Re: [PATCH 1/2] mm: introduce MAP_FIXED_SAFE

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Wed, Dec 13, 2017 at 10:25:49AM +0100, Michal Hocko wrote:
> +++ b/mm/mmap.c
> @@ -1342,6 +1342,10 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>  		if (!(file && path_noexec(&file->f_path)))
>  			prot |= PROT_EXEC;
>  
> +	/* force arch specific MAP_FIXED handling in get_unmapped_area */
> +	if (flags & MAP_FIXED_SAFE)
> +		flags |= MAP_FIXED;
> +
>  	if (!(flags & MAP_FIXED))
>  		addr = round_hint_to_min(addr);
>  

We're up to 22 MAP_ flags now.  We'll run out soon.  Let's preserve half
of a flag by giving userspace the definition:

#define MAP_FIXED_SAFE	(MAP_FIXED | _MAP_NOT_HINT)

then in here:

	if ((flags & _MAP_NOT_HINT) && !(flags & MAP_FIXED))
		return -EINVAL;

Now we can use _MAP_NOT_HINT all by itself in the future to mean
something else.