samples/landlock: Don't error out if a file path cannot be opened

Instead of creating a hard error and aborting the sandbox creation,
accept file path not usable in the LL_FS_RO and LL_FS_RW environment
variables but only print a warning.  This makes it easier to test, for
instance with LL_FS_RO="${PATH}:/usr/lib:/lib"

Print that we are going to execute the command in the sandbox before
doing so.

Rename "launch" to "execute", and improve header description.

Reviewed-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20240307143849.1517218-1-mic@digikod.net
[mic: Improve header description as suggested by Günther]
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: l0kod

samples/landlock/sandboxer.c

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: BSD-3-Clause
 /*
- * Simple Landlock sandbox manager able to launch a process restricted by a
- * user-defined filesystem access control policy.
+ * Simple Landlock sandbox manager able to execute a process restricted by
+ * user-defined file system and network access control policies.
  *
  * Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
  * Copyright © 2020 ANSSI
@@ -120,9 +120,11 @@ static int populate_ruleset_fs(const char *const env_var, const int ruleset_fd,
 		if (path_beneath.parent_fd < 0) {
 			fprintf(stderr, "Failed to open \"%s\": %s\n",
 				path_list[i], strerror(errno));
-			goto out_free_name;
+			continue;
 		}
 		if (fstat(path_beneath.parent_fd, &statbuf)) {
+			fprintf(stderr, "Failed to stat \"%s\": %s\n",
+				path_list[i], strerror(errno));
 			close(path_beneath.parent_fd);
 			goto out_free_name;
 		}
@@ -227,7 +229,7 @@ int main(const int argc, char *const argv[], char *const *const envp)
 			ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
 			ENV_TCP_CONNECT_NAME, argv[0]);
 		fprintf(stderr,
-			"Launch a command in a restricted environment.\n\n");
+			"Execute a command in a restricted environment.\n\n");
 		fprintf(stderr,
 			"Environment variables containing paths and ports "
 			"each separated by a colon:\n");
@@ -248,7 +250,7 @@ int main(const int argc, char *const argv[], char *const *const envp)
 			ENV_TCP_CONNECT_NAME);
 		fprintf(stderr,
 			"\nexample:\n"
-			"%s=\"/bin:/lib:/usr:/proc:/etc:/dev/urandom\" "
+			"%s=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
 			"%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
 			"%s=\"9418\" "
 			"%s=\"80:443\" "
@@ -383,6 +385,7 @@ int main(const int argc, char *const argv[], char *const *const envp)
 
 	cmd_path = argv[1];
 	cmd_argv = argv + 1;
+	fprintf(stderr, "Executing the sandboxed command...\n");
 	execvpe(cmd_path, cmd_argv, envp);
 	fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
 		strerror(errno));