[PATCH] string: Fix OOB read on generic strncmp

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

For unaligned case, reading ahead can only be done if parting reads
matches the aligned input.

Also extend the stratcliff tests to check such cases.

Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64-linux-gnu,
and powerpc-linux-gnu by removing the arch-specific assembly
implementation and disabling multi-arch (it covers both LE and BE
for 64 and 32 bits).
---
 string/stratcliff.c | 17 +++++++++++++++++
 string/strncmp.c    | 13 ++++++++++++-
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/string/stratcliff.c b/string/stratcliff.c
index 74d64cc03d..864d856921 100644
--- a/string/stratcliff.c
+++ b/string/stratcliff.c
@@ -409,6 +409,23 @@ do_test (void)
 	      }
 	  }
 
+      for (outer = 1; outer < 32; ++outer)
+	for (middle = 0; middle < 16; ++middle)
+	  {
+	    MEMSET (adr + middle, L('T'), 256);
+	    adr[256] = L('\0');
+	    MEMSET (dest + nchars - outer, L('T'), outer - 1);
+	    dest[nchars - outer] = L('U');
+
+	    if (STRNCMP (adr + middle, &dest[nchars - middle - 1], outer) > 0)
+	      {
+		printf ("%s 1 flunked for outer = %zu, middle = %zu, "
+			"inner = %zu\n",
+			STRINGIFY (STRNCMP), outer, middle, inner);
+		result = 1;
+	      }
+	  }
+
       /* strncpy/wcsncpy tests */
       adr[nchars - 1] = L('T');
       for (outer = nchars; outer >= max128; --outer)
diff --git a/string/strncmp.c b/string/strncmp.c
index 4c8bf36bb9..751bf53d55 100644
--- a/string/strncmp.c
+++ b/string/strncmp.c
@@ -73,7 +73,11 @@ strncmp_unaligned_loop (const op_t *x1, const op_t *x2, op_t w1, uintptr_t ofs,
   uintptr_t sh_2 = sizeof(op_t) * CHAR_BIT - sh_1;
 
   op_t w2 = MERGE (w2a, sh_1, (op_t)-1, sh_2);
-  if (!has_zero (w2) && n > (sizeof (op_t) - ofs))
+
+  /* Reading ahead is wrong if w1 and w2 already differs.  */
+  op_t w1a = MERGE (w1, 0, (op_t)-1, sh_2);
+
+  if (!has_zero (w2) && w2 == w1a && n >= (sizeof (op_t) - ofs))
     {
       op_t w2b;
 
@@ -90,6 +94,13 @@ strncmp_unaligned_loop (const op_t *x1, const op_t *x2, op_t w1, uintptr_t ofs,
 	  if (has_zero (w2b) || n <= (sizeof (op_t) - ofs))
 	    break;
 	  w1 = *x1++;
+
+	  /* Reading ahead is wrong if w1 and w2 already differs.  */
+	  w2 = MERGE (w2b, sh_1, (op_t)-1, sh_2);
+	  w1a = MERGE (w1, 0, (op_t)-1, sh_2);
+	  if (w2 != w1a)
+	    return final_cmp (w1a, w2, n);
+
 	  w2a = w2b;
 	}
 
-- 
2.34.1