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Date: Wed, 17 Feb 2016 22:03:59 -0500
From: Daniel Micay <>
Subject: Re: Address Sanitizer local root

On Wed, 2016-02-17 at 17:24 -0800, Konstantin Serebryany wrote:
> Sadly MPX is too slow, too memory-hungry, and does not protect from
> use-after-free at all.

MPX is definitely problematic (performance, memory usage, false
positives with some atomic data structures, false positives without
using it everywhere - essentially a new ABI) but I don't think the lack
of coverage for lifetime issues is a major issue.

The malloc implementation can do a good job at mitigating lifetime
issues though. It can't detect 100% of UAF issues, but it can force
usage of pointers to fault (via proper junk filling) and detect write
after free via a comparable quarantine technique + validating that the
junk data is unaltered when allocations leave the quarantine. It can be
just as good at detecting double-free.

See the follow-up email:

It's extremely painful to actually debug the aborts and faults produced
from this kind of hardening, so it doesn't really displace ASan at all
even for the bits where it can be as reliable, and it doesn't cover the
read-after-free case in the same way.
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