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@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>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name mm-mmap.c-expand_downwards-don-t-require-the-gap-if-.patch

diff --git a/mm/mmap.c b/mm/mmap.c
index 82b17dacda1d65b59a7b62e0a317f0ce8d641395..66ff511df1b660982f5fdbeec0cfbc1d6528cfae 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2312,7 +2312,6 @@ 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;
@@ -2321,15 +2320,12 @@ 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. */