linux-kernel - Re: generic uaccess.h

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Date:	Fri, 24 Jul 2009 11:20:22 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	monstr@...str.eu
Cc:	Linux Kernel list <linux-kernel@...r.kernel.org>,
	John Williams <john.williams@...alogix.com>
Subject: Re: generic uaccess.h

On Friday 24 July 2009, Michal Simek wrote:
> I have just look at asm-generic uaccess.h and there is one thing which
> seems to me wrong.
> 
> For put_user macro - you use __copy_to_user but you have for 64bit case
> ifdef CONFIG_64BIT
> but  look at fs/eventfd: eventfd_read function. At least for this
> function(syscall) is necessary "return" 64bit
> value on 32bit machines too.
> IMHO that ifdef CONFIG_64BIT shouldn't be there.
> 
> What do you think?
> If you agree with me, I'll generate proper patch with description.

The code was intentional, because 32 bit architectures normally
don't acces u64 values efficiently. I would expect the memcpy
to produce better object code in that case.
Did you see an actual bug in my version or are you only
guessing that the assignment should work better than the
memcpy?

What object code do you get with

int test(unsigned long long __user *out, unsigned long long in)
{
	return put_user(in, ptr);
}

in both cases?

	Arnd <><
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