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

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

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

quoted

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

quoted

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

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