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

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Message-ID: <CAKgNAkiyD_2tAxrBxirxViViMUsfLRRqQp5HowM58dG21LAa7Q@mail.gmail.com>
Date:	Wed, 1 Jun 2016 19:11:30 -0500
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Dave Hansen <dave@...1.net>
Cc:	Jonathan Corbet <corbet@....net>,
	lkml <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Linux API <linux-api@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 5/8] x86, pkeys: allocation/free syscalls

Hi Dave,

On 1 June 2016 at 14:32, Dave Hansen <dave@...1.net> 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/