selftests/exec: include cwd in long path calculation

When creating a pathname close to PATH_MAX to test execveat, factor in
the current working directory path otherwise we end up with an absolute
path that is longer than PATH_MAX. While execveat() may succeed, subsequent
calls to the kernel from the runtime environment which are required to
successfully execute the test binary/script may fail because of this.

To keep the semantics of the test the same, rework the relative pathname
part of the test to be relative to the root directory so it isn't
decreased by the length of the current working directory path.

Signed-off-by: Steve Muckle <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/exec/execveat.c

···
 }
 
 #define XX_DIR_LEN 200
-static int check_execveat_pathmax(int dot_dfd, const char *src, int is_script)
+static int check_execveat_pathmax(int root_dfd, const char *src, int is_script)
 {
 	int fail = 0;
 	int ii, count, len;
···
 
 	if (*longpath == '\0') {
 		/* Create a filename close to PATH_MAX in length */
+		char *cwd = getcwd(NULL, 0);
+
+		if (!cwd) {
+			printf("Failed to getcwd(), errno=%d (%s)\n",
+			       errno, strerror(errno));
+			return 2;
+		}
+		strcpy(longpath, cwd);
+		strcat(longpath, "/");
 		memset(longname, 'x', XX_DIR_LEN - 1);
 		longname[XX_DIR_LEN - 1] = '/';
 		longname[XX_DIR_LEN] = '\0';
-		count = (PATH_MAX - 3) / XX_DIR_LEN;
+		count = (PATH_MAX - 3 - strlen(cwd)) / XX_DIR_LEN;
 		for (ii = 0; ii < count; ii++) {
 			strcat(longpath, longname);
 			mkdir(longpath, 0755);
 		}
-		len = (PATH_MAX - 3) - (count * XX_DIR_LEN);
+		len = (PATH_MAX - 3 - strlen(cwd)) - (count * XX_DIR_LEN);
 		if (len <= 0)
 			len = 1;
 		memset(longname, 'y', len);
 		longname[len] = '\0';
 		strcat(longpath, longname);
+		free(cwd);
 	}
 	exe_cp(src, longpath);
 
···
 	}
 
 	/*
-	 * Execute as a long pathname relative to ".".  If this is a script,
+	 * Execute as a long pathname relative to "/".  If this is a script,
 	 * the interpreter will launch but fail to open the script because its
 	 * name ("/dev/fd/5/xxx....") is bigger than PATH_MAX.
 	 *
···
 	 * the exit status shall be 126."), so allow either.
 	 */
 	if (is_script)
-		fail += check_execveat_invoked_rc(dot_dfd, longpath, 0,
+		fail += check_execveat_invoked_rc(root_dfd, longpath + 1, 0,
 						  127, 126);
 	else
-		fail += check_execveat(dot_dfd, longpath, 0);
+		fail += check_execveat(root_dfd, longpath + 1, 0);
 
 	return fail;
 }
···
 	int subdir_dfd_ephemeral = open_or_die("subdir.ephemeral",
 					       O_DIRECTORY|O_RDONLY);
 	int dot_dfd = open_or_die(".", O_DIRECTORY|O_RDONLY);
+	int root_dfd = open_or_die("/", O_DIRECTORY|O_RDONLY);
 	int dot_dfd_path = open_or_die(".", O_DIRECTORY|O_RDONLY|O_PATH);
 	int dot_dfd_cloexec = open_or_die(".", O_DIRECTORY|O_RDONLY|O_CLOEXEC);
 	int fd = open_or_die("execveat", O_RDONLY);
···
 	/* Attempt to execute relative to non-directory => ENOTDIR */
 	fail += check_execveat_fail(fd, "execveat", 0, ENOTDIR);
 
-	fail += check_execveat_pathmax(dot_dfd, "execveat", 0);
-	fail += check_execveat_pathmax(dot_dfd, "script", 1);
+	fail += check_execveat_pathmax(root_dfd, "execveat", 0);
+	fail += check_execveat_pathmax(root_dfd, "script", 1);
 	return fail;
 }