[musl] Re: Thread pointer changes

[Russell King - ARM Linux]

On Fri, Jun 27, 2014 at 05:37:43PM -0400, Rich Felker wrote:
> On Fri, Jun 27, 2014 at 12:27:45PM -0700, Andy Lutomirski wrote:
> > Hi ARM people and Kees-
> > 
> > The "vectors" page appears to be an abomination that's a lot like the
> > x86_64 vsyscall page.  IMO it should be phased out.
> 
> I'm not a fan of this extreme approach, but if it's taken, there needs
> to be some way to continue to make universal binaries which work
> safely on:
> 
> - Pre-v6, v6, and v7+ hardware.

And that is to make use of the kuser page.

> - Pre-removal and post-removal kernels.

Please, don't get stuff confused.

The kuser page isn't going anywhere _except_ for one situation: where
the system integrator has determined that they have built their
userspace not to require the kuser helpers, and wish to have greater
security and performance through doing so.

What we *aren't* doing with the kernel is removing the page entirely
for ARMv7+ CPUs.  We are merely giving system integrators the option
to increase security /provided/ their userspace is built appropriately.

If the userspace requires the kuser helpers, then the page *must* be
provided by the kernel.

It is a system integrator/distro bug to provide a kernel with the kuser
helpers disabled, but then to provide a libc which requires the page.

The system integrator has a choice to make:

1. Have the kuser helper page, have libc support for it, and have
   compatibility for all ARM CPUs - but have the (minor) security
   issues raised last year over the page.

or

2. Disable the kuser helper page on later CPUs which provide /all/
   the facilities userspace needs in hardware, and build for a
   minimum of those CPUs.

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