[musl] Re: Thread pointer changes

[Russell King - ARM Linux]

On Fri, Jun 27, 2014 at 11:09:31PM +0200, Szabolcs Nagy wrote:
> i thought the helpers in the kernel can avoid certain memory
> barriers that the userspace has to do on armv6 for atomics
> (and those barriers are deprecated on armv7 so i thought the
> kuser page was better for portable binaries)

The helpers are provided so that libc can be independent of the CPU
facilities in the machine.  The key word there is _libc_, not
applications.

So, a libc can be built to support the lowest architecture that
someone deems to support, and it may make use of the kuser helpers.
If it does, then you have a libc which requires that the kuser
helpers are always provided by the kernel, and the KUSER_HELPERS
option must never be disabled.  If it is disabled, then the libc
will be useless against that kernel.

However, a libc built against modern architectures should not be
making use of the kuser helpers.  We found last year that the
Ubuntu 12.04 glibc did still make use of one kuser helper, and
as such Ubuntu 12.04 also needs KUSER_HELPERS to remain enabled.

The last combination is that the libc is built for modern architectures
without needing any kuser helpers at all.  In this case - and only this
case - the kernel's KUSER_HELPERS option can be disabled should the
system integrator want to increase security.

> > Due to that, any ARMv5 or earlier CPU will always have the kuser helper
> > page.  ARMv6 and later may or may not have the kuser helper page, but
> > there you're really building for a different ABI anyway (VFP-based) and
> > you also know that you have the thread registers.
> 
> so is it expected that the libc makes no attempt to provide
> portable binary interface for armv5 and armv6?

The libc interface that applications make use should not have any
dependence on whether KUSER_HELPERS is enabled or disabled, the
presence of that page should be totally invisible to applications.

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