Re: [PATCH] lib/decompress_bunzip2: fix off-by-one in run-length bounds check

[Andrew Morton]

On Sat, 12 Sep 2026 14:17:35 -0400 Matt Turner <mattst88@xxxxxxxxx> wrote:

> The run-length path rejects a block when dbufCount+t equals dbufSize,
> but the loop that follows writes exactly t bytes starting at dbufCount,
> so a block that fills the buffer exactly is legal. bzip2 allows it too:
> its decompressor bounds a block at 100000 * blockSize100k and checks
> that limit per byte appended. Use > instead of >=.
> 
> bzip2's encoder stops filling a block 19 bytes early, so nothing it
> produces ever reaches the limit and the bug stays hidden. Compressors
> that use the full block size do reach it: an lbzip2 -9 image whose block
> ends on a run fails to decode, and a self-extracting kernel built that
> way does not boot.
> 
> This code came from busybox, which fixed the same line in 2013 in commit
> 932e233a491b ("bunzip2: fix off-by-one check").
> 
>
> ...
>
> -			if (dbufCount+t >= dbufSize)
> +			if (dbufCount+t > dbufSize)

Sashiko thinks there's also a potential overflow here which could be
addressed in this patch.

	https://sashiko.dev/#/patchset/20260912-b4-bunzip2-blocksize-fix-v1-1-c7384bbfc954@xxxxxxxxx