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[Launchpad Bug Tracker <1854054@xxxxxxxxxxxxxxxxxx>]

This bug was fixed in the package cachefilesd -
0.10.10-0.1ubuntu0.18.04.1

---------------
cachefilesd (0.10.10-0.1ubuntu0.18.04.1) bionic; urgency=medium

  * Avoid counter underflow, leading to segfault (LP: #1854054)

 -- Dan Streetman <ddstreet@xxxxxxxxxxxxx>  Tue, 26 Nov 2019 08:01:23
-0500

** Changed in: cachefilesd (Ubuntu Bionic)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1854054

Title:
  nr_in_ready_table and nr_in_build_table can underflow in if statement

Status in cachefilesd package in Ubuntu:
  Fix Released
Status in cachefilesd source package in Xenial:
  Invalid
Status in cachefilesd source package in Bionic:
  Fix Released
Status in cachefilesd source package in Disco:
  Fix Committed
Status in cachefilesd source package in Eoan:
  Fix Released
Status in cachefilesd source package in Focal:
  Fix Released
Status in cachefilesd package in Debian:
  New

Bug description:
  [impact]

  the build_cull_table() function scans through elements up to
  nr_in_ready_table/nr_in_build_table, and performs actions if a match
  was found; however the match detection logic simply compares the for
  loop index against nr_in_*_table - 1, which underflows when 0,
  resulting in incorrect if section being run and then segfaulting.

  [test case]

  this is difficult to reproduce and it's unclear the specific
  conditions that can reproduce it, but it has been reported to happen
  and review of the code shows it clearly could happen.

  [regression potential]

  this simply moves the -1 over to the for loop counter as a +1, so the
  most likely regression would be a for loop counter overflow.  However
  that should not happen as the culltable_size is limited to 4096, and
  the for loop counter is unsigned int; so it should be safe from
  overflow.  Any other regression would likely involve a similar result
  as the current bug, a segfault.

  [other info]

  this bug does not exist in Xenial, as the counters there are signed
  ints, so underflow (from 0) does not happen.

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