Re: [BUG -next] "futex: switch to USER_DS for futex test" breaks s390

[Heiko Carstens <heiko.carstens@de.ibm.com>]

On Fri, Jan 03, 2014 at 04:41:10PM +0100, Andreas Schwab wrote:
> Heiko Carstens <heiko.carstens@de.ibm.com> writes:
> 
> > There is also other code that relies on this: e.g. copy_mount_options() my be
> > called with KERNEL_DS.
> 
> With KERNEL_DS you can *only* access kernel memory, which is unpagable.
> If you want to access user memory, you _must_ use USER_DS.

I didn't say anything else. copy_mount_options() will be called with KERNEL_DS
from e.g. do_mount_root().

> > If DEBUG_PAGEALLOC is turned on, it would crash badly in kernel space
> > if it crosses page boundaries and touches an invalid page, even though
> > it should survive...
> 
> Accessing an invalid page in kernel space is _always_ a bug.

Even though the current futex check relies on working exception handling
for this case.

If the patch I posted gets merged as well, it really doesn't matter for me.

Martin and I discussed this today and we will change the s390 code so that
it will also survive very early USER_DS accesses (without valid current->mm)
since we also discovered a couple of other oddities in our code.

But theses changes would be too complex for -stable, imho.