[Bug 1874521] [NEW] lttng-modules 2.10.8-1ubuntu2~18.04.1 ADT test failure with linux-kvm 4.15.0-1059.60

[Launchpad Bug Tracker]

You have been subscribed to a public bug by Stefan Bader (smb):

Testing failed on:
    amd64: https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-bionic/bionic/amd64/l/lttng-modules/20200423_185643_adec8@/log.gz

[SRU Justification]

== Impact ==

In linux 5.6 upstream replaced vmalloc_sync_all() by
vmalloc_sync_mappings() and vmalloc_sync_unmappings(). Lttng uses
vmalloc_sync_all() from a wrapper to prevent recursive page faults. The
call is either made directly in the code or, if kallsyms is available,
by looking up its address and then jumping indirectly.

Recent stable updates backported that change into 4.4, 4.15, 5.3, and
5.4. This affects lttng-modules. Either the build fails completely (if
kallsyms is not enabled). Or there will be warnings at runtime:

  LTTng: vmalloc_sync_all symbol lookup failed.
  Page fault handler and NMI tracing might trigger faults.

== Fix ==

Backporting 4 patches from upstream lttng fixes the issue for 4.4 and
5.4 (as those allow matching against linux kernel version). For 4.15 and
5.3 and additional patch adds Ubuntu specific version ranges.

Note, that this last patch also adds a range for 5.0 which was Disco.
This should not hurt and also indirectly is needed for some backport
kernels in 18.04/Bionic. But I want to try keeping the patches as
similar as possible across releases.

== Testcase ==

Installation of lttng-modules-dkms, depending on kernel configuration either the build will fail or loading lttng-test (or checking dmesg after reboot) will show warnings.
With the changes applied the build succeeds in all cases and no warnings appear.

== Regression Potential ==

Low, without the new function name present, everything would be
unchanged and once the change is there, functionality already is
regressed.

** Affects: lttng-modules (Ubuntu)
     Importance: Medium
         Status: Won't Fix

** Affects: lttng-modules (Ubuntu Xenial)
     Importance: Medium
     Assignee: Stefan Bader (smb)
         Status: In Progress

** Affects: lttng-modules (Ubuntu Bionic)
     Importance: Medium
     Assignee: Stefan Bader (smb)
         Status: In Progress

** Affects: lttng-modules (Ubuntu Eoan)
     Importance: Medium
     Assignee: Stefan Bader (smb)
         Status: In Progress

** Affects: lttng-modules (Ubuntu Focal)
     Importance: Medium
     Assignee: Stefan Bader (smb)
         Status: In Progress


** Tags: kernel-adt-failure patch
-- 
lttng-modules 2.10.8-1ubuntu2~18.04.1 ADT test failure with linux-kvm 4.15.0-1059.60
https://bugs.launchpad.net/bugs/1874521
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