tests: Fix race condition in aslr_setuid

Use a cloexec pipe to block the parent until the child is ready.

While here, redirect the output from ping to /dev/null, and mark the
test as requiring the inet feature since we ping the IPv4 loopback.

PR:		296116
MFC after:	1 week
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D57734

tests/sys/kern/Makefile

@@ -92,6 +92,7 @@ PROGS+=		coredump_phnum_helper
 PROGS+=		pdeathsig_helper
 PROGS+=		sendfile_helper
 
+CFLAGS.aslr+=				-I${SRCTOP}/tests
 LIBADD.aslr+=				util
 LIBADD.copy_file_range+=		md
 LIBADD.jail_lookup_root+=		jail util

tests/sys/kern/aslr.c

@@ -11,6 +11,7 @@
 #include <sys/user.h>
 #include <sys/wait.h>
 
+#include <fcntl.h>
 #include <libutil.h>
 #include <pwd.h>
 #include <signal.h>
@@ -19,6 +20,7 @@
 #include <unistd.h>
 
 #include <atf-c.h>
+#include "freebsd_test_suite/macros.h"
 
 /*
  * Spawn an unprivileged child with ASLR force-disabled, which then execs
@@ -30,16 +32,21 @@ spawn_ping(const atf_tc_t *tc)
 	const char *user;
 	struct passwd *passwd;
 	pid_t child;
-	int arg, error;
+	int arg, error, ctl[2];
+	char dummy;
 
 	user = atf_tc_get_config_var(tc, "unprivileged_user");
 	passwd = getpwnam(user);
 	ATF_REQUIRE(passwd != NULL);
 
+	ATF_REQUIRE(pipe2(ctl, O_CLOEXEC) == 0);
 	child = fork();
 	ATF_REQUIRE(child >= 0);
 	if (child == 0) {
-		if (seteuid(passwd->pw_uid) != 0)
+		if (close(ctl[0]) != 0 ||
+		    close(STDOUT_FILENO) != 0 ||
+		    open("/dev/null", O_WRONLY | O_APPEND) != STDOUT_FILENO ||
+		    seteuid(passwd->pw_uid) != 0)
 			_exit(1);
 
 		arg = PROC_ASLR_FORCE_DISABLE;
@@ -50,7 +57,9 @@ spawn_ping(const atf_tc_t *tc)
 		execl("/sbin/ping", "ping", "127.0.0.1", NULL);
 		_exit(127);
 	}
-	usleep(500000); /* XXX-MJ */
+	ATF_REQUIRE(close(ctl[1]) == 0);
+	ATF_REQUIRE(read(ctl[0], &dummy, 1) == 0);
+	ATF_REQUIRE(close(ctl[0]) == 0);
 
 	return (child);
 }
@@ -98,6 +107,7 @@ ATF_TC_BODY(aslr_setuid, tc)
 	pid_t child, pid;
 	int arg, error, st;
 
+	ATF_REQUIRE_FEATURE("inet");
 	if (!atf_tc_has_config_var(tc, "unprivileged_user"))
 		atf_tc_skip("unprivileged_user not set");