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From: Andreas Schwab <schwab@suse.de>
To: Timur Tabi <timur@freescale.com>
Subject: Re: Confused about usercopy_64.c
References: <47D56419.6000704@freescale.com>
Date: Mon, 10 Mar 2008 17:53:31 +0100
In-Reply-To: <47D56419.6000704@freescale.com> (Timur Tabi's message of "Mon\,
	10 Mar 2008 11\:38\:49 -0500")
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Timur Tabi <timur@freescale.com> writes:

> I'm confused about something in usercopy_64.c:
>
> unsigned long copy_from_user(void *to, const void __user *from, unsigned long n)
> {
> 	if (likely(access_ok(VERIFY_READ, from, n)))
> 		n = __copy_from_user(to, from, n);
> 	else
> 		memset(to, 0, n);
> 	return n;
> }
>
> If access_ok() returns false, then that means that we cannot copy the data from
> user-space.  So why are we returning 'n'?

To tell the caller that the function failed.  The result is the number
of bytes _left_ to be copying, ie. zero means success.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
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