Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1755222AbbBPINR (ORCPT <rfc822;w@1wt.eu>);
	Mon, 16 Feb 2015 03:13:17 -0500
Received: from mx1.redhat.com ([209.132.183.28]:55348 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1754817AbbBPINO (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Mon, 16 Feb 2015 03:13:14 -0500
Date: Mon, 16 Feb 2015 16:12:06 +0800
From: Fam Zheng <famz@redhat.com>
To: "Seymour, Shane M" <shane.seymour@hp.com>
Cc: Jonathan Corbet <corbet@lwn.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, "x86@kernel.org" <x86@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@amacapital.net>,
        David Herrmann <dh.herrmann@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>,
        Miklos Szeredi <mszeredi@suse.cz>,
        David Drysdale <drysdale@google.com>, Oleg Nesterov <oleg@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Vivek Goyal <vgoyal@redhat.com>, Mike Frysinger <vapier@gentoo.org>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Rashika Kheria <rashika.kheria@gmail.com>,
        Hugh Dickins <hughd@google.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Omar Sandoval <osandov@osandov.com>
Subject: Re: [PATCH RFC v3 0/7] epoll: Introduce new syscalls,
 epoll_ctl_batch and epoll_pwait1
Message-ID: <20150216081119.GA9964@cpc-pc.redhat.com>
References: <1423818243-15410-1-git-send-email-famz@redhat.com>
 <20150215150011.0340686c@lwn.net>
 <20150216010224.GA32421@ad.nay.redhat.com>
 <DDB9C85B850785449757F9914A034FCB3BF41130@G9W0766.americas.hpqcorp.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <DDB9C85B850785449757F9914A034FCB3BF41130@G9W0766.americas.hpqcorp.net>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 3092
Lines: 85

Hi Seymour,

On Mon, 02/16 07:25, Seymour, Shane M wrote:
> I found the manual pages really confusing so I had a go at rewriting
> them - there were places in the manual page that didn't match the
> functionality provided by your code as well as I could tell).

Could you point which places don't match the code?

> 
> My apologies for a few formatting issues though. I still don't like
> parts of epoll_pwait1 but it's less confusing than it was.

Any other than the timespec question don't you like?

> 
> You are free to take some or all or none of the changes.
> 
> I did have a question I marked with **** below about what you
> describe and what your code does.
> 

<snip>

>        The timeout member specifies the minimum time that epoll_wait(2) will
>        block. The time spent waiting will be rounded up to the clock
>        granularity. Kernel scheduling delays mean that the blocking
>        interval may overrun by a small amount. Specifying a -1 for either
>        tv_sec or tv_nsec member of the struct timespec timeout will cause
>        causes epoll_pwait1(2) to block indefinitely. Specifying a timeout
>        equal to zero (both tv_sec or tv_nsec member of the struct timespec
>        timeout are zero) causes epoll_wait(2) to return immediately, even
>        if no events are available.
> 
> **** Are you really really sure about this for the -1 stuff? your code copies
> in the timespec and just passes it to timespec_to_ktime:
> 
> +	if (copy_from_user(&p, params, sizeof(p)))
> +		return -EFAULT;
> ...
> +	kt = timespec_to_ktime(p.timeout);
> 
> Compare that to something like the futex syscall which does this:
> 
> 		if (copy_from_user(&ts, utime, sizeof(ts)) != 0)
> 			return -EFAULT;
> 		if (!timespec_valid(&ts))
> 			return -EINVAL;
> 
> 		t = timespec_to_ktime(ts);
> 
> If the timespec is not valid it returns -EINVAL back to user space. With your
> settings of tv_sec and/or tv_usec to -1 are you relying on a side effect of
> the conversion that could break your code in the future if in the unlikely
> event someone changes timespec_to_ktime() and should it be:
> 
> +	if (copy_from_user(&p, params, sizeof(p)))
> +		return -EFAULT;
> +       if ((p.timeout.tv_sec == -1) || (p.timeout.tv_nsec == -1)) {
> +  /* this is off the top of my head no idea if it will compile */
> +		p.timeout.tv_sec = KTIME_SEC_MAX;
> +		p.timeout.tv_nsec = 0;
> +	}
> +       if (!timespec_valid(&p.timeout))
> +       	return -EINVAL;
> ...
> +	kt = timespec_to_ktime(p.timeout);

OK. timespec_valid() is clear about this: negative tv_sec is invalid, so I
don't think accepting -1 from user is the right thing to do.

We cannot do pointer check as ppoll already because the structure is embedded
in epoll_wait_params.

Maybe it's best to use a flags bit (#define EPOLL_PWAIT1_BLOCK 1).  What do you
think?

Fam

<snip>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/