[PATCH v6] arm: Preserve the user r/w register TPIDRURW on context, switch and fork

[Will Deacon]

On Mon, Jun 17, 2013 at 11:13:48PM +0100, André Hentschel wrote:
> From: André Hentschel <nerv at dawncrow.de>
> 
> Since commit 6a1c53124aa1 the user writeable TLS register was zeroed to
> prevent it from being used as a covert channel between two tasks.
> 
> There are more and more applications coming to Windows RT,
> Wine could support them, but mostly they expect to have
> the thread environment block (TEB) in TPIDRURW.
> 
> This patch preserves that register per thread instead of clearing it.
> Unlike the TPIDRURO, which is already switched, the TPIDRURW
> can be updated from userspace so needs careful treatment in the case that we
> modify TPIDRURW and call fork(). To avoid this we must always read
> TPIDRURW in copy_thread.
> 
> Signed-off-by: André Hentschel <nerv at dawncrow.de>
> Signed-off-by: Will Deacon <will.deacon at arm.com>
> Signed-off-by: Jonathan Austin <jonathan.austin at arm.com> 
> 
> ---
> This patch is against Linux 3.10-rc6 (7d132055814ef17a6c7b69f342244c410a5e000f)
> 
> v2: rework and fixup of v1, based on a suggested patch by Will Deacon
> v3: total rework and fixup of v2
> v4: removed condition on assembler instruction,
>     adapted my code to kernel-style, both based on comments by Will Deacon
> v5: rebased v4 on 3.10-rc2 and adding this version history
> v6: moved loading the TLS registers to the macros
>     (fixing the "LDRD is not supported on all the CPUs we have" problem)

You've changed quite a lot with this version, including the way the macro
parameters are passed. Why not just replace the problematic ldrd with two
ldr instructions and be done with it? I don't think the simple build error
warrants an overhaul of the code we already had.

Cheers,

Will