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Re: [PATCH 2/2] y2038: linux: Provide __clock_getres64 implementation


Hi Joseph,

> On Fri, 18 Oct 2019, Lukasz Majewski wrote:
> 
> > When working on systems still supporting 32 bit time (__TIMESIZE !=
> > 64) without Y2038 time support the clock_getres returns error when
> > received tv_sec excess time_t range (i.e. overflow 32 bit type).
> > Moreover, the correctness of tv_nsec is checked.  
> 
> It's definitely not necessary to check if the kernel returned a valid 
> tv_nsec value; that can be assumed if the syscall succeeded at all.

Ok.

> I'm doubtful about the check for overflow in this case (a clock whose 
> resolution exceeds 68 years does not seem a very useful clock), but
> that check is at least theoretically relevant.

We can skip those two checks altogether (for clock_getres) and instead
use valid_timespec64_to_timespec() conversion helper function. 
(then we would rely on kernel providing valid range or return syscall
with error).

> 
> A stronger justification for the helper macro in patch 1/2 would be
> if you have a patch that uses it to check a timespec64 value coming
> from the *user* rather than from the kernel (that is, coming from the
> caller to one of the __*64 functions that will end up being directly
> called by code built with _TIME_BITS=64).

Yes, I do agree. One most apparent example is the clock_nanosleep
conversion, which would need such check on passed *request value (const
struct __timespec64 *request).

> 
> If a function needs to check the nanoseconds value (rather than
> deferring to a kernel check of that value), I'd expect it to need to
> so such a check regardless of whether it needs to convert 64-bit time
> to 32-bit time.  For example, that's what __clock_settime64 does -
> checks nanoseconds before calling into the kernel at all.  So it's
> not clear to me that there is actually a use case for a macro that
> combines a check of the nanoseconds value with a conversion from
> 64-bit to 32-bit time.

Ok.

> 
> There *is* definitely a use for a macro such as the
> IS_VALID_NANOSECONDS macro in the first patch.  Perhaps it would make
> sense to factor out and send a patch that just adds that macro and
> makes existing code with such checks use it.  (See io/ppoll.c,
> nptl/sem_clockwait.c, nptl/sem_timedwait.c,
> sysdeps/htl/pt-cond-timedwait.c, sysdeps/htl/pt-mutex-timedlock.c,
> sysdeps/htl/pt-rwlock-timedrdlock.c,
> sysdeps/htl/pt-rwlock-timedwrlock.c, sysdeps/htl/sem-timedwait.c,
> sysdeps/mach/nanosleep.c, sysdeps/pthread/timer_settime.c,
> sysdeps/sparc/sparc32/lowlevellock.c, sysdeps/unix/clock_nanosleep.c,
> sysdeps/unix/clock_settime.c,
> sysdeps/unix/sysv/linux/clock_settime.c, time/clock_nanosleep.c for
> examples of files with checks that look like they could use such a
> macro - not necessarily an exhaustive list.  Note that a few of those
> files use __builtin_expect, thus suggesting that the macro definition
> could use __glibc_likely to reflect that nanoseconds values almost
> certainly are valid in real use.)
> 

If we rely on kernel, then the IS_VALID_NANOSECONDS is not required for
clock_getres(). However, it would be beneficial to have it as static
inline function to be used in glibc.



Best regards,

Lukasz Majewski

--

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