Re: pselect/etc semantics (Was: [PATCH v2] signal: Adjust error codes according to restore_user_sigmask())

[Eric Wong]

David Laight <David.Laight@xxxxxxxxxx> wrote:
> From: Oleg Nesterov
> > Sent: 29 May 2019 17:12
> > Al, Linus, Eric, please help.
> > 
> > The previous discussion was very confusing, we simply can not understand each
> > other.
> > 
> > To me everything looks very simple and clear, but perhaps I missed something
> > obvious? Please correct me.
> > 
> > I think that the following code is correct
> > 
> > 	int interrupted = 0;
> > 
> > 	void sigint_handler(int sig)
> > 	{
> > 		interrupted = 1;
> > 	}
> > 
> > 	int main(void)
> > 	{
> > 		sigset_t sigint, empty;
> > 
> > 		sigemptyset(&sigint);
> > 		sigaddset(&sigint, SIGINT);
> > 		sigprocmask(SIG_BLOCK, &sigint, NULL);
> > 
> > 		signal(SIGINT, sigint_handler);
> > 
> > 		sigemptyset(&empty);	// so pselect() unblocks SIGINT
> > 
> > 		ret = pselect(..., &empty);
>                                 ^^^^^ sigint
> > 
> > 		if (ret >= 0)		// sucess or timeout
> > 			assert(!interrupted);
> > 
> > 		if (interrupted)
> > 			assert(ret == -EINTR);
> > 	}
> > 
> > IOW, if pselect(sigmask) temporary unblocks SIGINT according to sigmask, this
> > signal should not be delivered if a ready fd was found or timeout. The signal
> > handle should only run if ret == -EINTR.
> 
> Personally I think that is wrong.
> Given code like the above that has:
> 		while (!interrupted) {
> 			pselect(..., &sigint);
> 			// process available data
> 		}
> 
> You want the signal handler to be executed even if one of the fds
> always has available data.
> Otherwise you can't interrupt a process that is always busy.

Agreed...  I believe cmogstored has always had a bug in the way
it uses epoll_pwait because it failed to check interrupts if:

a) an FD is ready + interrupt
b) epoll_pwait returns 0 on interrupt

The bug remains in userspace for a), which I will fix by adding
an interrupt check when an FD is ready.  The window is very
small for a) and difficult to trigger, and also in a rare code
path.

The b) case is the kernel bug introduced in 854a6ed56839a40f
("signal: Add restore_user_sigmask()").

I don't think there's any disagreement that b) is a kernel bug.

So the confusion is for a), and POSIX is not clear w.r.t. how
pselect/poll works when there's both FD readiness and an
interrupt.

Thus I'm inclined to believe *select/*poll/epoll_*wait should
follow POSIX read() semantics:

       If a read() is interrupted by a signal before it reads any data, it shall
       return â1 with errno set to [EINTR].

       If  a  read()  is  interrupted by a signal after it has successfully read
       some data, it shall return the number of bytes read.

> One option is to return -EINTR if a signal is pending when the mask
> is updated - before even looking at anything else.
>
> Signals that happen later on (eg after a timeout) need not be reported
> (until the next time around the loop).

I'm not sure that's necessary and it would cause delays in
signal handling.