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From: Jeffrey Walton <noloader@gmail.com>
To: musl@lists.openwall.com
Subject: Re: [musl] Re: MT fork and key_lock in pthread_key_create.c
Date: Thu, 6 Oct 2022 16:04:48 -0400	[thread overview]
Message-ID: <CAH8yC8=NmM8=4F_ZFsPbwZrBO8-HaA0pNQPxapu=b5jr-KRWgg@mail.gmail.com> (raw)
In-Reply-To: <20221006192042.GW29905@brightrain.aerifal.cx>

On Thu, Oct 6, 2022 at 3:21 PM Rich Felker <dalias@libc.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

  parent reply	other threads:[~2022-10-06 20:05 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-10-06  6:37 [musl] " Alexey Izbyshev
2022-10-06  7:02 ` [musl] " Alexey Izbyshev
2022-10-06 19:20   ` Rich Felker
2022-10-06 19:50     ` Rich Felker
2022-10-07  1:26       ` Rich Felker
2022-10-07 10:53         ` Alexey Izbyshev
2022-10-07 21:18           ` Rich Felker
2022-10-08 16:07             ` Alexey Izbyshev
2022-10-07  8:18       ` Alexey Izbyshev
2022-10-06 20:04     ` Jeffrey Walton [this message]
2022-10-06 20:09       ` Rich Felker
2022-10-06 18:21 ` [musl] " Rich Felker
2022-10-08  1:36   ` Rich Felker
2022-10-08 17:03     ` Alexey Izbyshev
2022-10-11 17:50       ` Rich Felker

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