group.of.nepali.translators team mailing list archive

[Po-Hsu Lin <po-hsu.lin@xxxxxxxxxxxxx>]

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

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https://bugs.launchpad.net/bugs/1755627

Title:
  ibrs/ibpb fixes result in excessive kernel logging

Status in linux package in Ubuntu:
  Fix Released
Status in linux source package in Trusty:
  Fix Released
Status in linux source package in Xenial:
  Fix Released
Status in linux source package in Artful:
  Fix Released

Bug description:
  Since at least kernel 4.4.0-116, every invocation of `sysctl -a`
  results in kernel logs similar to the following:

  % sysctl -a &>/dev/null; dmesg -T | tail -8
  [Wed Mar 14 00:06:36 2018] sysctl_ibrs_enabled = 0, sysctl_ibpb_enabled = 0
  [Wed Mar 14 00:06:36 2018] use_ibrs = 4, use_ibpb = 4
  [Wed Mar 14 00:06:36 2018] read cpu 0 ibrs val 0
  [Wed Mar 14 00:06:36 2018] read cpu 1 ibrs val 0
  [Wed Mar 14 00:06:36 2018] sysctl_ibrs_enabled = 0, sysctl_ibpb_enabled = 0
  [Wed Mar 14 00:06:36 2018] use_ibrs = 4, use_ibpb = 4
  [Wed Mar 14 00:06:36 2018] read cpu 0 ibrs val 0
  [Wed Mar 14 00:06:36 2018] read cpu 1 ibrs val 0

  The output varies with the number of CPUs.

  After digging a bit, it turns out this is triggered upon every read of
  `kernel.ibrs_dump`:

  % for i in {1..3}; do sysctl kernel.ibrs_dump; dmesg -T | tail -8; echo; sleep 1; done
  kernel.ibrs_dump = 0
  [Wed Mar 14 00:08:48 2018] sysctl_ibrs_enabled = 0, sysctl_ibpb_enabled = 0
  [Wed Mar 14 00:08:48 2018] use_ibrs = 4, use_ibpb = 4
  [Wed Mar 14 00:08:48 2018] read cpu 0 ibrs val 0
  [Wed Mar 14 00:08:48 2018] read cpu 1 ibrs val 0
  [Wed Mar 14 00:08:48 2018] sysctl_ibrs_enabled = 0, sysctl_ibpb_enabled = 0
  [Wed Mar 14 00:08:48 2018] use_ibrs = 4, use_ibpb = 4
  [Wed Mar 14 00:08:48 2018] read cpu 0 ibrs val 0
  [Wed Mar 14 00:08:48 2018] read cpu 1 ibrs val 0

  kernel.ibrs_dump = 0
  [Wed Mar 14 00:08:49 2018] sysctl_ibrs_enabled = 0, sysctl_ibpb_enabled = 0
  [Wed Mar 14 00:08:49 2018] use_ibrs = 4, use_ibpb = 4
  [Wed Mar 14 00:08:49 2018] read cpu 0 ibrs val 0
  [Wed Mar 14 00:08:49 2018] read cpu 1 ibrs val 0
  [Wed Mar 14 00:08:49 2018] sysctl_ibrs_enabled = 0, sysctl_ibpb_enabled = 0
  [Wed Mar 14 00:08:49 2018] use_ibrs = 4, use_ibpb = 4
  [Wed Mar 14 00:08:49 2018] read cpu 0 ibrs val 0
  [Wed Mar 14 00:08:49 2018] read cpu 1 ibrs val 0

  kernel.ibrs_dump = 0
  [Wed Mar 14 00:08:50 2018] sysctl_ibrs_enabled = 0, sysctl_ibpb_enabled = 0
  [Wed Mar 14 00:08:50 2018] use_ibrs = 4, use_ibpb = 4
  [Wed Mar 14 00:08:50 2018] read cpu 0 ibrs val 0
  [Wed Mar 14 00:08:50 2018] read cpu 1 ibrs val 0
  [Wed Mar 14 00:08:50 2018] sysctl_ibrs_enabled = 0, sysctl_ibpb_enabled = 0
  [Wed Mar 14 00:08:50 2018] use_ibrs = 4, use_ibpb = 4
  [Wed Mar 14 00:08:50 2018] read cpu 0 ibrs val 0
  [Wed Mar 14 00:08:50 2018] read cpu 1 ibrs val 0

  
  Those tests were against an EC2 instance running Ubuntu 4.4.0-116.140-generic 4.4.98 per /proc/version_signature

  Normally this would not be the biggest concern but we have tooling
  that gathers instance info on a schedule, including sysctl output,
  thus resulting in the kernel ring buffer being full of nothing but
  said output in most cases and hindering live troubleshooting as a
  result.

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