mm/fremap.c: fix possible oops on error path

The vm_flags introduced in 6d7825b10dbe ("mm/fremap.c: fix oops on error
path") is supposed to avoid a compiler warning about unitialized
vm_flags without changing the generated code.

However I am concerned that this is going to be very brittle, and fail
with some compiler versions. The failure could be either of:

- compiler could actually load vma->vm_flags before checking for the
  !vma condition, thus reintroducing the oops

- compiler could optimize out the !vma check, since the pointer just got
  dereferenced shortly before (so the compiler knows it can't be NULL!)

I propose reversing this part of the change and initializing vm_flags to 0
just to avoid the bogus uninitialized use warning.

Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: Tommi Rantala <tt.rantala@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/fremap.c b/mm/fremap.c
index 6a8da7ee85fd58e7d0857a185239ef6af57a61d8..4723ac8d2fc200d5990f830b6f4c8c0fd2a9bbcf 100644 (file)
--- a/mm/fremap.c
+++ b/mm/fremap.c
@@ -129,7 +129,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
        struct vm_area_struct *vma;
        int err = -EINVAL;
        int has_write_lock = 0;
-       vm_flags_t vm_flags;
+       vm_flags_t vm_flags = 0;
 
        if (prot)
                return err;
@@ -163,8 +163,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
         * and that the remapped range is valid and fully within
         * the single existing vma.
         */
-       vm_flags = vma->vm_flags;
-       if (!vma || !(vm_flags & VM_SHARED))
+       if (!vma || !(vma->vm_flags & VM_SHARED))
                goto out;
 
        if (!vma->vm_ops || !vma->vm_ops->remap_pages)