Armaan Sandhu
87d3c04672
decompress_startswith() decodes just enough of a blob to compare its leading bytes against a prefix, and grows the output buffer whenever a round of decoding came back with less than prefix_len + 1 bytes decoded. That growth is only the right answer while the decoder actually wants more output room. If it instead ran out of input, i.e. the stream is truncated, then no amount of extra room will help, and the gzip and bzip2 paths double their allocation on every iteration until the process is out of memory. Neither decoder lets us tell the two cases apart by return code alone: inflate() reports the stalled stream as Z_BUF_ERROR, which we cannot simply refuse since that is also how it asks for a bigger output buffer, and BZ2_bzDecompress() does not distinguish them at all, it just keeps saying BZ_OK. The output space left over does tell us, though: a decoder that stopped without filling the buffer we gave it is waiting for input that is not coming, so report the stream as corrupt instead of growing. A 10 byte gzip header, or a 4 byte bzip2 header, with nothing behind it was enough to reach a single 4 GiB allocation this way. The xz and zstd paths already terminate, the former because liblzma turns a stalled stream into LZMA_BUF_ERROR. Note that no caller can reach this today: the only non-test caller is decompress_startswith_journal(), and compression_supported_journal() admits only NONE/XZ/LZ4/ZSTD. It is a trap for whoever wires up the other two, so let's close it now.
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