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cgifsave: fix heap-use-after-free #2849

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cgifsave: fix heap-use-after-free #2849

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kleisauke
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liq_set_dithering_level could somehow free the palette, so
do the transparency check prior to that.

Note that GCC still prints this warning:

In function ‘vips_foreign_save_cgif_write_frame’,
    inlined from ‘vips_foreign_save_cgif_sink_disc’ at ../libvips/foreign/cgifsave.c:587:8:
../libvips/foreign/cgifsave.c:401:35: warning: ‘pal_local’ may be used uninitialized [-Wmaybe-uninitialized]
  401 |                 for( i = 0; i < lp->count; i++ ) {
      |                                 ~~^~~~~~~
../libvips/foreign/cgifsave.c: In function ‘vips_foreign_save_cgif_sink_disc’:
../libvips/foreign/cgifsave.c:261:54: note: ‘pal_local’ was declared here
  261 |         const VipsQuantisePalette *lp, *pal_global, *pal_local;
      |                                                      ^~~~~~~~~

(probably a false positive)

@kleisauke
cgifsave: fix heap-use-after-free
8afdbbe 
`liq_set_dithering_level` could somehow free the palette, so
do the transparency check prior to that.
@kleisauke kleisauke marked this pull request as draft June 10, 2022 10:32
@kleisauke
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Marking as draft, since the palette is still fetched after vips__quantise_set_dithering_level.

libvips/libvips/foreign/cgifsave.c

Line 487 in 8afdbbe

trans_index = lp->count;

This seems to cause a new ASan failure:
https://github.com/libvips/libvips/runs/6828919349?check_suite_focus=true#step:18:327

@jcupitt
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jcupitt commented Jun 10, 2022

I'm half-way through reworking cgifsave, I should have a PR with a rewritten (and easier to maintain) version later today.

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jcupitt commented Jun 10, 2022

Should be fixed with #2853

@jcupitt jcupitt closed this Jun 10, 2022
@kleisauke kleisauke deleted the cgifsave-fix-asan branch June 11, 2022 08:15
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@kleisauke @jcupitt