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

Dave Hansen
2018-11-08T21:48:54Z

On 11/8/18 1:22 PM, Andy Lutomirski wrote:
>> +struct cet_kernel_state {
>> + u64 kernel_ssp; /* kernel shadow stack */
>> + u64 pl1_ssp; /* ring-1 shadow stack */
>> + u64 pl2_ssp; /* ring-2 shadow stack */
>> +} __packed;
>> +
> Why are these __packed? It seems like it'll generate bad code for no
> obvious purpose.

It's a hardware-defined in-memory structure. Granted, we'd need a
really wonky compiler to make that anything *other* than a nicely-packed
24-byte structure, but the __packed makes it explicit.

It is probably a really useful long-term thing to stop using __packed
and start using "__hw_defined" or something that #defines down to __packed.