linux-kernel - Re: mmap syscall problem

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Message-Id: <200907061705.50308.arnd@arndb.de>
Date:	Mon, 6 Jul 2009 17:05:50 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	michal.simek@...alogix.com
Cc:	Paul Mundt <lethal@...ux-sh.org>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	LTP <ltp-list@...ts.sourceforge.net>,
	John Williams <john.williams@...alogix.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	subrata@...ux.vnet.ibm.com
Subject: Re: mmap syscall problem

On Monday 06 July 2009, Michal Simek wrote:
> >       *p1 = 0xaa; *p2 = 0x55;
> >   
> I closed fd too.
>     close(fd);
> >       return *p1; /* returns 0xaa if broken, 0x55 if correct */
> > }

If you close the fd between the assignment and reading
from the pointer again, the test case becomes invalid because of
timing. Closing the fd before the '*p1 = 0xaa' should be fine,
but unnecessary.

I also realized that you might need to mark the pointers
as 'volatile' so that the compiler has to do the operations
in order.

> >   
> # ls -la existing-4k-file
> -rw-rw-r--    1 monstr   monstr       4096 Jul  6  2009 existing-4k-file
> 
> # ./test-arnd
> # echo $?
> 85

Ok, so inside a single task, this does not happen.

> # dd if=existing-4k-file of=/dev/console count=1 2>/dev/null
> U#
> 
> in file is first char U (0x55) which is IMO correct.

Right, though that was not part of the test, I'd expect this in
the file even if the return value was broken.

	Arnd <><
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