On 11/25/2017 07:45 PM, Helge Deller wrote:

On 22.11.2017 20:53, Matthew Wilcox wrote:

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On Wed, Nov 22, 2017 at 08:52:37AM -0800, Dan Williams wrote:

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On Wed, Nov 22, 2017 at 4:02 AM, Vlastimil Babka [off-list ref] wrote:

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On 11/01/2017 04:36 PM, Jan Kara wrote:

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From: Dan Williams <redacted>

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/

Thanks. BTW there doesn't seem to be much interest making MAP_FIXED_SAFE
a flag modifier after all, so MAP_PRIVATE_VALIDATE wouldn't get
immediate users.

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 */

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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. 

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

Yes, please.

Helge