Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack

[Matthew Wilcox]

On Fri, Nov 09, 2018 at 09:13:32AM -0800, Dave Hansen wrote:
> On 11/8/18 4:32 PM, Matthew Wilcox wrote:
> >> Now, looking at Yu-cheng's specific example, it doesn't matter.  We've
> >> got 64-bit types and natural 64-bit alignment.  Without __packed, we
> >> need to look out for natural alignment screwing us up.  With __packed,
> >> it just does what it *looks* like it does.
> > The question is whether Yu-cheng's struct is ever embedded in another
> > struct.  And if so, what does the hardware do?
> 
> It's not really.
> 
> +struct cet_user_state {
> +	u64 u_cet;	/* user control flow settings */
> +	u64 user_ssp;	/* user shadow stack pointer */
> +} __packed;
> 
> This ends up embedded in 'struct fpu'.  The hardware tells us what the
> sum of all the sizes of all the state components are, and also tells us
> the offsets inside the larger buffer.
> 
> We double-check that the structure sizes exactly match the sizes that
> the hardware tells us that the buffer pieces are via XCHECK_SZ().
> 
> But, later versions of the hardware have instructions that don't have
> static offsets for the state components (when the XSAVES/XSAVEC
> instructions are used).  So, for those, the structure embedding isn't
> used at *all* since some state might not be present.

But *when present*, this structure is always aligned on an 8-byte
boundary, right?