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

[Arnd Bergmann <arnd@arndb.de>]

On Fri, Jan 12, 2018 at 8:49 PM, Jeff Moyer <jmoyer@redhat.com> wrote:
> Matthew Wilcox <willy@infradead.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@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?
>
> Any update on this?  Willy, I'd be okay with your get_valid_timespec64
> patch if you wanted to formally submit that.

I had suggested a more complete helper function at some point,
to take care of all combinations of checking/non-checking, 32/64
bit, microsecond/nanosecond, and zeroing/checking the upper 32 bits
of nanoseconds before comparing against 1 billion, but Deepa
thought that was overkill, so I didn't continue that.

For all I can tell, the get_timespec64() helper should almost always
include the check, the one exception I know is utimensat() and related
functions that may encode the special UTIME_NOW and UTIME_OMIT
constants in the nanoseconds.

        Arnd