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Fix out of bounds write im->alpha[im->transparent] #785

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merged 2 commits into from Oct 9, 2021
Merged

Fix out of bounds write im->alpha[im->transparent] #785

merged 2 commits into from Oct 9, 2021

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bathterror
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Fixes #784

vapier
vapier requested changes Oct 9, 2021
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@vapier vapier merged commit ba14dec into libgd:master Oct 9, 2021
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vapier commented Oct 9, 2021

thx bud

pierrejoye pushed a commit that referenced this pull request Jan 29, 2022
@bathterror @pierrejoye
Fix out of bounds write im->alpha[im->transparent] (#785)
e050c86 
Since #737 gdImageColorTransparent does not correctly handle the case that im->transparent = -1
(which is the initial value and used to indicate no transparent colour has been set).

This leads to undefined behaviour via an out-of-bound write:
im->alpha[im->transparent] = gdAlphaOpaque;
(in practice I assume this merely overwrites an earlier struct member)

This can be triggered via loading a gif through gdImageCreateFromGifPtr

third_party/gd/source/gd.c:922:2: runtime error: index -1 out of bounds for type 'int [256]'
    #0 0x5629c034a839 in gdImageColorTransparent third_party/gd/source/gd.c:922:29
    #1 0x5629c034ebf0 in gdImageCreateFromGifCtx third_party/gd/source/gd_gif_in.c:328:4
    #2 0x5629c034f14f in gdImageCreateFromGifPtr third_party/gd/source/gd_gif_in.c:186:7

Fixes #784.
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[regression] undefined behaviour gdImageColorTransparent
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