Re: [PATCH 01/18] mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags

From: Helge Deller <deller@gmx.de>
To: Matthew Wilcox <willy@infradead.org>,
	Dan Williams <dan.j.williams@intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>, Jan Kara <jack@suse.cz>,
	Ross Zwisler <ross.zwisler@linux.intel.com>,
	Christoph Hellwig <hch@infradead.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
	Linux MM <linux-mm@kvack.org>,
	Linux API <linux-api@vger.kernel.org>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	"Darrick J . Wong" <darrick.wong@oracle.com>,
	Arnd Bergmann <arnd@arndb.de>, Andy Lutomirski <luto@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Florian Weimer <fweimer@redhat.com>,
	John Hubbard <jhubbard@nvidia.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	linux-parisc@vger.kernel.org
Subject: Re: [PATCH 01/18] mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags
Date: Sat, 25 Nov 2017 19:45:21 +0100	[thread overview]
Message-ID: <09f54d38-7cb5-343d-a017-2d71a793d05c@gmx.de> (raw)
In-Reply-To: <20171122195318.GA29485@bombadil.infradead.org>

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 <vbabka@suse.cz> wrote:
>>> On 11/01/2017 04:36 PM, Jan Kara wrote:
>>>> From: Dan Williams <dan.j.williams@intel.com>
>>>>
>>>> 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:
https://patchwork.kernel.org/patch/9970553/

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.

Helge

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