commit 2d1c89a5d7c872a1109768f50e2508cf9a4b0348
Author: Florian Weimer <fweimer@redhat.com>
Date:   Wed Jun 20 09:45:19 2018 +0200

    libio: Avoid ptrdiff_t overflow in IO_validate_vtable

    If the candidate pointer is sufficiently far away from
    __start___libc_IO_vtables, the result might not fit into ptrdiff_t.

diff --git a/libio/libioP.h b/libio/libioP.h
index b60244ac5fc3d908..f1576381500ffc85 100644
--- a/libio/libioP.h
+++ b/libio/libioP.h
@@ -957,8 +957,8 @@ IO_validate_vtable (const struct _IO_jump_t *vtable)
   /* Fast path: The vtable pointer is within the __libc_IO_vtables
      section.  */
   uintptr_t section_length = __stop___libc_IO_vtables - __start___libc_IO_vtables;
-  const char *ptr = (const char *) vtable;
-  uintptr_t offset = ptr - __start___libc_IO_vtables;
+  uintptr_t ptr = (uintptr_t) vtable;
+  uintptr_t offset = ptr - (uintptr_t) __start___libc_IO_vtables;
   if (__glibc_unlikely (offset >= section_length))
     /* The vtable pointer is not in the expected section.  Use the
        slow path, which will terminate the process if necessary.  */