Re: [PATCH v6 3/5] mm: introduce mmap3 for safely defining new mmap flags

[Christoph Hellwig]

On Wed, Aug 23, 2017 at 04:48:51PM -0700, Dan Williams wrote:
> The mmap(2) syscall suffers from the ABI anti-pattern of not validating
> unknown flags. However, proposals like MAP_SYNC and MAP_DIRECT need a
> mechanism to define new behavior that is known to fail on older kernels
> without the support. Define a new mmap3 syscall that checks for
> unsupported flags at syscall entry and add a 'mmap_supported_mask' to
> 'struct file_operations' so generic code can validate the ->mmap()
> handler knows about the specified flags. This also arranges for the
> flags to be passed to the handler so it can do further local validation
> if the requested behavior can be fulfilled.

What is the reason to not go with __MAP_VALID hack?  Adding new
syscalls is extremely painful, it will take forever to trickle this
through all architectures (especially with the various 32-bit
architectures having all kinds of different granularities for the
offset) and then the various C libraries, never mind applications.