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 1be7107 ("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/[email protected] Signed-off-by: Oleg Nesterov <[email protected]> Acked-by: Michal Hocko <[email protected]> Cc: Hugh Dickins <[email protected]> Cc: Larry Woodman <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
olerem · Jul 10, 2017 · 32e4e6d · 32e4e6d
1 parent 561b5e0
commit 32e4e6d
Showing 1 changed file with 3 additions and 7 deletions.
diff --git a/mm/mmap.c b/mm/mmap.c
@@ -2316,7 +2316,6 @@ int expand_downwards(struct vm_area_struct *vma,
 {
 	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_struct *vma,
 		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. */