linux-kernel - Re: [RFC PATCH 1/5] signal: Teach sigsuspend to use set_user

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Date:   Thu, 13 Jun 2019 11:43:25 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "'Eric W. Biederman'" <ebiederm@...ssion.com>,
        'Andrew Morton' <akpm@...ux-foundation.org>,
        'Deepa Dinamani' <deepa.kernel@...il.com>,
        "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
        "'arnd@...db.de'" <arnd@...db.de>,
        "'dbueso@...e.de'" <dbueso@...e.de>,
        "'axboe@...nel.dk'" <axboe@...nel.dk>,
        "'dave@...olabs.net'" <dave@...olabs.net>,
        "'e@...24.org'" <e@...24.org>,
        "'jbaron@...mai.com'" <jbaron@...mai.com>,
        "'linux-fsdevel@...r.kernel.org'" <linux-fsdevel@...r.kernel.org>,
        "'linux-aio@...ck.org'" <linux-aio@...ck.org>,
        "'omar.kilani@...il.com'" <omar.kilani@...il.com>,
        "'tglx@...utronix.de'" <tglx@...utronix.de>,
        'Al Viro' <viro@...IV.linux.org.uk>,
        'Linus Torvalds' <torvalds@...ux-foundation.org>,
        "'linux-arch@...r.kernel.org'" <linux-arch@...r.kernel.org>
Subject: Re: [RFC PATCH 1/5] signal: Teach sigsuspend to use set_user_sigmask

On 06/13, David Laight wrote:
>
> I tested NetBSD last night.
> pselect() always calls the signal handlers even when an fd is ready.
> I'm beginning to suspect that this is the 'standards conforming' behaviour.

May be. May be not. I have no idea.

> > The ToG page for pselect() http://pubs.opengroup.org/onlinepubs/9699919799/functions/pselect.html
> > says:
> >     "If sigmask is not a null pointer, then the pselect() function shall replace
> >     the signal mask of the caller by the set of signals pointed to by sigmask
> >     before examining the descriptors, and shall restore the signal mask of the
> >     calling thread before returning."

> > Note that it says 'before examining the descriptors' not 'before blocking'.

And you interpret this as if a pending signal should be delivered in any case,
even if pselect succeeds. Again, perhaps you are right, but to me this is simply
undocumented.

However, linux never did this. Until the commit 854a6ed56839 ("signal: Add
restore_user_sigmask()"). This commit caused regression. We had to revert it.

> > If nothing else the man pages need a note about the standards and portability.

Agreed.

Oleg.