linux-kernel - Re: [musl] Re: [PATCH v8 00/38] arm64/gcs: Provide support for GCS in userspace

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Message-ID: <c2426b69-5411-45fe-851a-219e1f4f9d3b@sirena.org.uk>
Date: Thu, 22 Feb 2024 13:57:53 +0000
From: Mark Brown <broonie@...nel.org>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [musl] Re: [PATCH v8 00/38] arm64/gcs: Provide support for GCS
 in userspace

On Wed, Feb 21, 2024 at 07:22:21PM +0000, Edgecombe, Rick P wrote:
> On Wed, 2024-02-21 at 14:06 -0500, dalias@...c.org wrote:

> > It's fine to turn RDSSP into an actual emulated read of the SSP, or
> > at
> > least an emulated load of zero so that uninitialized data is not left
> > in the target register.

> We can't intercept RDSSP, but it becomes a NOP by default. (disclaimer
> x86-only knowledge).

For arm64 we have a separate control GCSCRE0_EL1.nTR for access to
GCSPR_EL0 (our SSP equivalent) we can use.

> > I have not looked at all the instructions that become #UD but I
> > suspect they all have reasonable trivial ways to implement a
> > "disabled" version of them that userspace can act upon reasonably.

> This would have to be thought through functionally and performance
> wise. I'm not opposed if can come up with a fully fleshed out plan. How
> serious are you in pursuing musl support, if we had something like
> this?

Same here, we have to be careful since it's defining ABI in a way that
we don't normally provide ABI but if there's a clear case for doing it
then...

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