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Re: Thread safety of coroutine-sigaltstack

From: Laszlo Ersek
Subject: Re: Thread safety of coroutine-sigaltstack
Date: Thu, 21 Jan 2021 14:34:49 +0100

On 01/21/21 10:27, Max Reitz wrote:
> On 20.01.21 18:25, Laszlo Ersek wrote:
>> On 01/20/21 17:26, Max Reitz wrote:
>>> Hi,
>>> I’ve run into trouble with Vladimir’s async backup series on MacOS,
>>> namely that iotest 256 fails with qemu exiting because of a SIGUSR2.
>>> Turns out this is because MacOS (-xcode) uses coroutine-sigaltstack,
>>> when I use this on Linux, I get the same error.
>>> (You can find the series applied on my block branch e.g. here:
>>> https://github.com/XanClic/qemu.git block
>>> )
>>> Some debugging later I found that the problem seems to be two threads
>>> simultaneously creating a coroutine.  It makes sense that this case
>>> would appear with Vladimir’s series and iotest 256, because 256 runs two
>>> backup jobs in two different threads in a transaction, i.e. they’re
>>> launched simultaneously.  The async backup series makes backup use many
>>> concurrent coroutines and so by default launches 64+x coroutines when
>>> the backup is started.  Thus, the case of two coroutines created
>>> concurrently in two threads is very likely to occur.
>>> I think the problem is in coroutine-sigaltstack’s qemu_coroutine_new().
>>> It sets up a SIGUSR2 handler, then changes the signal handling stack,
>>> then raises SIGUSR2, then reverts the signal handling stack and the
>>> SIGUSR2 handler.  As far as I’m aware, setting up signal handlers and
>>> changing the signal handling stack are both process-global operations,
>>> and so if two threads do so concurrently, they will interfere with each
>>> other.
>> Signal action (disposition) is process-wide.
>> Signal mask and signal stack are thread-specific.
> Ah, OK.  Thanks for the insight!
>> A signal may be pending for the whole process, or for a specific thread.
>> In the former case, the signal is delivered to one of the threads that
>> are not blocking the signal.
>>> What usually happens is that one thread sets up everything,
>>> while the other is already in the process of reverting its changes: So
>>> the second thread reverts the SIGUSR2 handler to the default, and then
>>> the first thread raises SIGUSR2, thus making qemu exit.
>> I agree. The way SIGUSR2 is blocked (for the thread), made pending (for
>> the thread), and then allowed to be delivered (consequently, to the
>> thread), looks OK. But by the time it is delivered, the action has been
>> changed.
>>> (Could be worse though.  Both threads could set up the sigaltstack, then
>>> both raise SIGUSR2, and then we get one coroutine_trampoline()
>>> invocation in each thread, but both would use the same stack.  But I
>>> don’t think I’ve ever seen that happen, presumably because the race time
>>> window is much shorter.)
>> No, the "alternate stack for signal handlers" that sigaltstack()
>> configures is thread-specific. (I mean one could theoretically mess it
>> up if the stack were located in the same place between different
>> threads, but we call qemu_alloc_stack(), so that doesn't happen.)
>> https://pubs.opengroup.org/onlinepubs/9699919799/functions/sigaltstack.html
> Explains why I haven’t seen it. :)
>>> Now, this all seems obvious to me, but I’m wondering...  If
>>> coroutine-sigaltstack really couldn’t create coroutines concurrently,
>>> why wouldn’t we have noticed before?  I mean, this new backup case is
>>> kind of a stress test, yes, but surely we would have seen the problem
>>> already, right?  That’s why I’m not sure whether my analysis is correct.
>>> Anyway, I’ve attached a patch that wraps the whole SIGUSR2 handling
>>> section in a mutex, and that makes 256 pass reliably with Vladimir’s
>>> async backup series.  Besides being unsure whether the problem is really
>>> in coroutine-sigaltstack, I also don’t know whether getting out the big
>>> guns and wrapping everything in the mutex is the best solution.  So,
>>> it’s an RFC, I guess.
>> A simple grep for SIGUSR2 seems to indicate that SIGUSR2 is not used by
>> system emulation for anything else, in practice. Is it possible to
>> dedicate SIGUSR2 explicitly to coroutine-sigaltstack, and set up the
>> action beforehand, from some init function that executes on a "central"
>> thread, before qemu_coroutine_new() is ever called?
> Doesn’t sound unreasonable, but wouldn’t the signal handler then have to
> check whether the SIGUSR2 comes from coroutine-sigaltstack or from the
> outside?  Or should we then keep SIGUSR2 blocked all the time?

