[poppler] Compiling poppler with clang

[Albert Astals Cid]

El Dilluns, 27 d'agost de 2012, a les 22:31:13, Albert Astals Cid va escriure:
> El Dilluns, 27 d'agost de 2012, a les 08:29:40, Thomas Freitag va escriure:
> > On 27.08.2012 00:56, Albert Astals Cid wrote:
> > > El Diumenge, 26 d'agost de 2012, a les 15:48:37, He Liu va escriure:
> > >>>> 5. vtable pointer will be overwritten
> > >>>> Function.cc:422:10: warning: destination for this 'memcpy' call is a
> > >>>> pointer to dynamic class 'SampledFunction'; vtable pointer will be
> > >>>> overwritten [-Wdynamic-class-memaccess]
> > >>>> 
> > >>>>    memcpy(this, func, sizeof(SampledFunction));
> > >>>>    ~~~~~~ ^
> > >>>> 
> > >>>> Function.cc:422:10: note: explicitly cast the pointer to silence this
> > >>>> warning
> > >>>> 
> > >>>> At least categrory 5. sound serious to me, I would never have copied
> > >>>> instances of C++ objects in that way, because it depends on the
> > >>>> compiler
> > >>>> and the class if this causes problems on runtime, s. i.e.
> > >>>> http://weseetips.com/tag/afx_zero_init_object/,
> > >>> 
> > >>> Note this is memset-ing to 0, not memcpy-ing a class to itself. To be
> > >>> honest i agree memcpy'in a SampledFunction to a SampledFunction is
> > >>> ugly,
> > >>> but i fail to see why it would not work.
> > 
> > It works, at least with the actual used compilers. But it works only,
> > because the allocated members of SampledFunction are overwriten after
> > doing the memcpy. And this behaviour makes it just more ugly in my eyes.
> > And this is the same with the other memcpy's in ExponentialFunction,
> > StitchingFunction and PostScriptFunction. And it will work till such
> > time as everybody who changes the class will not forget to do it in the
> > same way. So if You are not willing to change it (or let somebody else
> > change it, I know, never change running code), we could use the hint:
> > explicitly cast the pointer to silence this warning.
> 
> I'm fine with someone fixing the code. Less warnings is always good, but for
> master, there's no need to introduce a pontential breaker just because
> clang complains.

So at the end noone volunteered to re-rewite those functions not to use memcpy 
over classes with virtuals?

Cheers,
  Albert

> 
> Cheers,
>   Albert
> 
> > Cheers,
> > Thomas
> > 
> > >> Hi Albert,
> > >> 
> > >> :-)
> > >> 
> > >> A pointer of type SampleFunction* could be pointing to an instance of a
> > >> SampleFunction sub-class, which has different vtable contents.
> > > 
> > > No, it could not, SampledFunction does not have any childs and the
> > > function
> > > doing that memcopy is private anyway.
> > > 
> > >> As a result,
> > >> one could construct a SampleFunction with SampleFunction(SampleFunction
> > >> *)
> > >> using a pointer to a sub-class instance, and overwrite the
> > >> SampleFunction's
> > >> vtable address with the sub-class's vtable address.
> > >> 
> > >> I am not sure if it will lead to any bugs/vulnerabilities in this case,
> > >> but
> > >> it is not safe practice in general.
> > > 
> > > Sure, i never said it was. I'm just saying i don't see why it would not
> > > work in our case.
> > > 
> > >> Since the vtable structure depends on
> > >> how the compiler is implemented, memcpy or memset on object pointers
> > >> will
> > >> generally lead to undefined behaviors.
> > > 
> > > I'm far from a compiler expert, but one would hope that for a given
> > > class
> > > the compiler stores always the "stuff" in the same order in memory, so
> > > again, i fail to see why this should fail in our case.
> > > 
> > > Cheers,
> > > 
> > >    Albert
> > >> 
> > >> Thanks.
> > >> _______________________________________________
> > >> poppler mailing list
> > >> poppler at lists.freedesktop.org
> > >> http://lists.freedesktop.org/mailman/listinfo/poppler
> > > 
> > > _______________________________________________
> > > poppler mailing list
> > > poppler at lists.freedesktop.org
> > > http://lists.freedesktop.org/mailman/listinfo/poppler
> > > 
> > > .
> > 
> > _______________________________________________
> > poppler mailing list
> > poppler at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/poppler
> 
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