mm/mmap: Fix error return in do_vmi_align_munmap()

If mas_store_gfp() in the gather loop failed, the 'error' variable that
ultimately gets returned was not being set. In many cases, its original
value of -ENOMEM was still in place, and that was fine. But if VMAs had
been split at the start or end of the range, then 'error' could be zero.

Change to the 'error = foo(); if (error) goto …' idiom to fix the bug.

Also clean up a later case which avoided the same bug by *explicitly*
setting error = -ENOMEM right before calling the function that might
return -ENOMEM.

In a final cosmetic change, move the 'Point of no return' comment to
*after* the goto. That's been in the wrong place since the preallocation
was removed, and this new error path was added.

Fixes: 606c812 ("mm/mmap: Fix error path in do_vmi_align_munmap()")
Signed-off-by: David Woodhouse <[email protected]>
Cc: [email protected]
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>

mm/mmap.c

@@ -2387,7 +2387,8 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
  }
  vma_start_write(next);
  mas_set_range(&mas_detach, next->vm_start, next->vm_end - 1);
- if (mas_store_gfp(&mas_detach, next, GFP_KERNEL))
+ error = mas_store_gfp(&mas_detach, next, GFP_KERNEL);
+ if (error)
  goto munmap_gather_failed;
  vma_mark_detached(next, true);
  if (next->vm_flags & VM_LOCKED)
@@ -2436,12 +2437,12 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
  BUG_ON(count != test_count);
  }
 #endif
- /* Point of no return */
- error = -ENOMEM;
  vma_iter_set(vmi, start);
- if (vma_iter_clear_gfp(vmi, start, end, GFP_KERNEL))
+ error = vma_iter_clear_gfp(vmi, start, end, GFP_KERNEL);
+ if (error)
  goto clear_tree_failed;
 
+ /* Point of no return */
  mm->locked_vm -= locked_vm;
  mm->map_count -= count;
  /*