tests: add a basic test for argc == 0

The kernel should reject such exec()s now, early on. Instead of adding
the needed boilerplate to write a test in C, just add an -n argument for
"(n)ull argv" to the execve helper and exec this other helper that just
exits silently with argv count.

Reviewed by:	emaste, kib, markj (all previous version)
Differential Revision:	https://reviews.freebsd.org/D34045

tests/sys/kern/execve/Makefile

@@ -10,6 +10,7 @@ ATF_TESTS_SH+=	execve_test
 
 PROGS+=		good_aout
 PROGS+=		execve_helper
+PROGS+=		execve_argc_helper
 
 LDFLAGS.goodaout+=	-static
 

tests/sys/kern/execve/execve_argc_helper.c

@@ -0,0 +1,15 @@
+/*
+ * This file is in the public domain.
+ */
+
+#include <sys/cdefs.h>
+
+#include <stdio.h>
+
+int
+main(int argc, char **argv __unused)
+{
+
+	printf("%d\n", argc);
+	return (0);
+}

tests/sys/kern/execve/execve_helper.c

@@ -38,17 +38,24 @@
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 
+/* Passing -n == null_argv */
+static char * const null_argv[] = { NULL };
+
 int
 main(int argc, char **argv)
 {
 
-	if (argc != 2) {
-		fprintf(stderr, "usage: %s <progname>\n", argv[0]);
+	if (argc == 2) {
+		execve(argv[1], &argv[1], NULL);
+	} else if (argc == 3 && strcmp(argv[1], "-n") == 0) {
+		execve(argv[2], null_argv, NULL);
+	} else {
+		fprintf(stderr, "usage: %s [-n] <progname>\n", argv[0]);
 		exit(2);
 	}
 
-	execve(argv[1], &argv[1], NULL);
 	err(1, "execve failed");
 }

tests/sys/kern/execve/execve_test.sh

@@ -99,6 +99,23 @@ trunc_aout_body()
 	    -x "cd $(atf_get_srcdir) && ./execve_helper trunc_aout"
 }
 
+empty_args_head()
+{
+	atf_set "descr" "Empty argv behavior"
+}
+empty_args_body()
+{
+	atf_check -o inline:"1\n" \
+	    -x "cd $(atf_get_srcdir) && ./execve_helper execve_argc_helper"
+
+	# Historically we allowed argc == 0, while execve(2) claimed we didn't.
+	# execve() should kick back an EINVAL now.  We verified the helper was
+	# there/working in the check just above.
+	atf_check -s exit:1 \
+	    -e match:".+Invalid argument$" \
+	    -x "cd $(atf_get_srcdir) && ./execve_helper -n execve_argc_helper"
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case bad_interp_len
@@ -111,5 +128,6 @@ atf_init_test_cases()
 	atf_add_test_case script_arg_nospace
 	atf_add_test_case sparse_aout
 	atf_add_test_case trunc_aout
+	atf_add_test_case empty_args
 
 }