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

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 11 Aug 2010 21:22:51 -0700 "H. Peter Anvin" <hpa@zytor.com> wrote:

> [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 d__j__ 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.

It occurs so rarely that it's probably not worth bothering about, IMO.

However we should arrange for it to fail at compile time rather than
at link time, please.