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

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Date:   Tue, 28 May 2019 09:02:32 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Deepa Dinamani' <deepa.kernel@...il.com>,
        Oleg Nesterov <oleg@...hat.com>
CC:     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()

From: Deepa Dinamani
> Sent: 24 May 2019 18:02
...
> Maybe a crude userspace application could do something like this:
> 
> sig_handler()
> {
>   set global abort = 1
> }
> 
> poll_the_fds()
> {
>         ret = epoll_pwait()
>         if (ret)
>                 return ret
>         if (abort)
>                 // but this abort should be ignored if ret was 0.
>                 return try_again
> 
> }

As an application writer I'd want to see 'abort == 1' even
if epoll_pwait() returned that an fd was 'ready'.

So the code above should probably be:
    wait_again:
        ret = epoll_pwait();
        if (abort)
            process_abort();
        if (ret <= 0) {
            if (ret == 0)
                process_timeout();
            if (ret == 0 || errno == EINTR)
                goto wait_again;
            // Something went horribly wrong in epoll_pwait()
            return ret;
        }
        // process the 'ready' fds

It would be non-unreasonable for the application to have
all signals blocked except during the epoll_pwait().
So the application needs the signal handler for SIG_INT (etc)
to be called even if one of the fd is always 'ready'.

    David

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