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

[Matthew Wilcox <willy@infradead.org>]

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@infradead.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?