Re: [RFC PATCH v8 2/2] mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags

Jan Kara
2017-09-11T09:47:16Z

On Fri 08-09-17 12:35:13, 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 MAP_SHARED_VALIDATE flag pattern that
> is guaranteed to fail on all legacy mmap implementations.
>
> With this in place new flags can be defined as:
>
> #define MAP_new (MAP_SHARED_VALIDATE | val)

Is this changelog stale? Given MAP_SHARED_VALIDATE will be new mapping
type, I'd expect we define new flags just as any other mapping flags...
I see no reason why MAP_SHARED_VALIDATE should be or'ed to that.

> 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_mask 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>
> ---
> arch/alpha/include/uapi/asm/mman.h | 1 +
> arch/mips/include/uapi/asm/mman.h | 1 +
> arch/parisc/include/uapi/asm/mman.h | 1 +
> arch/xtensa/include/uapi/asm/mman.h | 1 +
> include/linux/fs.h | 1 +
> include/linux/mman.h | 44 ++++++++++++++++++++++++++
> include/uapi/asm-generic/mman-common.h | 1 +
> mm/mmap.c | 10 +++++-
> tools/include/uapi/asm-generic/mman-common.h | 1 +
> 9 files changed, 60 insertions(+), 1 deletion(-)
>
> diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
> index 3b26cc62dadb..c32276c4196a 100644
> --- a/arch/alpha/include/uapi/asm/mman.h
> +++ b/arch/alpha/include/uapi/asm/mman.h
> @@ -14,6 +14,7 @@
> #define MAP_TYPE 0x0f /* Mask for type of mapping (OSF/1 is _wrong_) */
> #define MAP_FIXED 0x100 /* Interpret addr exactly */
> #define MAP_ANONYMOUS 0x10 /* don't use a file */
> +#define MAP_SHARED_VALIDATE (MAP_SHARED|MAP_PRIVATE) /* validate extension flags */

And I'd explicitely define MAP_SHARED_VALIDATE as the first unused value
among mapping types (which is in fact enum embedded inside mapping flags).
I.e. 0x03 on alpha, x86, and probably all other archs - it has nothing to
do with MAP_SHARED|MAP_PRIVATE - it is just another type of the mapping
which happens to have most of the MAP_SHARED semantics...

Honza

--
Jan Kara <jack@suse.com>
SUSE Labs, CR