[Fontconfig] Possible memory problem

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Hi Behdad,

Behdad Esfahbod wrote:
> /lib/libc.so.6[0xdc7124]
> /lib/libc.so.6(__libc_free+0x77)[0xdc765f]
> /home/behdad/.local/lib/libfontconfig.so.1(FcValueListDestroy+0x1d0)[0x7172ec]
> /home/behdad/.local/lib/libfontconfig.so.1(FcPatternDestroy+0x155)[0x717750]
> /usr/lib/firefox-1.0.7/components/libgfx_gtk.so[0x3f4a746]
> /usr/lib/firefox-1.0.7/components/libgfx_gtk.so[0x3f4a97a]
> /usr/lib/firefox-1.0.7/components/libgfx_gtk.so[0x3f4b698]
> /usr/lib/firefox-1.0.7/components/libgfx_gtk.so[0x3f4ea73]

Is it an invalid pointer rather than, say, a double free?  I'd be 
interested in knowing what the pointer actually looks like (i.e. is it 
like 0, or like some number which obviously isn't a pointer, like 0x25?) 
  This should never happen, because here .bank is saying that it's a 
dynamic FcValueListPtr and .u.dyn is not actually a pointer.

The code at fault has to be,
	if (l.bank == FC_BANK_DYNAMIC)

unless something is getting inlined.  Installing fontconfig compiled 
with -O0 would actually be helpful here, too, just so that I know it's 
actually FcValueListDestroy and not one of its callees.

> I also valgrinded pango/examples/pango-cairoview.  With branch, I
> get this:
> ==15686==  Address 0x1BE93AA0 is 6264 bytes inside a block of size 25876 alloc'd
> ==15686==    at 0x1B909222: malloc (vg_replace_malloc.c:130)
> ==15686==    by 0x1BABDAEC: FcDirCacheProduce (fccache.c:823)
> ==15686==    by 0x1BABDBFE: FcGlobalCacheUpdate (fccache.c:304)
> ==15686==    by 0x1BAC7D6D: FcDirScanConfig (fcdir.c:190)
> ==15686==    by 0x1BAC2412: FcConfigBuildFonts (fccfg.c:304)
> ==15686==    by 0x1BACB30C: FcInitLoadConfigAndFonts (fcinit.c:85)
> ==15686==    by 0x1BACB5A5: FcInit (fcinit.c:103)
> ==15686==    by 0x1BABFD51: FcConfigGetCurrent (fccfg.c:360)
> ==15686==    by 0x1BAC3E9D: FcConfigSubstituteWithPat (fccfg.c:1278)
> ==15686==    by 0x1BAC3F00: FcConfigSubstitute (fccfg.c:1490)
> ==15686==    by 0x1B972BAB: pango_cairo_fc_font_map_context_substitute (pangocairo-fcfontmap.c:94)
> ==15686==    by 0x1B94E427: pango_fc_font_map_load_fontset (pangofc-fontmap.c:958)

As Stephen says, I'm pretty sure this is unrelated to the crash; it's 
just because some bytes are skipped over in the global cache.


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