Public bug reported:
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.
** Affects: linux (Ubuntu)
Importance: Undecided
Status: Incomplete
** Tags: xenial
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https://bugs.launchpad.net/bugs/1755627
Title:
ibrs/ibpb fixes result in excessive kernel logging
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