* Confused about usercopy_64.c
@ 2008-03-10 16:38 Timur Tabi
2008-03-10 16:50 ` Olof Johansson
2008-03-10 16:53 ` Andreas Schwab
0 siblings, 2 replies; 3+ messages in thread
From: Timur Tabi @ 2008-03-10 16:38 UTC (permalink / raw)
To: linuxppc-dev
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'? Shouldn't we return zero, to let the
caller know that the function failed?
--
Timur Tabi
Linux kernel developer at Freescale
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Confused about usercopy_64.c
2008-03-10 16:38 Confused about usercopy_64.c Timur Tabi
@ 2008-03-10 16:50 ` Olof Johansson
2008-03-10 16:53 ` Andreas Schwab
1 sibling, 0 replies; 3+ messages in thread
From: Olof Johansson @ 2008-03-10 16:50 UTC (permalink / raw)
To: Timur Tabi; +Cc: linuxppc-dev
On Mon, Mar 10, 2008 at 11:38:49AM -0500, Timur Tabi wrote:
> 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'? Shouldn't we return zero, to let the
> caller know that the function failed?
copy_from_user() returns number of bytes _not_ copied.
-Olof
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Confused about usercopy_64.c
2008-03-10 16:38 Confused about usercopy_64.c Timur Tabi
2008-03-10 16:50 ` Olof Johansson
@ 2008-03-10 16:53 ` Andreas Schwab
1 sibling, 0 replies; 3+ messages in thread
From: Andreas Schwab @ 2008-03-10 16:53 UTC (permalink / raw)
To: Timur Tabi; +Cc: linuxppc-dev
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
PGP key fingerprint = 58CA 54C7 6D53 942B 1756 01D3 44D5 214B 8276 4ED5
"And now for something completely different."
^ permalink raw reply [flat|nested] 3+ messages in thread
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