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Date:   Sat, 25 Nov 2017 19:45:21 +0100
From:   Helge Deller <>
To:     Matthew Wilcox <>,
        Dan Williams <>
Cc:     Vlastimil Babka <>, Jan Kara <>,
        Ross Zwisler <>,
        Christoph Hellwig <>,
        linux-fsdevel <>,
        "" <>,
        Linux MM <>,
        Linux API <>,
        linux-ext4 <>,
        linux-xfs <>,
        "Darrick J . Wong" <>,
        Arnd Bergmann <>,
        Andy Lutomirski <>,
        Andrew Morton <>,
        Michal Hocko <>,
        Kees Cook <>,
        Florian Weimer <>,
        John Hubbard <>,
        "Kirill A. Shutemov" <>,
Subject: Re: [PATCH 01/18] mm: introduce MAP_SHARED_VALIDATE, a mechanism to
 safely define new mmap flags

On 22.11.2017 20:53, Matthew Wilcox wrote:
> On Wed, Nov 22, 2017 at 08:52:37AM -0800, Dan Williams wrote:
>> On Wed, Nov 22, 2017 at 4:02 AM, Vlastimil Babka <> wrote:
>>> On 11/01/2017 04:36 PM, Jan Kara wrote:
>>>> From: Dan Williams <>
>>>> The mmap(2) syscall suffers from the ABI anti-pattern of not validating
>>>> unknown flags. However, proposals like MAP_SYNC need a mechanism to
>>>> define new behavior that is known to fail on older kernels without the
>>>> support. Define a new MAP_SHARED_VALIDATE flag pattern that is
>>>> guaranteed to fail on all legacy mmap implementations.
>>> So I'm trying to make sense of this together with Michal's attempt for
>>> MAP_FIXED_SAFE [1] where he has to introduce a completely new flag
>>> instead of flag modifier exactly for the reason of not validating
>>> unknown flags. And my conclusion is that because MAP_SHARED_VALIDATE
>>> implies MAP_SHARED and excludes MAP_PRIVATE, MAP_FIXED_SAFE as a
>>> modifier cannot build on top of this. Wouldn't thus it be really better
>>> long-term to introduce mmap3 at this point? ...
>> We have room to define MAP_PRIVATE_VALIDATE in MAP_TYPE on every arch
>> except parisc. Can we steal an extra bit for MAP_TYPE from somewhere
>> else on parisc?
> It looks like 0x08 should work.

I posted an RFC to the parisc mailing list for that:

Basically this is (for parisc only):
-#define MAP_TYPE	0x03		/* Mask for type of mapping */
+#define MAP_TYPE	(MAP_SHARED|MAP_PRIVATE|MAP_RESRVD1|MAP_RESRVD2) /* Mask for type of mapping */
 #define MAP_FIXED	0x04		/* Interpret addr exactly */
+#define MAP_RESRVD1	0x08		/* reserved for 3rd bit of MAP_TYPE */
 #define MAP_ANONYMOUS	0x10		/* don't use a file */
+#define MAP_RESRVD2	0x20		/* reserved for 4th bit of MAP_TYPE */

> But I don't have an HPUX machine around
> to check that HP didn't use that bit for something else.

We completely dropped support for HPUX binaries, so it's not relvant any longer. 

> It'd probably help to cc the linux-parisc mailing list when asking
> questions about PARISC, eh?

Yes, please.


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