linux-kernel - Re: + drivers-acpi-apei-erst-dbgc-get_useru64-doesnt-work-on-i386.patch added to -mm tree

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Message-ID: <4C63771B.6030005@zytor.com>
Date:	Wed, 11 Aug 2010 21:22:51 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Randy Dunlap <randy.dunlap@...cle.com>,
	linux-kernel@...r.kernel.org, gcosta@...hat.com, lenb@...nel.org,
	mingo@...e.hu, tglx@...utronix.de, ying.huang@...el.com,
	Linux Arch Mailing List <linux-arch@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: + drivers-acpi-apei-erst-dbgc-get_useru64-doesnt-work-on-i386.patch
 added to -mm tree

[Adding Linux and linux-arch.  The context is that get_user/put_user
don't work on 64 bit values on i386.]

On 08/11/2010 05:33 PM, Andrew Morton wrote:
> 
> Anyway, this should be fixed in x86 core, I suspect.

After looking at it -- and suffering a bad case of déjà vu -- I'm
reluctant to change it, as get/put_user are specified to work only on
locally atomic data:

 * This macro copies a single simple variable from user space to kernel
 * space.  It supports simple types like char and int, but not larger
 * data types like structures or arrays.

Given that u64 is not a simple type on 32 bits, it would appear that the
behavior is intentional.

A user might very well find that supporting u64 and/or structure types
would be beneficial, but it would a) be a semantic change, and b) would
introduce the possibility of a partially completed transfer.  That is a
semantic change to the interface.  However, it may very well be nicer to
have a generally available get_user()/put_user() for the cases which
would just kick an EFAULT up the stack when they fail anyway.

If there is consensus for making get_user/put_user a general interface,
I'm more than willing to do the x86 changes, but I don't want to do them
a) unilaterally and b) for 2.6.36.  This seems like .37 material at this
point.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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