strange effect with --enable-fast-malloc=full

[Lubos Lunak]

On út 16. duben 2002 15:28, Falk Brettschneider wrote:
> Hi!
>
> Lubos Lunak wrote:
> >On út 16. duben 2002 14:07, Falk Brettschneider wrote:
> >>Hi!
> >>
> >>I tried to compile KDE_3_0_BRANCH with using the
> >>--enable-fast-malloc=full flag in configure, first for kdecore only,
> >>second for the whole kdesupport, kdelibs, kdebase.
> >>But now always I got a strange effect when closing my KDE application.
> >>Somehow it hangs forever in an infinite loop somewhere in malloc.
> >>Whenever I interrupt it in gdb, it shows the same callstack:
> >>  http://www.geocities.com/gigafalk/strange.jpg
> >
> > Hmm. The backtrace is completely useless - the only 3 things I can see is
> >that 1) the backtrace ends in malloc(), 2) you don't have libkdecore with
> >debug info
>
> sorry, now I recompiled kdecore with debug info and I saw it loops back
> in line 2963 of malloc.c because there's always a p != 0.
>
> > even though you have it in libqt and 3) you use Keramik. Could you
> >give me a testcase,
>
> it only happens in a closed-source KDE app of the company I work for
> here, I can't give you the code or the binary and plugins.
>
> >or find out where exactly in the malloc it loops?
>
> I saw in the debugger that nextp is equal to p->fd and p doesn't get a
> new pointer within the code block of the while statement. That's why it
> loops infinitely. I don't understand what that all means though... Do
> you have an idea?

 Not really, but I think p->fd is a pointer to the next free block (so 
nextp==p->fd should never happen). I think it's either deleting a block twice 
or a write out of bounds. Valgrind will tell for sure.

>
> > And first of all, could you run the application in valgrind? This malloc
> >implementation seems to be more vulnerable to heap corruption, so it may
> > be you have a bug somewhere and malloc in glibc gets over it somehow.
> > Your second mail just confirms my suspicion. Most probably you're trying
> > to delete something twice.
>
> I should install valgrind...

 Yes, you should. Unless you can prove the bug is not in the application, I'm 
considering this malloc() implementation to be Bug Free(tm).

-- 
 Lubos Lunak
 llunak at suse.cz ; l.lunak at kde.org
 http://dforce.sh.cvut.cz/~seli