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

Matthew Wilcox
2018-11-09T17:17:40Z

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?