tree 38c2fe1d57b68788f79ae018075b246f228310cc
parent 60461b2b23eb57319525a3e00d7ae57e51598ebc
author Lorenz Brun <lorenz@monogon.tech> 1699975104 +0100
committer Lorenz Brun <lorenz@monogon.tech> 1699980243 +0000

treewide: stop using LZ4 for initrd compression

There are two issues at play here: One is a bug in pierrec/lz4 when
using the legacy framing format [1]. This bit us when we hit a broken
size region with CL:2130, taking hours to debug.

The other is the fact that the Linux LZ4 frame format has significant
design issues [2], especially with concatenanted initrds.

The first issue could be fixed by switching to a different LZ4
implementation (we do even have the reference impl in the monorepo) but
there is no API to generate the legacy frame format and things like [3],
a patch carried by Ubuntu to fix more edge cases just do not inspire
confidence in such a solution.

Thus, this CL switches over to using zstd for compressing initrds.

Zstd is slower than LZ4 for decompressing, but it still decompresses at
multiple GB/s per core while having a much better compression ratio.
It also doesn't have any Linux-specific bits and Linux uses the
reference implementation for decoding, which should make it much more
robust. So overall I think this is a good tradeoff.

[1] https://github.com/pierrec/lz4/issues/156
[2] https://github.com/lz4/lz4/issues/956#issuecomment-736705712
[3] https://launchpadlibrarian.net/507407918/0001-unlz4-Handle-0-size-chunks-discard-trailing-padding-.patch

Change-Id: I69cf69f2f361de325f4b39f2d3644ee729643716
Reviewed-on: https://review.monogon.dev/c/monogon/+/2313
Tested-by: Jenkins CI
Reviewed-by: Serge Bazanski <serge@monogon.tech>