Re: [PATCH v8 01/14] mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags

[Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>]

On Tue 10-10-17 07:49:01, 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.
> 
> 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-AlSwsSmVLrQ@public.gmane.org>
> Cc: Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>
> Cc: Andy Lutomirski <luto-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
> Suggested-by: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
> Suggested-by: Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
> Signed-off-by: Dan Williams <dan.j.williams-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> ---
>  arch/alpha/include/uapi/asm/mman.h           |    1 +
>  arch/mips/include/uapi/asm/mman.h            |    1 +
>  arch/mips/kernel/vdso.c                      |    2 +
>  arch/parisc/include/uapi/asm/mman.h          |    1 +
>  arch/tile/mm/elf.c                           |    3 +-
>  arch/xtensa/include/uapi/asm/mman.h          |    1 +
>  include/linux/fs.h                           |    2 +
>  include/linux/mm.h                           |    2 +
>  include/linux/mman.h                         |   39 ++++++++++++++++++++++++++
>  include/uapi/asm-generic/mman-common.h       |    1 +
>  mm/mmap.c                                    |   21 ++++++++++++--
>  tools/include/uapi/asm-generic/mman-common.h |    1 +
>  12 files changed, 69 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
> index 3b26cc62dadb..92823f24890b 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 0x3		/* share + validate extension flags */

Just a nit but I'd put definition of MAP_SHARED_VALIDATE close to the
definition of MAP_SHARED and MAP_PRIVATE where it logically belongs (for
all archs).

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f8c10d336e42..5c4c98e4adc9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2133,7 +2133,7 @@ extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned lo
>  
>  extern unsigned long mmap_region(struct file *file, unsigned long addr,
>  	unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,
> -	struct list_head *uf);
> +	struct list_head *uf, unsigned long map_flags);
>  extern unsigned long do_mmap(struct file *file, unsigned long addr,
>  	unsigned long len, unsigned long prot, unsigned long flags,
>  	vm_flags_t vm_flags, unsigned long pgoff, unsigned long *populate,

I have to say I'm not very keen on passing down both vm_flags and map_flags
- vm_flags are almost a subset of map_flags but not quite and the ambiguity
which needs to be used for a particular check seems to open a space for
errors. Granted you currently only care about MAP_DIRECT in ->mmap_validate
and just pass map_flags through mmap_region() so there's no space for
confusion but future checks could do something different. But OTOH I don't
see a cleaner way of avoiding the need to allocate vma flag for something
you need to check down in ->mmap_validate so I guess I'll live with that
and if problems really happen, we may have cleaner idea what needs to be
done.

So overall feel free to add:

Reviewed-by: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>

								Honza
-- 
Jan Kara <jack-IBi9RG/b67k@public.gmane.org>
SUSE Labs, CR