cmp: avoid reading uninitialized memory

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, 2 insertions, 0 deletions

diff --git a/src/cmp.c b/src/cmp.c
index e2d6f25..de67f92 100644
--- a/src/cmp.c
+++ b/src/cmp.c
@@ -459,6 +459,8 @@ cmp (void)
           /* Insert sentinels for the block compare.  */
           buf0[read0] = ~buf1[read0];
           buf1[read1] = ~buf0[read1];
+          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);
         }