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@xxxxxxxx>
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@xxxxxxxx>
...... 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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