Shadow Stack Locking Semantics between arch's

Howdy folks,

If you haven't seen, I have been floating some patches to bring x86-64 
shadow stack controls over to prctl, and it's generally straight forward.
The larger motivation, is that I am doing all of this so we can place 
LSM controls on shadow stack manipulations. For x86 will we need to hook
the old and new implementations.

I have noticed some semantic differences between the arches. Given this 
example of current thread state, current thread locking state and the
new features requested, as shown below:

unsigned long locked = 0x2; // Kernel Task State -> LOCK WRITE
unsigned long cur_val = 0x3; // Kernel Task State -> WRITE and 
SHADOWSTACK ENABLED
unsigned long new_val = 0x0; // Userspace Feature Change via syscall -> 
DISABLE

| x86-64 | risc-v | arm64 |
| ---------- | -------- | --------- |
| Works  | Fails  | Fails     |

Besides the locking difference, I have also noticed a few other issues 
noted below:

Risc-v:
- shouldn't reject all bits in locking, it supports shadow stack.
- shouldn't return EINVAL on locking checks

See the code snippets for these below:

if (is_shstk_locked(t))
                 return -EINVAL;

int arch_lock_shadow_stack_status(struct task_struct *task,
                                   unsigned long arg)
{
         /* If shtstk not supported or not enabled on task, nothing to 
lock here */
         if (!is_user_shstk_enabled() ||
             !is_shstk_enabled(task) || arg != 0)
                 return -EINVAL;

         set_shstk_lock(task, true);

         return 0;
}

arm64:
1. Locking failures return -EBUSY vs -EPERM, I spoke with Mark (on CC) 
he seemed OK with this,

I would like to find a way to rectify this so userspace has a common API.

I am proposing and have questions over the following:
1. What should the behavior be if you had write locked and disable the 
shadow stack?
   - I can argue both ways here, -EPERM or success. I think I and most 
arches lead to failure.
2. riscv should check that the low bit is set in locking not just that 
its 0, it should be 1
3. riscv should return -EPERM vs -EINVAL
4. x86 locking state should fail

If we can all agree on item 1, that locked bits check can be refactored 
and shared in one of
two ways:
1. within prctl itself, before the arch hook is called, we would need 
helpers per-arch to extract the thread features and lock bits
2. as a helper where folks just pass the unsigned long of the bits to 
get the result

Thanks everyone


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