[Bug 1899621] Re: [Ubuntu 20.04] zlib: inflateSyncPoint() returns an incorrect result on z15

[Łukasz Zemczak]

I see that the burp armhf autopkgtest keeps on failing on the new zlib.
I wonder if the burp-unit-test suite is really flaky and the release-
pocket run only passed by chance?

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Title:
  [Ubuntu 20.04] zlib: inflateSyncPoint() returns an incorrect result on
  z15

Status in Ubuntu on IBM z Systems:
  Fix Committed
Status in zlib package in Ubuntu:
  Fix Released
Status in zlib source package in Focal:
  Fix Committed
Status in zlib source package in Groovy:
  Fix Released

Bug description:
  Description:   zlib: inflateSyncPoint() returns an incorrect result on z15
  Symptom:       Certain rsync builds fail with "error in rsync protocol data
                 stream" on z15.
  Problem:       inflateSyncPoint() does not take into account the hardware
                 compression state and returns an incorrect result.
  Solution:      Make inflateSyncPoint() fail if the hardware compression is on.
                 The hardware does not provide enough information in order to
                 implement this function.

  [Impact]

   * Certain rsync builds fail with "error in rsync protocol data
  stream" on z15.

   * On ubuntu, rsync normally uses zstd or lz4. But when rsync is
  forced to use non-default zlib compression (-z flag) it uses
  inflateSyncPoint() API. This can also happen when rsync on the the
  other end doesn't support zstd & lz4.

   * inflateSyncPoint() does not take into account the hardware
  compression state and returns an incorrect result.

   * Make inflateSyncPoint() fail if the hardware compression is on. The
  hardware does not provide enough information in order to implement
  this function.

   * Above makes rsync to succeed, when zlib uses hardware compression.

  [Test Case]

  Reproduction:  z15 only: printf 1 >1 && rsync -z 1 2

  [Regression Potential]

   * inflateSyncPoint API is rarely used. Scanning codesearch, there is
  a lot of false positives due to embedded copies of zlib in all the
  things. I do see that both rsync and backuppc-rsync use it. It used
  during a safety check to ensure that algorithm is not at a sync point
  (or that cannot be determined). Nonetheless, returning error is a
  safer implementation for this API call.

  [Other Info]

   * This is a regression introduced by adding & enabling zlib hw
  acceleration by default on z15; and discovering using rsync that one
  API is implemented incorrectly.

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