linux-kernel - Re: compat_sys_times() bogus until jiffies >= 0.

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Message-ID: <20071108060029.GB3307@old.davidb.org>
Date:	Wed, 7 Nov 2007 22:00:29 -0800
From:	David Brown <lkml@...idb.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Ulrich Drepper <drepper@...hat.com>,
	Michael Kerrisk <mtk-manpages@....net>
Subject: Re: compat_sys_times() bogus until jiffies >= 0.

On Wed, Nov 07, 2007 at 03:28:33PM -0800, Andrew Morton wrote:
>> On Wed, 7 Nov 2007 14:47:22 -0800 David Brown <lkml@...idb.org> wrote:

>> will return '-1' to user space and set the negated clock_t value to errno.
>>
>> At minimum, perhaps it should return a sane errno value.
>
>RETURN VALUE
>       times()  returns  the  number of clock ticks that have elapsed since an
>       arbitrary point in the past.  For Linux 2.4 and earlier this  point  is
>       the  moment  the  system  was  booted.   Since Linux 2.6, this point is
>       (2^32/HZ) - 300 (i.e., about 429 million) seconds  before  system  boot
>       time.   The  return  value  may  overflow  the  possible  range of type
>       clock_t.  On error, (clock_t) -1 is returned, and errno is  set  appro-
>       priately.

The strange -1 behavior is enshrined in history.  I think a better answer
is to tell people to use getrusage() if they want a return result without
this problem.

Adding INITIAL_JIFFIES will fix the case where an embedded system is booted
up to run a test and then shut down, and the mask, although it causes
discontinuities periodically at least moves them away from the early boot.

INITIAL_JIFFIES was a good idea, but it is probably best to keep it inside
of the kernel.

David Brown
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