Re: [RFC PATCH 0/2] ARM: Fix unparseable signal frame with CONFIG_IWMMXT

On Wed, Jun 21, 2017 at 04:46:01PM +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.
> 
> I don't know whether any userspace has implemented any sort of
> workaround for this, but the ABI by itself is insufficient anyway.
> 
> This series attempts to omit the spurious iWMMXt record when
> appropriate.
> 
> Not extensively tested, and the ABI impact is unknown for now.

Hmm, I would actually suggest that we poke in a correct size for the
missing iWMMXt record, and an invalid magic number as the "simple"
solution for this - that doesn't make any layout changes to the
data structures, and is probably the safest solution for backporting.

Going forward, I think something along the lines of your proposal is
okay.

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