From: Oleg Nesterov <oleg@xxxxxxxxxx>
Subject: mm/mmap.c: expand_downwards: don't require the gap if !vm_prev
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@xxxxxxxxxx
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Larry Woodman <lwoodman@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/mmap.c | 10 +++-------
1 file changed, 3 insertions(+), 7 deletions(-)
diff -puN mm/mmap.c~expand_downwards-dont-require-the-gap-if-vm_prev mm/mmap.c
--- a/mm/mmap.c~expand_downwards-dont-require-the-gap-if-vm_prev
+++ a/mm/mmap.c
@@ -2316,7 +2316,6 @@ int expand_downwards(struct vm_area_stru
{
struct mm_struct *mm = vma->vm_mm;
struct vm_area_struct *prev;
- unsigned long gap_addr;
int error;
address &= PAGE_MASK;
@@ -2325,15 +2324,12 @@ 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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