linux-kernel - Re: [PATCH] x86: unify copy_from

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Message-Id: <5260241602000078000FBDD9@nat28.tlf.novell.com>
Date:	Thu, 17 Oct 2013 16:53:26 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Arjan van de Ven" <arjan@...ux.intel.com>
Cc:	<mingo@...e.hu>, <tglx@...utronix.de>, <linux@...ck-us.net>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH] x86: unify copy_from_user() checking

>>> On 17.10.13 at 17:45, Arjan van de Ven <arjan@...ux.intel.com> wrote:
> for me, the value of the feature overall is this range checking, not the 
> fixed size part.
> for fixed size... the chance of the programmer getting it wrong is near 
> zero.
> the chance of getting one of the checks wrong is much higher
> (we've had cases of wrong sign in the checks, off by ones in the checks etc)
> and that is what it was supposed to find.
> If that's not possible due practical issues (like the inline case above but 
> more
> the compiler practicalities).... removing the warning part entirely is 
> likely just better.

But it would at least cover the case where, for some pointer,
someone mixes up sizeof(ptr) and sizeof(*ptr). So I think - it
being cheap - the current constant size check could stay, ...

> Having a runtime check for the case where the argument is not constant but 
> we know the buffer
> size... is likely still clear value... cheap (perfect branch prediction 
> unless disaster hits!)
> and the failure case is obviously the disaster case.

... and the non-constant case be taken care of at run time.
That's precisely what the patch does.

Jan

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