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Message-ID: <CAH8yC8=NmM8=4F_ZFsPbwZrBO8-HaA0pNQPxapu=b5jr-KRWgg@mail.gmail.com>
Date: Thu, 6 Oct 2022 16:04:48 -0400
From: Jeffrey Walton <noloader@...il.com>
To: musl@...ts.openwall.com
Subject: Re: Re: MT fork and key_lock in pthread_key_create.c
On Thu, Oct 6, 2022 at 3:21 PM Rich Felker <dalias@...c.org> wrote:
> On Thu, Oct 06, 2022 at 10:02:11AM +0300, Alexey Izbyshev wrote:
> > On 2022-10-06 09:37, Alexey Izbyshev wrote:
> > >...
> > >Also, I looked at how __aio_atfork() handles a similar case with
> > >maplock, and it seems wrong. It takes the read lock and then simply
> > >unlocks it both in the parent and in the child. But if there were
> > >other holders of the read lock at the time of fork(), the lock won't
> > >end up in the unlocked state in the child. It should probably be
> > >completely nulled-out in the child instead.
> > >
> > Looking at aio further, I don't understand how it's supposed to work
> > with MT fork at all. __aio_atfork() is called in _Fork() when the
> > allocator locks are already held. Meanwhile another thread could be
> > stuck in __aio_get_queue() holding maplock in exclusive mode while
> > trying to allocate, resulting in deadlock.
>
> Indeed, this is messy and I don't think it makes sense to be doing
> this at all. The child is just going to throw away the state so the
> parent shouldn't need to synchronize at all, but if we walk the
> multi-level map[] table in the child after async fork, it's possible
> that the contents seen are inconsistent, even that the pointers are
> only half-written or something.
>
> I see a few possible solutions:
>
> 1. Just set map = 0 in the child and leak the memory. This is not
> going to matter unless you're doing multiple generations of fork
> with aio anyway.
This may make security testing and evaluation trickier, like when
using -fanalyze=memory.
I think it is better to work with the tools nowadays.
Jeff
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