Re: [PATCH v6 3/4] vfs: Add timestamp_truncate() api

[Arnd Bergmann <arnd@arndb.de>]

On Tue, Jan 23, 2018 at 5:25 PM, Deepa Dinamani <deepa.kernel@gmail.com> wrote:
> On Mon, Jan 22, 2018 at 12:25 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>> On Sun, Jan 21, 2018 at 6:04 PM, Deepa Dinamani <deepa.kernel@gmail.com> wrote:
>>> +               t.tv_nsec -= t.tv_nsec % gran;
>>
>> This doesn't actuall ywork if tv_nsec is negative.
>
> Right.
>
>> Which may not be an issue in most cases, but did somebody check
>> utimensat() or whatever?
>
> I checked POSIX again. There is no mention of tv_nsec being positive
> always for utimes.
> And, the long term plan is to replace all the callers of
> timespec_trunc() to use this new api instead for filesystems.
> So this will need to be fixed. I will fix this and post an update.

I found this on
http://pubs.opengroup.org/onlinepubs/9699919799/functions/utimes.html

ERRORS
These functions shall fail if:
...
[EINVAL]
 Either of the times argument structures specified a tv_nsec value that was
 neither UTIME_NOW nor UTIME_OMIT, and was a value less than zero or
 greater than or equal to 1000 million.

which is the same as the Linux man page and what the kernel actually
does for all the syscalls. The POSIX description seems a bit ambiguous
to whether it also expects or allows EINVAL for utimes() with a tv_usec
over 1000000 microseconds, or if it just applies to the utimensat and
futimens(). Older descriptions that only explain utimes() don't mention
the range check on tv_usec either.

          Arnd