686

[Jakub Bogusz]

On Sun, Sep 19, 2021 at 08:17:47PM +0200, Jan Rękorajski wrote:
> On Thu, 19 Aug 2021, Jakub Bogusz wrote:
> 
> > On Wed, Aug 18, 2021 at 10:34:48PM +0200, Jan Rękorajski wrote:
> > > New builds of qt4 on i686 exhibit crashes (ex. linguist in avogadro), or
> > > infinite looping (ex. meinproc4 in kde4-kig).
> > > 
> > > I'm running out of ideas[1] and time to troubleshoot this and would
> > > appreciate if anyone would be willing to try and figure out WTF is
> > > broken there.
> > > 
> > > [1] neither -O0, nor -std=gnu98 seem to do the trick, it could be a
> > > glibc 2.34 issue, but I don't have resources at hand to validate it.
> > 
> > I don't know yet if it's related to glibc, gcc or binutils, but simple
> > testcase is searching in empty QMap:
> > 
> > ```
> > #include <QtCore/QMap>
> > 
> > int main()
> > {
> >     QMap<int, std::string> mm;
> >     mm.constFind(999);
> > }
> > ```
> > 
> > It hangs even on carme-x86_64.
> > 
> > Issue is probably related to shared_null static field (SIOF?)
> 
> My take is on gcc 11. I downgraded everything but gcc and it still crashed.
> 
> To verify that it's indeed gcc I need to rebuild it (gcc) locally due to
> intertwined deps preventing simple package downgrade.

I tried to investigate the issue deeper - and the last I found was:

The symbol _ZN8QMapData11shared_nullE (demangled: QMapData::shared_null)
resides both in executable (objdump -T from i686):

0804c040 g    DO .bss   00000048  Base        _ZN8QMapData11shared_nullE

and libQtCore.so.4:

002fa460 g    DO .data  00000048  Base        _ZN8QMapData11shared_nullE


When compiled in current Th, the code in executable sees symbol mapped
from executable and the library sees symbol mapped from library.

IIRC when compiled on slightly older system (qt4 built with gcc 7 or 8,
glibc 2.33 etc.) in both cases symbol address was the same.

But in my current system (i686, glibc-2.34, binutils-2.37, gcc-8.4.0, qt4
rebuilt in this environment) the result is the same as in Th: there are
two instances of this symbol and testcase hangs.


-- 
Jakub Bogusz    http://qboosh.pl/