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

[Dave Hansen <dave.hansen@intel.com>]

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.