# Re: \[PATCH v28 26/32\] x86/cet/shstk: Introduce shadow stack token setup/verify routines

- From: Yu, Yu-cheng \<hidden\>
- Date: 2021-07-23 18:01:40 
- List: linux-arch
- Page: https://ratatoskr.run/linux-arch/2021/07/1456230
- Thread: https://ratatoskr.run/linux-doc/2021/07/1455208/t.md

```text
On 7/22/2021 2:15 PM, Dave Hansen wrote:
> On 7/22/21 1:52 PM, Yu-cheng Yu wrote:
>> +	if (fpregs_state_valid(fpu, smp_processor_id())) {
>> +		rdmsrl(MSR_IA32_PL3_SSP, ssp);
>> +	} else {
>> +		struct cet_user_state *p;
>> +
>> +		/*
>> +		 * When !fpregs_state_valid() and get_xsave_addr() returns
>> +		 * null, XFEAUTRE_CET_USER is in init state.  Shadow stack
>> +		 * pointer is null in this case, so return zero.
>> +		 */
>> +		p = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
>> +		if (p)
>> +			ssp = p->user_ssp;
>> +	}
>> +
>> +	fpregs_unlock();
>
> Why are we even calling into this code if shadow stacks might be
> disabled?  Seems like we should have just errored out long before
> getting here.
>

That is true.  When this function is called, shadow stack is enabled. If get_xsave_addr() returns null, it is possible xstates is messed up. Maybe I can update the comments to explain it?

Thanks,
Yu-cheng
```