Malloc broke Serenity's JPGLoader, or: how to win the lottery
An investigation in SerenityOS found a bug causing color distortion in JPG images due to a memory allocation change. The bug was fixed by adjusting color order, emphasizing the importance of careful coding practices.
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The investigation into a bug affecting the decoding of JPG images in SerenityOS revealed a color distortion issue, which was resolved by adjusting the color order in the code. The bug was traced back to a change related to memory allocation functions, specifically malloc_good_size(), which inadvertently altered the order of components in the JPGLoader code. This change led to non-deterministic iteration over components, causing the color distortion in images. After extensive debugging and multiple rebuilds, a commit was made to fix the bug by ensuring deterministic iteration over components. The bug had been masked by fortuitous circumstances until the memory allocation change exposed the underlying issue. The resolution highlights the importance of meticulous coding practices to prevent unexpected bugs, emphasizing the need for consistent and deterministic processes in software development.
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18 commentsIt also very nicely prevents security issues, since if the hashing algorithm is fixed, it can be exploited for denial of service by coming up with keys that all fall into the same bucket.
I have been in this business for decades and I have run into the situation where changing the shape of memory uncovers bugs. Every time it causes many hours and days of debugging.
If programming weren’t hard, they wouldn’t need us to do it. (I’m not sure how much longer that phrase will hold up under large language models.)
I think SerenityOS has some folks that help each other out with resources and PCs for testing purposes.
I mean, this is like trying to do Windows Vista development with a computer released in the timeframe between Windows 3.1 and Windows 95
> Someone used a HashTable to store objects that should be ordered, then iterated over it using the basic HashTable iterator
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