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Date:   Thu, 23 May 2019 18:36:04 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     'Deepa Dinamani' <deepa.kernel@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Arnd Bergmann <arnd@arndb.de>,
        "dbueso@suse.de" <dbueso@suse.de>,
        "axboe@kernel.dk" <axboe@kernel.dk>,
        Davidlohr Bueso <dave@stgolabs.net>, Eric Wong <e@80x24.org>,
        Jason Baron <jbaron@akamai.com>,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        linux-aio <linux-aio@kvack.org>,
        Omar Kilani <omar.kilani@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: Re: [PATCH v2] signal: Adjust error codes according to
 restore_user_sigmask()
Message-ID: <20190523163604.GE23070@redhat.com>
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On 05/23, David Laight wrote:
>
> From: Oleg Nesterov
> > On 05/23, David Laight wrote:
> > >
> > > I'm confused...
> >
> > Me too. To clarify, the current code is obviously buggy, pselect/whatever
> > shouldn't return 0 (or anything else) if it was interrupted and we are going
> > to deliver the signal.
>
> If it was interrupted the return value has to be EINTR.

Yes, and this is what we need to fix.

> Whether any signal handlers are called is a separate matter.

Not really... because in this case we know that the signal will be delivered,

> > 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?

Suppose pselect() finds a ready fd, and this races with SIG_URG.

Why do you think the handler should run?

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?

Oleg.