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Re: Please, use --check=crc32 or switch to a safe format

From: Antonio Diaz Diaz
Subject: Re: Please, use --check=crc32 or switch to a safe format
Date: Fri, 07 Apr 2017 01:17:50 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i586; en-US; rv: Gecko/20110420 SeaMonkey/2.0.14

Michael Stone wrote:
On Wed, Apr 05, 2017 at 04:16:53PM -0300, Matias Fonzo wrote:
There's nothing wrong in want to provide a "second" format, this
increases the chances for accessing the project, inspect, study,

Exactly zero people have reported being unable to access the project
because of xz. This is really getting silly.

You are right that the case of coreutils is not one of access, just of (reduced) safety in the access.

I would say that the lack of integrity checking in lzma-alone was a bad thing, but at least it was a known fact. With xz the situation is kind of deceptive. The user is induced to think that xz is safe (it even provides SHA256!), but depending on how the file was created and what decompressor is available, the integrity check in xz is sometimes performed and sometimes not. In particular, the case of decompressing coreutils tarballs with busybox is comparable to using lzma-alone.

But other projects may have in fact access problems. As Lasse Collin said[1], "XZ Embedded is also very limited. It cannot decompress all .xz files".


At least some projects using optional xz features have already suffered (subtle) problems[2]:

"But speaking as the Squashfs author, the lack of BCJ support for an architecture creates a subtle failure mode in Squashfs, this is because not all blocks in a Squashfs filesystem get compressed with a BCJ filter. At compression time each block is compressed without any BCJ filter, and then with the BCJ filter(s) selected on the command line, and the best compression for *that* block is chosen. What this means is kernels without a particular BCJ filter can still read the Squashfs metadata (mount, ls etc.) and read many of the files, it is only some files that mysteriously fail with decompression error. As such this will be (and has been) invariably treated as a bug in Squashfs."

One advantage of lzip (or gzip, or bzip2) over xz is that even the tiny educational decompressor lzd can decompress and check the integrity of even the largest files created with the mighty plzip on the largest computers in the world. From the point of view of the typical user, the lzip format just works and is safe, no strings attached.

Best regards,

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