linux-kernel - Re: [PATCH v2] signal: Adjust error codes according to restore_user

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Date:   Fri, 24 May 2019 15:29:12 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Deepa Dinamani' <deepa.kernel@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        "dbueso@...e.de" <dbueso@...e.de>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        Davidlohr Bueso <dave@...olabs.net>, Eric Wong <e@...24.org>,
        Jason Baron <jbaron@...mai.com>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        linux-aio <linux-aio@...ck.org>,
        Omar Kilani <omar.kilani@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH v2] signal: Adjust error codes according to
 restore_user_sigmask()

It seems that we all are just trying to confuse each other. I got lost.

On 05/23, David Laight wrote:
>
> From: Oleg Nesterov
> > Sent: 23 May 2019 17:36
> > On 05/23, David Laight wrote:
> > >
> > > From: Oleg Nesterov
> > > > On 05/23, David Laight wrote:
> ...
> > > > Not sure I understand... OK, suppose that you do
> > > >
> > > > 	block-all-signals;
> > > > 	ret = pselect(..., sigmask(SIG_URG));
> > > >
> > > > if it returns success/timeout then the handler for SIG_URG should not be called?
> > >
> > > Ugg...
> > > Posix probably allows the signal handler be called at the point the event
> > > happens rather than being deferred until the system call completes.
> > > Queueing up the signal handler to be run at a later time (syscall exit)
> > > certainly makes sense.
> > > Definitely safest to call the signal handler even if success/timeout
> > > is returned.
> >
> > Why?
> >
> > > pselect() exists to stop the entry race, not the exit one.
> >
> > pselect() has to block SIG_URG again before it returns to user-mode, right?
>
> Yep.
> So the signal handler can't be called for a signal that happens after
> pselect() returns.

Yes. And "after pselect() returns" actually means "after pselect() restores
the old sigmask while it returns to user mode".

> > Suppose pselect() finds a ready fd, and this races with SIG_URG.
>
> You mean if SIG_URG is raised after a ready fd is found (or even timeout)?
> So the return value isn't EINTR.

Yes.

> (If an fd is readable on entry, the SIG_URG could have happened much earlier.)

Why not? See the pseudo code above. It was blocked before pselect() was called.
So SIG_URG can be already pending when pselect() is called but since an fd is
already ready on entry pselect() restores the old sigmask (and thus blocks SIG_URG
again) and returns success. The handler is not called.

However, if there is no a ready fd, pselect won't block. It will notice SIG_URG,
deliver this signal, and return -EINTR.


> > Why do you think the handler should run?
>
> Think of the application code loop.
> Consider what happens if the signal is SIG_INT - to request the program
> stop.

SIG_INT or SIG_URG ? Again, please look at the pseudo code above. SIG_INT is
blocked and never unblocked.

> After every pselect() call the application looks to see if the handler
> has been called.
> If one of the fds is always readable pselect() will never return EINTR
> but you want the SIG_INT handler run so that the loop gets terminated.
> If you only call the signal handler when EINTR is returned the process
> will never stop.
> So you need to call the handler even when pselect() succeeds/time out.

Then do not block SIG_INT ?

	block-all-signals-except-SIG_INT;
	ret = pselect(..., sigmask{SIG_URG, SIG_INT});


> > What if SIG_URG comes right after pselect() blocks SIG_URG again? I mean,
> > how this differs the case when it comes before, but a ready fd was already
> > found?
>
> I suspect you need to defer the re-instatement of the original mask
> to the code that calls the signal handlers (which probably should
> be called with the programs signal mask).

This is what the kernel does when the signal is delivered, the original mask
is restored after the signal handler runs.

> So that particular window doesn't exist.

Which window???

Oleg.