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SubjectRe: linux-next: Tree for Nov 7
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On 11/13/2017 09:06 AM, Michal Hocko wrote:
> OK, so this one should take care of the backward compatibility while
> still not touching the arch code
> ---
> commit 39ff9bf8597e79a032da0954aea1f0d77d137765
> Author: Michal Hocko <mhocko@suse.com>
> Date: Mon Nov 13 17:06:24 2017 +0100
>
> mm: introduce MAP_FIXED_SAFE
>
> MAP_FIXED is used quite often but it is inherently dangerous because it
> unmaps an existing mapping covered by the requested range. While this
> might be might be really desidered behavior in many cases there are
> others which would rather see a failure than a silent memory corruption.
> Introduce a new MAP_FIXED_SAFE flag for mmap to achive this behavior.
> It is a MAP_FIXED extension with a single exception that it fails with
> ENOMEM if the requested address is already covered by an existing
> mapping. We still do rely on get_unmaped_area to handle all the arch
> specific MAP_FIXED treatment and check for a conflicting vma after it
> returns.
>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
>
> ...... deleted .......
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 680506faceae..aad8d37f0205 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1358,6 +1358,10 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
> if (mm->map_count > sysctl_max_map_count)
> return -ENOMEM;
>
> + /* force arch specific MAP_FIXED handling in get_unmapped_area */
> + if (flags & MAP_FIXED_SAFE)
> + flags |= MAP_FIXED;
> +
> /* Obtain the address to map to. we verify (or select) it and ensure
> * that it represents a valid section of the address space.
> */

Do you need to move this code above:

if (!(flags & MAP_FIXED))
addr = round_hint_to_min(addr);

/* Careful about overflows.. */
len = PAGE_ALIGN(len);
if (!len)
return -ENOMEM;

Not doing that might mean the hint address will end up being rounded for
MAP_FIXED_SAFE which would change the behavior from MAP_FIXED.

--
Khalid

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