linux-kernel - Re: [PATCH 3/6] y2038: linux: Provide __clock

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Date:   Sat, 20 Apr 2019 13:21:12 +0200
From:   Lukasz Majewski <lukma@...x.de>
To:     Stepan Golosunov <stepan@...osunov.pp.ru>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        libc-alpha@...rceware.org, Paul Eggert <eggert@...ucla.edu>,
        Joseph Myers <joseph@...esourcery.com>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] y2038: linux: Provide __clock_settime64
 implementation

Hi Stepan,

> 15.04.2019 в 00:08:38 +0200 Lukasz Majewski написал:
> > +# if defined __NR_clock_settime64
> > +  /* Make sure that passed __timespec64 struct pad is 0.  */
> > +  struct __timespec64 ts = *tp;
> > +  ts.tv_pad = 0;
> > +  return INLINE_SYSCALL_CALL (clock_settime64, clock_id, &ts);  
> 
> Isn't kernel supposed to zero out padding on its own?
> At least comment in kernel's get_timespec64 says so:
> 
> 	/* Zero out the padding for 32 bit systems or in compat mode
> */ if (IS_ENABLED(CONFIG_64BIT_TIME) && in_compat_syscall())
> 		kts.tv_nsec &= 0xFFFFFFFFUL;
> 

For ARM (and x86) 32 bit machines I do use following syscalls (like
clock_settime64):
https://elixir.bootlin.com/linux/v5.1-rc4/source/arch/arm/tools/syscall.tbl#L420

which are providing 64 bit time support on 32 bit systems.

Yes. In those systems the upper part (32 bits) of tv_nsec is cleared up
with mask in the kernel. However, I would prefer not to pass random data
to the kernel, and hence I do clear it up explicitly in glibc.

> The code looks buggy though. It fails to zero out the padding in
> 32-bit kernels.

For the 32 bit systems without Y2038 support enabled in glibc - the
clock_settime would be used, which corresponds to sys_clock_settime32()
in the kernel.

> That part is probably broken since
> 98f76206b3350 ("compat: Cleanup in_compat_syscall() callers").
> 
> And, hmm, is CONFIG_64BIT_TIME enabled anywhere?

When I do use clock_settime64 on the glibc side (with _TIME_BITS=64), I
do not need to enable such config in the kernel. 

If the kernel supports this call (5.1+), then use it, otherwise
fallback to clock_settime().

For 64 bit systems, I do not change the execution path.

If you are interested, please look on the following repo (which has
some more commits than those posted to the mailing list):
https://github.com/lmajewski/y2038_glibc/commits/Y2038-2.29-glibc-__clock-internal-struct-timespec-v1

And meta layer for testing.

https://github.com/lmajewski/meta-y2038

Best regards,

Lukasz Majewski

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