Re: [PATCH 01/17] mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Tue, Oct 24, 2017 at 05:23:58PM +0200, 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.
> 
> It is worth noting that the original proposal was for a standalone
> MAP_VALIDATE flag. However, when that  could not be supported by all
> archs Linus observed:
> 
>     I see why you *think* you want a bitmap. You think you want
>     a bitmap because you want to make MAP_VALIDATE be part of MAP_SYNC
>     etc, so that people can do
> 
>     ret = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED
> 		    | MAP_SYNC, fd, 0);
> 
>     and "know" that MAP_SYNC actually takes.
> 
>     And I'm saying that whole wish is bogus. You're fundamentally
>     depending on special semantics, just make it explicit. It's already
>     not portable, so don't try to make it so.
> 
>     Rename that MAP_VALIDATE as MAP_SHARED_VALIDATE, make it have a value
>     of 0x3, and make people do
> 
>     ret = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED_VALIDATE
> 		    | MAP_SYNC, fd, 0);
> 
>     and then the kernel side is easier too (none of that random garbage
>     playing games with looking at the "MAP_VALIDATE bit", but just another
>     case statement in that map type thing.
> 
>     Boom. Done.
> 
> Similar to ->fallocate() we also want the ability to validate the
> support for new flags on a per ->mmap() 'struct file_operations'
> instance basis.  Towards that end arrange for flags to be generically
> validated against a mmap_supported_flags exported by 'struct
> file_operations'. By default all existing flags are implicitly
> supported, but new flags require MAP_SHARED_VALIDATE and
> per-instance-opt-in.
> 
> Cc: Jan Kara <jack@suse.cz>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Suggested-by: Christoph Hellwig <hch@lst.de>
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Jan Kara <jack@suse.cz>

Looks great.

Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>

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