Add support for syscall failure tests

tests/common/link_failure.c

@@ -0,0 +1,51 @@
+/* 
+ * gcc link_failure.c -o link_failure.so -shared -ldl 
+ * LD_PRELOAD=./link_failure.so FAILURE_PATH=/etc/shadow ./test /etc/shadow
+ */
+
+#define _GNU_SOURCE
+#include <dlfcn.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <stdarg.h>
+#include <fcntl.h>
+#include <string.h>
+#include <errno.h>
+#include <assert.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+
+typedef int (*link_type) (const char *oldpath, const char *newpath);
+static link_type next_link;
+
+static const char *failure_path = NULL;
+
+int link (const char *oldpath, const char *newpath)
+{
+	if (NULL == next_link)
+	{
+		next_link = dlsym (RTLD_NEXT, "link");
+		assert (NULL != next_link);
+	}
+	if (NULL == failure_path) {
+		failure_path = getenv ("FAILURE_PATH");
+		if (NULL == failure_path) {
+			fputs ("No FAILURE_PATH defined\n", stderr);
+		}
+	}
+
+	if (   (NULL != newpath)
+	    && (NULL != failure_path)
+	    && (strcmp (newpath, failure_path) == 0))
+	{
+		fprintf (stderr, "link FAILURE %s %s\n", oldpath, newpath);
+		errno = EIO;
+		return -1;
+	}
+
+	return next_link (oldpath, newpath);
+}
+