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Re: SMDoubleSlider usability on GNUstep


From: David Chisnall
Subject: Re: SMDoubleSlider usability on GNUstep
Date: Tue, 20 Feb 2018 12:46:32 +0000

On 20 Feb 2018, at 12:33, Yavor Doganov <address@hidden> wrote:
> 
> #1  0x00007ffff77d2015 in -[NSCell doubleValue] (self=0x5555562ef7f0, 
> _cmd=<optimized out>) at NSCell.m:269
> #2  0x00007ffff778362a in -[NSActionCell doubleValue] (self=0x5555562ef7f0, 
> _cmd=<optimized out>) at NSActionCell.m:187
> #3  0x000055555555bfdb in -[SMDoubleSliderCell doubleHiValue] 
> (self=0x5555562ef7f0, _cmd=<optimized out>) at SMDoubleSliderCell.m:448

Skimming the code, it looks as if their -doubleValue method calls their 
-doubleHiValue method, which calls the superclass’s -doubleValue method.  In 
GNUstep, the superclass has another call to its superclass method:

https://github.com/gnustep/libs-gui/blob/master/Source/NSActionCell.m#L187

This then checks whether the object responds to -doubleValue, and if it does 
calls that:

https://github.com/gnustep/libs-gui/blob/master/Source/NSCell.m#L265

Unfortunately, in this case, it appears that the object value is self, so you 
get infinite recursion.

This looks like a bug elsewhere, as setting a cell’s object value to the cell 
itself is likely to cause problems.  I don’t see any calls to -setObjectValue: 
in the SMDoubleSlider code that you’ve linked to, so something else must be 
doing this.  It looks quite a bit like a use-after-free bug.

David




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