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

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Date:   Thu, 7 Dec 2017 11:14:27 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Matthew Wilcox <willy@...radead.org>,
        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

On Wed, Dec 6, 2017 at 9:46 PM, Michael Ellerman <mpe@...erman.id.au> wrote:
> Matthew Wilcox <willy@...radead.org> writes:
>
>> On Tue, Dec 05, 2017 at 08:54:35PM -0800, Matthew Wilcox wrote:
>>> On Wed, Dec 06, 2017 at 03:51:44PM +1100, Michael Ellerman wrote:
>>> > Cyril Hrubis <chrubis@...e.cz> writes:
>>> >
>>> > > Hi!
>>> > >> > MAP_FIXED_UNIQUE
>>> > >> > MAP_FIXED_ONCE
>>> > >> > MAP_FIXED_FRESH
>>> > >>
>>> > >> Well, I can open a poll for the best name, but none of those you are
>>> > >> proposing sound much better to me. Yeah, naming sucks...
>>> > >
>>> > > Given that MAP_FIXED replaces the previous mapping MAP_FIXED_NOREPLACE
>>> > > would probably be a best fit.
>>> >
>>> > Yeah that could work.
>>> >
>>> > I prefer "no clobber" as I just suggested, because the existing
>>> > MAP_FIXED doesn't politely "replace" a mapping, it destroys the current
>>> > one - which you or another thread may be using - and clobbers it with
>>> > the new one.
>>>
>>> It's longer than MAP_FIXED_WEAK :-P
>>>
>>> You'd have to be pretty darn strong to clobber an existing mapping.
>>
>> I think we're thinking about this all wrong.  We shouldn't document it as
>> "This is a variant of MAP_FIXED".  We should document it as "Here's an
>> alternative to MAP_FIXED".
>>
>> 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".
>>
>> Now, we should implement MAP_REQUIRED as having each architecture
>> define _MAP_NOT_A_HINT, and then #define MAP_REQUIRED (MAP_FIXED |
>> _MAP_NOT_A_HINT), but that's not information to confuse users with.
>>
>> Also, that lets us add a third option at some point that is Yet Another
>> Way to interpret the 'addr' argument, by having MAP_FIXED clear and
>> _MAP_NOT_A_HINT set.
>>
>> I'm not set on MAP_REQUIRED.  I came up with some awful names
>> (MAP_TODDLER, MAP_TANTRUM, MAP_ULTIMATUM, MAP_BOSS, MAP_PROGRAM_MANAGER,
>> etc).  But I think we should drop FIXED from the middle of the name.
>
> 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.

-Kees

-- 
Kees Cook
Pixel Security