Blocking SIGUSR2 in all threads at all times, except when temporarily
unblocking it with sigsuspend(), is one approach, but I don't think it
would necessarily be 100% safe against other processes sending SIGUSR2
asynchronously. And IMO that's not even a goal -- sending a signal
requires permission:


    For a process to have permission to send a signal to a process
    designated by pid, unless the sending process has appropriate
    privileges, the real or effective user ID of the sending process
    shall match the real or saved set-user-ID of the receiving process.

(I assume (hope) that SELinux / sVirt further restricts this, so one
qemu process couldn't simply signal another, due to their different labels.)

Thus, when the host kernel permits a different process to generate
SIGUSR2 for QEMU, it's OK to let things just crash and burn. Every other
process with such a permission should *know better* than to send an
unsolicited SIGUSR2 to QEMU.

I mean, what happens if you send an external SIGUSR2 to QEMU right now?
The default action for SIGUSR2 is to terminate the process:


(It's possible that I missed some already existent treatment of SIGUSR2
in QEMU that's different from coroutine-sigaltstack, for example via
sigfillset(). But that seems unlikely, due to the comment on
qemu_init_main_loop() in "include/qemu/main-loop.h" which indicates
SIGUSR2 is currently available to threads for custom use.)

>> ... I've tried to see if POSIX says anything on signals being delivered
>> with mutexen held. I can't find anything specific (the spec seems to
>> talk about delivery of a signal while the thread waits in
>> pthread_mutex_lock(), but that's not what we care about, here). I'm just
>> somewhat uncomfortable with bracketing this whole hackery into a mutex
>> even... Keeping sigaction() out of the picture could be a small
>> performance benefit, too.
> Speaking of signal being delivered in the mutexed section...  What would
> happen if we get an external signal after SIGUSR2 was delivered and
> coroutine_trampoline() set up the sigsetjmp(), but before the stack is
> switched back?  Wouldn’t this new signal then trash the stack?  Should
> we block all signals while using the alternate stack?
> (Looking at the x64 objdump, the stack actually seems to be used to
> store @self across sigsetjmp().)

I wouldn't worry about it. Signals are a crude interface for programs.
If a program documents that a particular signal can be sent to it for a
particular purpose (which implies the asynchronous generation of that
signal of course), then processes that have proper permission to send
that signal are *welcome* to send that signal at *any* time. If the
program mishandles the signal, that's a bug in the signalee.

Conversely, if a signal is not documented like that by the program, but
another process (having the needed permission) still sends the signal,
breakage is expected, and the signaler process is at fault. In my book,
it's no different from sending a signal that is simply neither caught
nor ignored nor blocked by the signalee process, and whose default
disposition is to terminate the process (marked "T" or "A" in the table
linked above). E.g., if you send a SIGILL to a process out of the blue,
the process is totally expected to blow up, or at least to misbehave.

>> The logic in the patch doesn't look broken, but the comments should be
>> updated minimally -- the signal stack is thread-specific (similarly to
>> how a thread has its own stack anyway, regardless of signals).
> Sure, I can do that.
> I agree that there probably are better solutions than to wrap everything
> in a lock.  OTOH, it looks to me like this lock is the most simple
> solution.  If Daniel is right[1] and we should drop
> coroutine-sigaltstack altogether (at some point...), perhaps it is best
> to go for the most simple solution now.
> [1]
> https://lists.nongnu.org/archive/html/qemu-block/2021-01/msg00808.html

SUSv3 marked ucontext functions obsolescent:


and they are entirely missing from SUSv4 (aka the latest POSIX):


So you can use ucontext if you #define _XOPEN_SOURCE as 500 or 600, but
(in theory) not if you #define it as 700. How this works out in practice
on OSX -- i.e. how long they intend to support _XOPEN_SOURCE 600 --, I
can't tell.

I don't disagree with Daniel though; you can always bring back
coroutine-sigaltstack from the git history, if Apple decides to drop

If you went for the mutex for the time being, I wouldn't try to nack it. :)


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