Confused about usercopy_64.c

[Andreas Schwab]

Timur Tabi <timur at 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.

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Andreas Schwab, SuSE Labs, schwab at suse.de
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