From kde-core-devel  Thu Nov 19 16:14:25 2009
From: Thiago Macieira <thiago () kde ! org>
Date: Thu, 19 Nov 2009 16:14:25 +0000
To: kde-core-devel
Subject: Re: mmap failures in KPixmapCache
Message-Id: <200911191714.25836.thiago () kde ! org>
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Em Quinta-feira 19 Novembro 2009, =E0s 14:49:06, voc=EA escreveu:
> Am Donnerstag 19 November 2009 13:57:18 schrieb Thiago Macieira:
> > Em Quarta-feira 18 Novembro 2009, =E0s 23:31:25, voc=EA escreveu:
> > > I see it here on "Linux parkotron 2.6.31-ARCH #1 SMP PREEMPT Tue Nov
> > > 10 19:01:40 CET 2009 x86_64 Intel(R) Core(TM) i5 CPU 750 @ 2.67GHz
> > > GenuineIntel GNU/Linux"
> > >
> > > Unfortunately, I know nothing about, mmapping or QFile::map. It
> > > doesn't makes sense for me to report it as I can't provide the
> > > relevant details. David, are you able to report it?
> >
> > We found the issue. It's a KDE bug.
> >
> > KPixmapCache was trying to extend the file by mapping a section of the
> > file past the end, then writing to it. That was never guaranteed to wor=
k.
> > In fact, it didn't work outside Linux.
> >
> > In Qt 4.6, when testing mmap on other platforms, we realised that this
> >  didn't work on Mac, so we introduced a check to ensure that you don't
> > rely on non- cross-platform behaviour. That's what KDE was tripping on.
>=20
> This is definitely not the case for KDevelop. On my system, alone this
>  simple test application fails:
>=20
> "
> #include <QFile>
>=20
> int main(int argc, char** argv)
> {
>   {
>     QFile f("maptest.data");
>     f.open(QIODevice::WriteOnly);
>     f.write("aaaaaaaaaaaaaaaaaaaa", 20);
>   }
>   {
>     QFile f("maptest.data");
>     f.open(QIODevice::ReadOnly);
>     Q_ASSERT(f.isOpen());
>     Q_ASSERT(f.map(0, f.size()));
>   }
> }
> "
>=20
> It happens with both kernel versions 2.6.31 and 2.6.27. I'm using a 64-bit
> kernel and userspace.
>=20
> Is this failure reproducible somewhere else? Unfortunately, I don't have
>  the qt sources installed, else I would dig into them and check for the
>  actual problem.

The testcase above works just fine here. No asserts.

The problem is when you allocate past the end of the file. You allocated ju=
st=20
the full file.

=2D-=20
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
  Senior Product Manager - Nokia, Qt Development Frameworks
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358

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