selftests/landlock: Add tests for execveat + AT_EXECVE_CHECK

Extend layout1.execute with the new AT_EXECVE_CHECK flag. The semantic
with AT_EXECVE_CHECK is the same as with a simple execve(2),
LANDLOCK_ACCESS_FS_EXECUTE is enforced the same way.

Cc: Günther Noack <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Paul Moore <[e-mail]>
Signed-off-by: Mickaël Salaün <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Kees Cook <[e-mail]>

tools/testing/selftests/landlock/fs_test.c

···
 #include <linux/fs.h>
 #include <linux/mount.h>
 
+/* Defines AT_EXECVE_CHECK without type conflicts. */
+#define _ASM_GENERIC_FCNTL_H
+#include <linux/fcntl.h>
+
 #include "common.h"
 
 #ifndef renameat2
···
 	};
 }
 
+static void test_check_exec(struct __test_metadata *const _metadata,
+			    const int err, const char *const path)
+{
+	int ret;
+	char *const argv[] = { (char *)path, NULL };
+
+	ret = execveat(AT_FDCWD, path, argv, NULL,
+		       AT_EMPTY_PATH | AT_EXECVE_CHECK);
+	if (err) {
+		EXPECT_EQ(-1, ret);
+		EXPECT_EQ(errno, err);
+	} else {
+		EXPECT_EQ(0, ret);
+	}
+}
+
 TEST_F_FORK(layout1, execute)
 {
 	const struct rule rules[] = {
···
 	copy_binary(_metadata, file1_s1d2);
 	copy_binary(_metadata, file1_s1d3);
 
+	/* Checks before file1_s1d1 being denied. */
+	test_execute(_metadata, 0, file1_s1d1);
+	test_check_exec(_metadata, 0, file1_s1d1);
+
 	enforce_ruleset(_metadata, ruleset_fd);
 	ASSERT_EQ(0, close(ruleset_fd));
 
 	ASSERT_EQ(0, test_open(dir_s1d1, O_RDONLY));
 	ASSERT_EQ(0, test_open(file1_s1d1, O_RDONLY));
 	test_execute(_metadata, EACCES, file1_s1d1);
+	test_check_exec(_metadata, EACCES, file1_s1d1);
 
 	ASSERT_EQ(0, test_open(dir_s1d2, O_RDONLY));
 	ASSERT_EQ(0, test_open(file1_s1d2, O_RDONLY));
 	test_execute(_metadata, 0, file1_s1d2);
+	test_check_exec(_metadata, 0, file1_s1d2);
 
 	ASSERT_EQ(0, test_open(dir_s1d3, O_RDONLY));
 	ASSERT_EQ(0, test_open(file1_s1d3, O_RDONLY));
 	test_execute(_metadata, 0, file1_s1d3);
+	test_check_exec(_metadata, 0, file1_s1d3);
 }
 
 TEST_F_FORK(layout1, link)