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

Hi Dave,

On 1 June 2016 at 14:32, Dave Hansen [off-list ref] wrote:

On 06/01/2016 11:37 AM, Jonathan Corbet wrote:

quoted

quoted

+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/