linux-kernel - Re: [PATCH 1/1] aio: make sure the input "timeout" value is valid

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Date:   Fri, 12 Jan 2018 14:49:33 -0500
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Leizhen \(ThunderTown\)" <thunder.leizhen@...wei.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Benjamin LaHaise <bcrl@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-aio <linux-aio@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Tianhong Ding <dingtianhong@...wei.com>,
        Hanjun Guo <guohanjun@...wei.com>,
        Libin <huawei.libin@...wei.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Deepa Dinamani <deepa.kernel@...il.com>
Subject: Re: [PATCH 1/1] aio: make sure the input "timeout" value is valid

Matthew Wilcox <willy@...radead.org> writes:

> On Thu, Dec 14, 2017 at 11:18:30AM +0800, Leizhen (ThunderTown) wrote:
>> On 2017/12/14 3:31, Matthew Wilcox wrote:
>> > On Wed, Dec 13, 2017 at 11:27:00AM -0500, Jeff Moyer wrote:
>> >> Matthew Wilcox <willy@...radead.org> writes:
>> >>
>> >>> On Wed, Dec 13, 2017 at 09:42:52PM +0800, Zhen Lei wrote:
>> >>>> Below information is reported by a lower kernel version, and I saw the
>> >>>> problem still exist in current version.
>> >>>
>> >>> I think you're right, but what an awful interface we have here!
>> >>> The user must not only fetch it, they must validate it separately?
>> >>> And if they forget, then userspace is provoking undefined behaviour?  Ugh.
>> >>> Why not this:
>> >>
>> >> Why not go a step further and have get_timespec64 check for validity?
>> >> I wonder what caller doesn't want that to happen...
>> I tried this before. But I found some places call get_timespec64 in the following function.
>> If we do the check in get_timespec64, the check will be duplicated.
>> 
>> For example:
>> static long do_pselect(int n, fd_set __user *inp, fd_set __user *outp,
>> ....
>> 	if (get_timespec64(&ts, tsp))
>> 		return -EFAULT;
>> 
>> 	to = &end_time;
>> 	if (poll_select_set_timeout(to, ts.tv_sec, ts.tv_nsec))
>> 
>> int poll_select_set_timeout(struct timespec64 *to, time64_t sec, long nsec)
>> {
>> 	struct timespec64 ts = {.tv_sec = sec, .tv_nsec = nsec};
>> 
>> 	if (!timespec64_valid(&ts))
>> 		return -EINVAL;
>
> The check is only two comparisons!  Why do we have an interface that can
> cause bugs for the sake of saving *two comparisons*?!  Can we talk about
> the cost of a cache miss versus the cost of executing these comparisons?

Any update on this?  Willy, I'd be okay with your get_valid_timespec64
patch if you wanted to formally submit that.

-Jeff