Re: [PATCH 1/2] syscalls: avoid time() using __cvdso_gettimeofday in use-level's VDSO

[Vincenzo Frascino <vincenzo.frascino@arm.com>]

Hi Thomas.

On 11/25/20 11:32 AM, Thomas Gleixner wrote:
[...]

>>> Here we propose to use '__NR_time' to invoke syscall directly that makes
>>> test all get real seconds via ktime_get_real_second.
> 
> This is a general problem and not really just for this particular test
> case.
> 
> Due to the internal implementation of ktime_get_real_seconds(), which is
> a 2038 safe replacement for the former get_seconds() function, this
> accumulation issue can be observed. (time(2) via syscall and newer
> versions of VDSO use the same mechanism).
> 
>      clock_gettime(CLOCK_REALTIME, &ts);
>      sec = time();
>      assert(sec >= ts.tv_sec);
> 
> That assert can trigger for two reasons:
> 
>  1) Clock was set between the clock_gettime() and time().
> 
>  2) The clock has advanced far enough that:
> 
>     timekeeper.tv_nsec + (clock_now_ns() - last_update_ns) > NSEC_PER_SEC
> 
> #1 is just a property of clock REALTIME. There is nothing we can do
>    about that.
> 
> #2 is due to the optimized get_seconds()/time() access which avoids to
>    read the clock. This can happen on bare metal as well, but is far
>    more likely to be exposed on virt.
> 
> The same problem exists for CLOCK_XXX vs. CLOCK_XXX_COARSE
> 
>      clock_gettime(CLOCK_XXX, &ts);
>      clock_gettime(CLOCK_XXX_COARSE, &tc);
>      assert(tc.tv_sec >= ts.tv_sec);
> 
> The _COARSE variants return their associated timekeeper.tv_sec,tv_nsec
> pair without reading the clock. Same as #2 above just extended to clock
> MONOTONIC.
> 
> There is no way to fix this except giving up on the fast accessors and
> make everything take the slow path and read the clock, which might make
> a lot of people unhappy.
> 
> For clock REALTIME #1 is anyway an issue, so I think documenting this
> proper is the right thing to do.
> 
> Thoughts?
>

I completely agree with your analysis and the fact that we should document this
information.

My proposal would be to use either the vDSO document present in the kernel [1]
or the man pages for time(2) and clock_gettime(2). Probably the second would be
more accessible to user space developers.

[1] Documentation/ABI/stable/vdso

> Thanks,
> 
>         tglx
> 

-- 
Regards,
Vincenzo