linux-kernel - Re: pselect/etc semantics (Was: [PATCH v2] signal: Adjust error codes according to restore_user

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Date:   Wed, 29 May 2019 11:26:02 -0700
From:   Deepa Dinamani <deepa.kernel@...il.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, dbueso@...e.de, 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
Subject: Re: pselect/etc semantics (Was: [PATCH v2] signal: Adjust error codes
 according to restore_user_sigmask())

On Wed, May 29, 2019 at 9:12 AM Oleg Nesterov <oleg@...hat.com> wrote:
>
> 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);
>
>                 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.

I do not think we discussed this part earlier. But, if this is true
then this is what is wrong as part of 854a6ed56839a. I missed that
before.

> (pselect() can be interrupted by any other signal which has a handler. In this
>  case the handler can be called even if ret >= 0. This is correct, I fail to
>  understand why some people think this is wrong, and in any case we simply can't
>  avoid this).

This patch is wrong because I did not know that it was ok to deliver a
signal and not set the errno before. I also admitted to this. And
proposed another way to revert the patch.:
https://lore.kernel.org/lkml/CABeXuvouBzZuNarmNcd9JgZgvonL1N_p21gat=O_x0-1hMx=6A@mail.gmail.com/

-Deepa