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Message-ID: <20221007012654.GZ29905@brightrain.aerifal.cx>
Date: Thu, 6 Oct 2022 21:26:54 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: Re: MT fork and key_lock in pthread_key_create.c
On Thu, Oct 06, 2022 at 03:50:54PM -0400, Rich Felker wrote:
> On Thu, Oct 06, 2022 at 03:20:42PM -0400, Rich Felker wrote:
> > On Thu, Oct 06, 2022 at 10:02:11AM +0300, Alexey Izbyshev wrote:
> > > On 2022-10-06 09:37, Alexey Izbyshev wrote:
> > > >Hi,
> > > >
> > > >I noticed that fork() doesn't take key_lock that is used to protect
> > > >the global table of thread-specific keys. I couldn't find mentions of
> > > >this lock in the MT fork discussion in the mailing list archive. Was
> > > >this lock overlooked?
> > > >
> > > >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.
> >
> > 2. The same, but be a little bit smarter. pthread_rwlock_tryrdlock in
> > the child, and if it succeeds, we know the map is consistent so we
> > can just zero it out the same as now. Still "leaks" but only on
> > contention to expand the map.
> >
> > 3. Getting a little smarter still: move the __aio_atfork for the
> > parent side from _Fork to fork, outside of the critical section
> > where malloc lock is held. Then proceed as in (2). Now, the
> > tryrdlock is guaranteed to succeed in the child. Leak is only
> > possible when _Fork is used (in which case the child context is an
> > async signal one, and thus calling any aio_* that would allocate
> > map[] again is UB -- note that in this case, the only reason we
> > have to do anything at all in the child is to prevent close from
> > interacting with aio).
> >
> > After writing them out, 3 seems like the right choice.
>
> Proposed patch attached.
> diff --git a/src/aio/aio.c b/src/aio/aio.c
> index fa24f6b6..4c3379e1 100644
> --- a/src/aio/aio.c
> +++ b/src/aio/aio.c
> @@ -401,11 +401,25 @@ void __aio_atfork(int who)
> if (who<0) {
> pthread_rwlock_rdlock(&maplock);
> return;
> + } else if (!who) {
> + pthread_rwlock_unlock(&maplock);
> + return;
> }
> - if (who>0 && map) for (int a=0; a<(-1U/2+1)>>24; a++)
> + if (pthread_rwlock_tryrdlock(&maplock)) {
> + /* Obtaining lock may fail if _Fork was called nor via
> + * fork. In this case, no further aio is possible from
> + * child and we can just null out map so __aio_close
> + * does not attempt to do anything. */
> + map = 0;
> + return;
> + }
> + if (map) for (int a=0; a<(-1U/2+1)>>24; a++)
> if (map[a]) for (int b=0; b<256; b++)
> if (map[a][b]) for (int c=0; c<256; c++)
> if (map[a][b][c]) for (int d=0; d<256; d++)
> map[a][b][c][d] = 0;
> - pthread_rwlock_unlock(&maplock);
> + /* Re-initialize the rwlock rather than unlocking since there
> + * may have been more than one reference on it in the parent.
> + * We are not a lock holder anyway; the thread in the parent was. */
> + pthread_rwlock_init(&maplock, 0);
> }
> diff --git a/src/process/_Fork.c b/src/process/_Fork.c
> index da063868..fb0fdc2c 100644
> --- a/src/process/_Fork.c
> +++ b/src/process/_Fork.c
> @@ -14,7 +14,6 @@ pid_t _Fork(void)
> pid_t ret;
> sigset_t set;
> __block_all_sigs(&set);
> - __aio_atfork(-1);
> LOCK(__abort_lock);
> #ifdef SYS_fork
> ret = __syscall(SYS_fork);
> @@ -32,7 +31,7 @@ pid_t _Fork(void)
> if (libc.need_locks) libc.need_locks = -1;
> }
> UNLOCK(__abort_lock);
> - __aio_atfork(!ret);
> + if (!ret) __aio_atfork(1);
> __restore_sigs(&set);
> return __syscall_ret(ret);
> }
> diff --git a/src/process/fork.c b/src/process/fork.c
> index ff71845c..80e804b1 100644
> --- a/src/process/fork.c
> +++ b/src/process/fork.c
> @@ -36,6 +36,7 @@ static volatile int *const *const atfork_locks[] = {
> static void dummy(int x) { }
> weak_alias(dummy, __fork_handler);
> weak_alias(dummy, __malloc_atfork);
> +weak_alias(dummy, __aio_atfork);
> weak_alias(dummy, __ldso_atfork);
>
> static void dummy_0(void) { }
> @@ -50,6 +51,7 @@ pid_t fork(void)
> int need_locks = libc.need_locks > 0;
> if (need_locks) {
> __ldso_atfork(-1);
> + __aio_atfork(-1);
> __inhibit_ptc();
> for (int i=0; i<sizeof atfork_locks/sizeof *atfork_locks; i++)
> if (*atfork_locks[i]) LOCK(*atfork_locks[i]);
> @@ -75,6 +77,7 @@ pid_t fork(void)
> if (ret) UNLOCK(*atfork_locks[i]);
> else **atfork_locks[i] = 0;
> __release_ptc();
> + if (ret) __aio_atfork(0);
> __ldso_atfork(!ret);
> }
> __restore_sigs(&set);
There's at least one other related bug here: when __aio_get_queue has
to take the write lock, it does so without blocking signals, so close
called from a signal handler that interrupts will self-deadlock.
This is fixable by blocking signals for the write-lock critical
section, but there's still a potentially long window where we're
blocking forward progress of other threads.
I think the right thing to do here is add another lock for updating
the map that we can take while we still hold the rdlock. After taking
this lock, we can re-inspect if any work is needed. If so, do all the
work *still holding only the rdlock*, but not writing the results into
the existing map. After all the allocation is done, release the
rdlock, take the wrlock (with signals blocked), install the new
memory, then release all the locks. This makes it so the wrlock is
only hold momentarily, under very controlled conditions, so we don't
have to worry about messing up AS-safety of close.
As an alternative, the rwlock could be removed entirely in favor of
maing the whole map[] atomic. This has the amusing effect or making
what's conceptually one of the worst C types ever to appear in real
code:
static struct aio_queue *volatile *volatile *volatile *volatile *volatile map;
although it would actually have to be declared void *volatile map;
with lots of type conversions to navigate through it, in order to
respect type rules for a_cas_p. Then all that would be needed is one
normal lock governing modifications.
Given that this code *tries* to mostly use high-level synchronization
primitives, my leaning would be not to do that.
FWIW, note that all of this was a problem irrespective of MT-fork. So
thankfully it was not a hellish unforseen consequence of adopting
MT-fork, just an existing bug.
Rich
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