[RFC PATCH v2 2/2] ARM: signal: Fix unparseable iwmmxt_sigframe in uc_regspace[]

[Russell King - ARM Linux]

On Tue, Jun 27, 2017 at 06:04:07PM +0100, Dave Martin wrote:
> In kernels with CONFIG_IWMMXT=y running on non-iWMMXt hardware, the
> signal frame can be left partially uninitialised in such a way
> that userspace cannot parse uc_regspace[] safely.  In particular,
> this means that the VFP registers cannot be located reliably in the
> signal frame when a multi_v7_defconfig kernel is run on the
> majority of platforms.
> 
> The cause is that the uc_regspace[] is laid out statically based on
> the kernel config, but the decision of whether to save/restore the
> iWMMXt registers must be a runtime decision.
> 
> To minimise breakage of software that may assume a fixed layout,
> this patch emits a dummy block of the same size as iwmmxt_sigframe,
> for non-iWMMXt threads.  However, the magic and size of this block
> are now filled in to help parsers skip over it.  A new DUMMY_MAGIC
> is defined for this purpose.
> 
> It is probably legitimate (if non-portable) for userspace to
> manufacture its own sigframe for sigreturn, and there is no obvious
> reason why userspace should be required to insert a DUMMY_MAGIC
> block when running on non-iWMMXt hardware, when omitting it has
> worked just fine forever in other configurations.  So in this case,
> sigreturn does not require this block to be present.
> 
> Reported-by: Edmund Grimley-Evans <Edmund.Grimley-Evans at arm.com>
> Signed-off-by: Dave Martin <Dave.Martin at arm.com>

This looks fine to me.  Please drop it in the patch system, thanks.

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