* Ram Pai:
> On Thu, Nov 08, 2018 at 09:23:35PM +0100, Florian Weimer wrote:
>> * Ram Pai:
>>
>> > Florian,
>> >
>> > I can. But I am struggling to understand the requirement. Why is
>> > this needed? Are we proposing a enhancement to the sys_pkey_alloc(),
>> > to be able to allocate keys that are initialied to disable-read
>> > only?
>>
>> Yes, I think that would be a natural consequence.
>>
>> However, my immediate need comes from the fact that the AMR register can
>> contain a flag combination that is not possible to represent with the
>> existing PKEY_DISABLE_WRITE and PKEY_DISABLE_ACCESS flags. User code
>> could write to AMR directly, so I cannot rule out that certain flag
>> combinations exist there.
>>
>> So I came up with this:
>>
>> int
>> pkey_get (int key)
>> {
>> if (key < 0 || key > PKEY_MAX)
>> {
>> __set_errno (EINVAL);
>> return -1;
>> }
>> unsigned int index = pkey_index (key);
>> unsigned long int amr = pkey_read ();
>> unsigned int bits = (amr >> index) & 3;
>>
>> /* Translate from AMR values. PKEY_AMR_READ standing alone is not
>> currently representable. */
>> if (bits & PKEY_AMR_READ)
>
> this should be
> if (bits & (PKEY_AMR_READ|PKEY_AMR_WRITE))
This would return zero for PKEY_AMR_READ alone.
>> return PKEY_DISABLE_ACCESS;
>
>
>> else if (bits == PKEY_AMR_WRITE)
>> return PKEY_DISABLE_WRITE;
>> return 0;
>> }
It's hard to tell whether PKEY_DISABLE_ACCESS is better in this case.
Which is why I want PKEY_DISABLE_READ.
>> And this is not ideal. I would prefer something like this instead:
>>
>> switch (bits)
>> {
>> case PKEY_AMR_READ | PKEY_AMR_WRITE:
>> return PKEY_DISABLE_ACCESS;
>> case PKEY_AMR_READ:
>> return PKEY_DISABLE_READ;
>> case PKEY_AMR_WRITE:
>> return PKEY_DISABLE_WRITE;
>> case 0:
>> return 0;
>> }
>
> yes.
> and on x86 it will be something like:
> switch (bits)
> {
> case PKEY_PKRU_ACCESS :
> return PKEY_DISABLE_ACCESS;
> case PKEY_AMR_WRITE:
> return PKEY_DISABLE_WRITE;
> case 0:
> return 0;
> }
x86 returns the PKRU bits directly, including the nonsensical case
(PKEY_DISABLE_ACCESS | PKEY_DISABLE_WRITE).
> But for this to work, why do you need to enhance the sys_pkey_alloc()
> interface? Not that I am against it. Trying to understand if the
> enhancement is really needed.
sys_pkey_alloc performs an implicit pkey_set for the newly allocated key
(that is, it updates the PKRU/AMR register). It makes sense to match
the behavior of the userspace implementation.
Thanks,
Florian
