Re: [PATCH 5/8] x86, pkeys: allocation/free syscalls

Michael Kerrisk (man-pages)
2016-06-02T00:11:30Z

Hi Dave,

On 1 June 2016 at 14:32, Dave Hansen <dave-gkUM19QKKo4@public.gmane.org> wrote:
> On 06/01/2016 11:37 AM, Jonathan Corbet wrote:
>>> +static inline
>>> +int mm_pkey_free(struct mm_struct *mm, int pkey)
>>> +{
>>> + /*
>>> + * pkey 0 is special, always allocated and can never
>>> + * be freed.
>>> + */
>>> + if (!pkey || !validate_pkey(pkey))
>>> + return -EINVAL;
>>> + if (!mm_pkey_is_allocated(mm, pkey))
>>> + return -EINVAL;
>>> +
>>> + mm_set_pkey_free(mm, pkey);
>>> +
>>> + return 0;
>>> +}
>>
>> If I read this right, it doesn't actually remove any pkey restrictions
>> that may have been applied while the key was allocated. So there could be
>> pages with that key assigned that might do surprising things if the key is
>> reallocated for another use later, right? Is that how the API is intended
>> to work?
>
> Yeah, that's how it works.
>
> It's not ideal. It would be _best_ if we during mm_pkey_free(), we
> ensured that no VMAs under that mm have that vma_pkey() set. But, that
> search would be potentially expensive (a walk over all VMAs), or would
> force us to keep a data structure with a count of all the VMAs with a
> given key.
>
> I should probably discuss this behavior in the manpages and address it

s/probably//

And, did I miss it. Was there an updated man-pages patch in the latest
series? I did not notice it.

> more directly in the changelog for this patch.

Cheers,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/