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Subject[PATCH 3.16 180/202] mm/mmap.c: expand_downwards: don't require the gap if !vm_prev
3.16.66-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Oleg Nesterov <oleg@redhat.com>

commit 32e4e6d5cbb0c0e427391635991fe65e17797af8 upstream.

expand_stack(vma) fails if address < stack_guard_gap even if there is no
vma->vm_prev. I don't think this makes sense, and we didn't do this
before the recent commit 1be7107fbe18 ("mm: larger stack guard gap,
between vmas").

We do not need a gap in this case, any address is fine as long as
security_mmap_addr() doesn't object.

This also simplifies the code, we know that address >= prev->vm_end and
thus underflow is not possible.

Link: http://lkml.kernel.org/r/20170628175258.GA24881@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Larry Woodman <lwoodman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
mm/mmap.c | 10 +++-------
1 file changed, 3 insertions(+), 7 deletions(-)

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2244,7 +2244,6 @@ int expand_downwards(struct vm_area_stru
unsigned long address)
{
struct vm_area_struct *prev;
- unsigned long gap_addr;
int error;

address &= PAGE_MASK;
@@ -2253,15 +2252,12 @@ int expand_downwards(struct vm_area_stru
return error;

/* Enforce stack_guard_gap */
- gap_addr = address - stack_guard_gap;
- if (gap_addr > address)
- return -ENOMEM;
prev = vma->vm_prev;
- if (prev && prev->vm_end > gap_addr &&
+ /* Check that both stack segments have the same anon_vma? */
+ if (prev && !(prev->vm_flags & VM_GROWSDOWN) &&
(prev->vm_flags & (VM_WRITE|VM_READ|VM_EXEC))) {
- if (!(prev->vm_flags & VM_GROWSDOWN))
+ if (address - prev->vm_end < stack_guard_gap)
return -ENOMEM;
- /* Check that both stack segments have the same anon_vma? */
}

/* We must make sure the anon_vma is allocated. */
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