linux-kernel - Re: [PATCH 0/2] mm: introduce MAP_FIXED

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Date:   Fri, 08 Dec 2017 22:08:07 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Matthew Wilcox <willy@...radead.org>,
        Kees Cook <keescook@...omium.org>
Cc:     Cyril Hrubis <chrubis@...e.cz>, Michal Hocko <mhocko@...nel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Florian Weimer <fweimer@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
        Joel Stanley <joel@....id.au>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE

Matthew Wilcox <willy@...radead.org> writes:

> On Thu, Dec 07, 2017 at 11:14:27AM -0800, Kees Cook wrote:
>> On Wed, Dec 6, 2017 at 9:46 PM, Michael Ellerman <mpe@...erman.id.au> wrote:
>> > Matthew Wilcox <willy@...radead.org> writes:
>> >> So, just like we currently say "exactly one of MAP_SHARED or MAP_PRIVATE",
>> >> we could add a new paragraph saying "at most one of MAP_FIXED or
>> >> MAP_REQUIRED" and "any of the following values".
>> >
>> > MAP_REQUIRED doesn't immediately grab me, but I don't actively dislike
>> > it either :)
>> >
>> > What about MAP_AT_ADDR ?
>> >
>> > It's short, and says what it does on the tin. The first argument to mmap
>> > is actually called "addr" too.
>> 
>> "FIXED" is supposed to do this too.
>> 
>> Pavel suggested:
>> 
>> MAP_ADD_FIXED
>> 
>> (which is different from "use fixed", and describes why it would fail:
>> can't add since it already exists.)
>> 
>> Perhaps "MAP_FIXED_NEW"?
>> 
>> There has been a request to drop "FIXED" from the name, so these:
>> 
>> MAP_FIXED_NOCLOBBER
>> MAP_FIXED_NOREPLACE
>> MAP_FIXED_ADD
>> MAP_FIXED_NEW
>> 
>> Could be:
>> 
>> MAP_NOCLOBBER
>> MAP_NOREPLACE
>> MAP_ADD
>> MAP_NEW
>> 
>> and we still have the unloved, but acceptable:
>> 
>> MAP_REQUIRED
>> 
>> My vote is still for "NOREPLACE" or "NOCLOBBER" since it's very
>> specific, though "NEW" is pretty clear too.
>
> How about MAP_NOFORCE?

It doesn't tell me that addr is not a hint. That's a crucial detail.

Without MAP_FIXED mmap never "forces/replaces/clobbers", so why would I
need MAP_NOFORCE if I don't have MAP_FIXED?

So it needs something in there to indicate that the addr is not a hint,
that's the only thing that flag actually *does*.


If we had a time machine, the right set of flags would be:

  - MAP_FIXED:   don't treat addr as a hint, fail if addr is not free
  - MAP_REPLACE: replace an existing mapping (or force or clobber)

But the two were conflated for some reason in the current MAP_FIXED.

Given we can't go back and fix it, the closest we can get is to add a
variant of MAP_FIXED which subtracts the "REPLACE" semantic.

ie: MAP_FIXED_NOREPLACE

cheers