cmp: avoid reading uninitialized memory

[This *is* useful, so reapply. ]
When comparing buffers a word at a time, cmp could read up to
sizeof (word) - 1 uninitialized bytes.
* src/cmp.c (cmp): Set not just a single guaranteed-differing
sentinel byte just beyond any final read byte, but also ensure
that any following bytes are defined, if those may be read via
block_compare's word-at-a-time comparison.  Reported by Bruno Haible
in https://lists.gnu.org/r/diffutils-devel/2021-07/msg00003.html

1 files changed, 5 insertions, 0 deletions

diff --git a/src/cmp.c b/src/cmp.c
index 65a20db..9953d14 100644
--- a/src/cmp.c
+++ b/src/cmp.c
@@ -463,6 +463,11 @@ cmp (void)
             buf0[read1] = 0x79; /* arbitrary and distinct from the above */
           buf0[read0] = ~buf1[read0];
           buf1[read1] = ~buf0[read1];
+          /* Ensure all bytes of a final word-read are initialized.  */
+          memset (buf0 + read0 + 1, 0,
+                  sizeof (word) - read0 % sizeof (word) - 1);
+          memset (buf1 + read1 + 1, 0,
+                  sizeof (word) - read1 % sizeof (word) - 1);
 
           first_diff = block_compare (buffer0, buffer1);
         }