linux-kernel - Re: [PATCH RFC v3 0/7] epoll: Introduce new syscalls, epoll_ctl_batch and epoll

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Message-ID: <20150216081119.GA9964@cpc-pc.redhat.com>
Date:	Mon, 16 Feb 2015 16:12:06 +0800
From:	Fam Zheng <famz@...hat.com>
To:	"Seymour, Shane M" <shane.seymour@...com>
Cc:	Jonathan Corbet <corbet@....net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Andy Lutomirski <luto@...capital.net>,
	David Herrmann <dh.herrmann@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	David Drysdale <drysdale@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Vivek Goyal <vgoyal@...hat.com>,
	Mike Frysinger <vapier@...too.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Rashika Kheria <rashika.kheria@...il.com>,
	Hugh Dickins <hughd@...gle.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	Josh Triplett <josh@...htriplett.org>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Omar Sandoval <osandov@...ndov.com>
Subject: Re: [PATCH RFC v3 0/7] epoll: Introduce new syscalls,
 epoll_ctl_batch and epoll_pwait1

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>
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