Re: [HACKERS] Custom compression methods (mac+lz4.h)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Justin Pryzby <pryzby(at)telsasoft(dot)com>
Cc: Dilip Kumar <dilipbalaut(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, David Steele <david(at)pgmasters(dot)net>, Ildus Kurbangaliev <i(dot)kurbangaliev(at)gmail(dot)com>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, Andrey Borodin <x4mmm(at)yandex-team(dot)ru>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: [HACKERS] Custom compression methods (mac+lz4.h)
Date: 2021-03-21 23:52:09
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Justin Pryzby <pryzby(at)telsasoft(dot)com> writes:
> On Sun, Mar 21, 2021 at 07:11:50PM -0400, Tom Lane wrote:
>> I hate to be the bearer of bad news, but this suggests that
>> LZ4_decompress_safe_partial is seriously broken in 1.9.2
>> as well:

> Ouch

Actually, after reading that closer, the problem only affects the
case where the compressed-data-length passed to the function is
a lie. So it shouldn't be a problem for our usage.

Also, after studying the documentation for LZ4_decompress_safe
and LZ4_decompress_safe_partial, I realized that liblz4 is also
counting on the *output* buffer size to not be a lie. So we
cannot pass it a number larger than the chunk's true decompressed
size. The attached patch resolves the issue I'm seeing.

regards, tom lane

Attachment Content-Type Size
fix-partial-lz4-decompression.patch text/x-diff 682 bytes

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