[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:
Status in “eglibc” source package in Lucid:
Status in “eglibc” source package in Oneiric:
Status in “eglibc” source package in Precise:
Status in “eglibc” source package in Quantal:
In processors with AVX support virtual machines running can cause the program to execute invalid opcodes, thus crashing a running program.
This has been fixed in eglibc in precise. It it present in Lucid, Natty and Oneiric.
A fix can be backported from the cvs-avx-detection.diff patch present in the precise version. This is provided in the below debdiff.
Please see how to reproduce.
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 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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