diff --git a/lib/libc/tests/string/strncmp_test.c b/lib/libc/tests/string/strncmp_test.c
--- a/lib/libc/tests/string/strncmp_test.c
+++ b/lib/libc/tests/string/strncmp_test.c
@@ -27,13 +27,61 @@
  */
 
 #include <sys/cdefs.h>
+#include <sys/param.h>
+#include <sys/mman.h>
 
-#include <atf-c.h>
+#include <assert.h>
 #include <dlfcn.h>
 #include <string.h>
 
+#include <atf-c.h>
+
 int (*volatile strncmp_fn)(const char *, const char *, size_t);
 
+static char *
+makebuf(size_t len, int guard_at_end)
+{
+	char *buf;
+	size_t alloc_size, page_size;
+
+	page_size = getpagesize();
+	alloc_size = roundup2(len, page_size) + page_size;
+
+	buf = mmap(NULL, alloc_size, PROT_READ | PROT_WRITE, MAP_ANON, -1, 0);
+	assert(buf);
+	if (guard_at_end) {
+		assert(munmap(buf + alloc_size - page_size, page_size) == 0);
+		return (buf + alloc_size - page_size - len);
+	} else {
+		assert(munmap(buf, page_size) == 0);
+		return (buf + page_size);
+	}
+}
+
+static void
+test_strncmp(const char *s)
+{
+  char *src, *dst;
+	size_t size, bufsize, x;
+	int i, j;
+
+	size = strlen(s) + 1;
+	for (i = 0; i <= 1; i++) {
+		for (j = 0; j <= 1; j++) {
+			for (bufsize = 0; bufsize <= size + 10; bufsize++) {
+				src = makebuf(size, i);
+				memcpy(src, s, size);
+				dst = makebuf(bufsize, j);
+				memset(dst, 'X', bufsize);
+				assert(strlcpy(dst, src, bufsize) == size-1);
+				assert(bufsize == 0 || strncmp_fn(src, dst, bufsize - 1) == 0);
+				for (x = size; x < bufsize; x++)
+					assert(dst[x] == 'X');
+			}
+		}
+	}
+}
+
 static void
 alignment_testcase(char *a, char *b, int want, size_t len)
 {
@@ -138,6 +186,18 @@
 					check_strncmp_alignments(a, b, a_off, b_off, len, pos);
 }
 
+ATF_TC_WITHOUT_HEAD(bounds);
+ATF_TC_BODY(bounds, tc)
+{
+	size_t i;
+	char buf[64];
+
+	for (i = 0; i < sizeof(buf) - 1; i++) {
+		buf[i] = ' ' + i;
+		test_strncmp(buf);
+	}
+}
+
 ATF_TC(strncmp_null);
 ATF_TC_HEAD(strncmp_null, tc)
 {
@@ -159,6 +219,7 @@
 		strncmp_fn = strncmp;
 
 	ATF_TP_ADD_TC(tp, strncmp_alignments);
+	ATF_TP_ADD_TC(tp, bounds);
 	ATF_TP_ADD_TC(tp, strncmp_null);
 
 	return atf_no_error();