diff --git a/ChangeLog b/ChangeLog
index cecbdf4985..69ee9d08ca 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2018-09-19  Wilco Dijkstra  <wdijkstr@arm.com>
+
+	[BZ #23637]
+	* string/test-strstr.c (pr23637): New function.
+	(test_main): Add tests with longer needles.
+	* string/strcasestr.c (AVAILABLE): Fix readahead distance.
+	* string/strstr.c (AVAILABLE): Likewise.
+
 2018-09-19  Szabolcs Nagy  <szabolcs.nagy@arm.com>
 
 	* sysdeps/ieee754/flt-32/e_powf.c (checkint): Fix documentation.
diff --git a/string/strcasestr.c b/string/strcasestr.c
index 1f6b7b846f..8aa76037dc 100644
--- a/string/strcasestr.c
+++ b/string/strcasestr.c
@@ -37,8 +37,9 @@
 /* Two-Way algorithm.  */
 #define RETURN_TYPE char *
 #define AVAILABLE(h, h_l, j, n_l)			\
-  (((j) + (n_l) <= (h_l)) || ((h_l) += __strnlen ((void*)((h) + (h_l)), 512), \
-			      (j) + (n_l) <= (h_l)))
+  (((j) + (n_l) <= (h_l)) \
+   || ((h_l) += __strnlen ((void*)((h) + (h_l)), (n_l) + 512), \
+       (j) + (n_l) <= (h_l)))
 #define CHECK_EOL (1)
 #define RET0_IF_0(a) if (!a) goto ret0
 #define CANON_ELEMENT(c) TOLOWER (c)
diff --git a/string/strstr.c b/string/strstr.c
index 33acdc5442..f74d7189ed 100644
--- a/string/strstr.c
+++ b/string/strstr.c
@@ -33,8 +33,9 @@
 
 #define RETURN_TYPE char *
 #define AVAILABLE(h, h_l, j, n_l)			\
-  (((j) + (n_l) <= (h_l)) || ((h_l) += __strnlen ((void*)((h) + (h_l)), 512), \
-			      (j) + (n_l) <= (h_l)))
+  (((j) + (n_l) <= (h_l)) \
+   || ((h_l) += __strnlen ((void*)((h) + (h_l)), (n_l) + 512), \
+       (j) + (n_l) <= (h_l)))
 #define CHECK_EOL (1)
 #define RET0_IF_0(a) if (!a) goto ret0
 #define FASTSEARCH(S,C,N) (void*) strchr ((void*)(S), (C))
diff --git a/string/test-strstr.c b/string/test-strstr.c
index 8d99716ff3..5861b01b73 100644
--- a/string/test-strstr.c
+++ b/string/test-strstr.c
@@ -151,6 +151,32 @@ check2 (void)
     }
 }
 
+#define N 1024
+
+static void
+pr23637 (void)
+{
+  char *h = (char*) buf1;
+  char *n = (char*) buf2;
+
+  for (int i = 0; i < N; i++)
+    {
+      n[i] = 'x';
+      h[i] = ' ';
+      h[i + N] = 'x';
+    }
+
+  n[N] = '\0';
+  h[N * 2] = '\0';
+
+  /* Ensure we don't match at the first 'x'.  */
+  h[0] = 'x';
+
+  char *exp_result = stupid_strstr (h, n);
+  FOR_EACH_IMPL (impl, 0)
+    check_result (impl, h, n, exp_result);
+}
+
 static int
 test_main (void)
 {
@@ -158,6 +184,7 @@ test_main (void)
 
   check1 ();
   check2 ();
+  pr23637 ();
 
   printf ("%23s", "");
   FOR_EACH_IMPL (impl, 0)
@@ -202,6 +229,9 @@ test_main (void)
 	do_test (15, 9, hlen, klen, 1);
 	do_test (15, 15, hlen, klen, 0);
 	do_test (15, 15, hlen, klen, 1);
+
+	do_test (15, 15, hlen + klen * 4, klen * 4, 0);
+	do_test (15, 15, hlen + klen * 4, klen * 4, 1);
       }
 
   do_test (0, 0, page_size - 1, 16, 0);