[Bug 979003] Re: libc incorrectly detects AVX support

Chris J Arges 979003 at bugs.launchpad.net
Tue Sep 4 16:09:14 UTC 2012

I've built a  do-no-harm fix just for the original AVX bug using the
patch identified in #1. If somebody can verify this on AMD  hardware
(with AVX extension) that would be much appreciated.

Here is the package in my PPA:

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  libc incorrectly detects AVX support

Status in “eglibc” package in Ubuntu:
  Fix Released
Status in “eglibc” source package in Lucid:
Status in “eglibc” source package in Oneiric:
Status in “eglibc” source package in Precise:
  In Progress
Status in “eglibc” source package in Quantal:
  Fix Released

Bug description:
  In processors with AVX support virtual machines running can cause the program to execute invalid opcodes, thus crashing a running program.

  [Development Fix]
  This has been fixed in eglibc in precise. It it present in Lucid, Natty and Oneiric.

  [Stable Fix]
  A fix can be backported from the cvs-avx-detection.diff patch present in the precise version. This is provided in the below debdiff.

  [Test Case]
  Please see how to reproduce.

  [Regression Potential]
  This patch affects amd64 versions of eglibc, and in particular processors that have the AVX extension. This patch adds more complete checks for AVX enablement.


  * Description of the problem:

  libc incorrectly detects if AVX is enabled. On processors with AVX
  support like the Xeon E31270, libc does not check sufficiently to
  determine if AVX is actually enabled. The problem is exhibited on
  virtual machines using the effected version of eglibc where the host
  machine is running Xen and has a AVX capable CPU.

  This bugreport explains the problem well: http://bugs.debian.org/cgi-

  * Versions Affected:

  The problem is in lucid, eglibc-2.11.1-0ubuntu7.10, x86_64.
  The problem is also in current versions of eglibc available for Natty, Maverick and Oneiric.

  The problem is patched upstream in debian unstable eglibc 2.13-22 which made it into precise eglibc 2.13-23ubuntu1:

  There is a patch backported for glibc 2.11 provided by avx-fix.patch here:

  * How to reproduce:

  The problem may only be reproducible on particular hardware that supports AVX being used as a Xen host.
  It is reproducible when trying to start apache. The program exits when execution of an AVX instruction fails.

  Start Apache on a Lucid guest VM where the host machine is running Xen
  and has a Xeon E31270 model cpu.

  * What happens?
  Apache exits with an error:
  apache2[858] trap invalid opcode ip:7ffcebfdf920 sp:7fffc6da6798 error:0 in ld-2.11.1.so[7ffcebfca000+20000]

  * What is expected?
  Apache starts normally.

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