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Date: Thu, 1 Jun 2017 12:16:38 -0400
From: Rich Felker <>
Subject: Re: Use-after-free in __unlock

On Thu, Jun 01, 2017 at 11:57:53AM -0400, Rich Felker wrote:
> Thanks for finding and reporting this. Indeed, __lock and __unlock are
> not made safe for use on dynamic-lifetime objects, and I was wrongly
> thinking they were only used for static-lifetime ones (various
> libc-internal locks).
> For infrequently-used locks, which these seem to be, I see no reason
> to optimize for contention by having a separate waiter count; simply
> having a single atomic int whose states are "unlocked", "locked with
> no waiters", and "locked maybe with waiters" is a simple solution and
> slightly shrinks the relevant structures anyway. It's possible that
> this is the right solution for all places where __lock and __unlock
> are used, but we should probably do a review and see which ones might
> reasonably be subject to high contention (where spurious FUTEX_WAKE is
> very costly).

Quick analysis (based on search for ./-> inside __unlock/UNLOCK args):

Places where __lock/LOCK is used incorrectly:

- pthread struct
- DIR objects (maybe)

And places where it's used correctly:

- tz
- random
- syslog
- static-linked no-free version of malloc
- gettext
- locale
- pthread_atfork
- sem_open
- synccall
- atexit & variants
- open file list

Of the latter, these seem like things you could reasonably hammer
where false contention would be costly:

- tz
- random
- gettext

There are others where you could hammer, but they already involve much
heavier syscalls than futex so it's unlikely to matter what lock
contention tracking method is used.

I actually don't think the current "incorrect" use in DIR objects
matters, since there's no way to observe that thread B can legally
close a dir until _after_ thread A's call that took the lock returns
and thread A synchronizes with thread B. That is, mere use of
__lock/__unlock in dynamic-storage objects is not actually wrong; the
problem is specific to pthread exit lock and the related logic
concerning object lifetime. So a local fix may actually be more


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