Re: [PATCH] pasta: Workaround: wait for execvp() to be done in child before entering netns

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 7 Nov 2022 14:17:44 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Fri, Nov 04, 2022 at 02:53:28AM +0100, Stefano Brivio wrote:
> > This happens about every third time on the two_guests/basic test, and
> > on that test only: we clone() twice, first to spawn a child, then to
> > spawn a thread to check that we can enter the target network
> > namespace.
> > 
> > In this thread, we open a file descriptor associated to the target
> > namespace. It might happen that it doesn't exist yet: the kernel can
> > legitimately take its time to create one, after clone(). In this
> > case, at least on a 5.15 Linux kernel, trying to open that file again
> > always yields EACCES, and we get stuck there.
> > 
> > This only occurs if we spawn two instances of pasta very close
> > together, as it's done in the two_guests/basic case.
> > 
> > I couldn't figure out what the race condition is, yet, and especially
> > if it's a kernel issue or something we're doing wrong. However, if we
> > wait until the execvp() in the child is done, the issue disappears.
> > I'm not sure yet if it's just because of timing and this is hiding an
> > unrelated race condition.
> > 
> > The workaround consists of checking /proc/PID/exe against our own. If
> > it's different, that means execvp() already completed and we can
> > proceed. It's rather ugly, but much better than the alternative.
> > Leave a FIXME there for the moment being.
> > 
> > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>  
> 
> Weird and ugly, but seems like we need it.
> 
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

Sorry, I forgot to mention: I didn't actually push this, because while
repeating that test over and over again I hit the same failure at some
point, even with this trick.

So this is probably just affecting the timing. I need to look into it
again.

-- 
Stefano