Comment 4 for bug 1020210

Created attachment 6833
Reproduction recipe

I reported the following bug in ubuntu's libc about 6 months ago where the ATOMIC_FASTBINS feature can crash or cause heap corruption.

https://bugs.launchpad.net/ubuntu/+source/eglibc/+bug/1020210

I'm pasting the content of that problem here and will attach the reproduction recipe. Unfortunately, the reproduction recipe is only reliable with the exact version of eglibc in ubuntu, given enough time to build an arbitrary version, I should be able to create one that works with any version.

----

We have an application which makes heavy allocation and de-allocation demands from multiple threads. We run this application continuously on many servers, and once every several CPU months or years, we were getting a crash in _int_free that did not look like vanilla heap corruption. I believe I have narrowed it down to a race condition in _int_free due to the ATOMIC_FASTBINS feature. Basically, in the lockless FASTBIN _int_free path, a chunk is pulled into a local variable with the intent to add it to the fastbins list. However, the heap consolidation/trim code can race with this, and can coalesce the entire block and/or give it back to the OS before _int_free has a chance to try and store it into the fastbins list.

The problem is very challenging to reproduce in situ, but using gdb I have a recipe which demonstrates the crash 100% of the time on my 12.04 x64 system running eglibc 2.15. It relies on malloc_trim, although in our in situ data, the consolidation is triggered as a result of a normal free. malloc_trim is just easier to control.

While I am not a glibc developer, I could not see any easy ways to fix the situation shy of disabling ATOMIC_FASTBINS.

I am attaching the reproduction source. Other pertinent information follows:

> jpieper@calculon:~/downloads$ lsb_release -rd
> Description: Ubuntu 12.04 LTS
> Release: 12.04

> jpieper@calculon:~/downloads$ apt-cache policy libc6
> libc6:
> Installed: 2.15-0ubuntu10
> Candidate: 2.15-0ubuntu10
> Version table:
> *** 2.15-0ubuntu10 0
> 500 http://us.archive.ubuntu.com/ubuntu/ precise/main amd64 Packages
> 100 /var/lib/dpkg/status

What I expect: I expect the attached application, when run using the gdb script in the comments, to complete with no failures.
What happened: A SIGSEGV after the final continue.