varnishtest: setuid(nobody) if running as root

If we were running as root, m00004 would fail as it would not be able
to read the files from the temporary directory.  To avoid this
problem, we now setuid to nobody if we are running as root.

bin/varnishtest/vtc_main.c

@@ -42,6 +42,11 @@ SVNID("$Id$")
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/wait.h>
+#include <pwd.h>
+
+#ifdef __linux__
+#include <sys/prctl.h>
+#endif
 
 #include "libvarnish.h"
 #include "vev.h"
@@ -364,6 +369,21 @@ main(int argc, char * const *argv)
 
 	vb = vev_new_base();
 
+	if (geteuid() == 0) {
+		struct passwd *pw;
+		pw = getpwnam("nobody");
+		assert(setgid(pw->pw_gid) == 0);
+		assert(setuid(pw->pw_uid) == 0);
+		/* On Linux >= 2.4, you need to set the dumpable flag
+		   to get core dumps after you have done a setuid. */
+#ifdef __linux__
+		if (prctl(PR_SET_DUMPABLE, 1) != 0) {
+		  printf("Could not set dumpable bit.  Core dumps turned "
+			 "off\n");
+		}
+#endif
+	}
+
 	i = 0;
 	while(!VTAILQ_EMPTY(&tst_head) || i) {
 		if (!VTAILQ_EMPTY(&tst_head) && njob < npar) {