[RFC PATCH v1 2/7] samples/landlock: Clarify option parsing behaviour

[Matthieu Buffet <matthieu@buffet.re>]

- Clarify which environment variables are optional, which ones are
  mandatory
- Clarify the difference between unset variables and empty ones
- Move the (larger) help message to a helper function

Signed-off-by: Matthieu Buffet <matthieu@buffet.re>
---
 samples/landlock/sandboxer.c | 86 ++++++++++++++++++++----------------
 1 file changed, 48 insertions(+), 38 deletions(-)

diff --git a/samples/landlock/sandboxer.c b/samples/landlock/sandboxer.c
index a84ae3a15482..08704504dc51 100644
--- a/samples/landlock/sandboxer.c
+++ b/samples/landlock/sandboxer.c
@@ -221,6 +221,53 @@ static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
 
 #define LANDLOCK_ABI_LAST 5
 
+static void print_help(const char *prog)
+{
+	fprintf(stderr,
+		"usage: %s=\"...\" %s=\"...\" [other environment variables] %s "
+		"<cmd> [args]...\n\n",
+		ENV_FS_RO_NAME, ENV_FS_RW_NAME, prog);
+	fprintf(stderr,
+		"Execute a command in a restricted environment.\n\n");
+	fprintf(stderr,
+		"Environment variables containing paths and ports "
+		"can be multi-valued, with a colon delimiter.\n"
+		"\n"
+		"Mandatory settings:\n");
+	fprintf(stderr,
+		"* %s: list of paths allowed to be used in a read-only way.\n",
+		ENV_FS_RO_NAME);
+	fprintf(stderr,
+		"* %s: list of paths allowed to be used in a read-write way.\n",
+		ENV_FS_RW_NAME);
+	fprintf(stderr,
+		"\n"
+		"Optional settings (when not set, their associated access "
+		"check is always allowed) (for lists, an empty string means "
+		"to allow nothing, e.g. %s=\"\"):\n",
+		ENV_TCP_BIND_NAME);
+	fprintf(stderr,
+		"* %s: list of ports allowed to bind (server).\n",
+		ENV_TCP_BIND_NAME);
+	fprintf(stderr,
+		"* %s: list of ports allowed to connect (client).\n",
+		ENV_TCP_CONNECT_NAME);
+	fprintf(stderr,
+		"\n"
+		"Example:\n"
+		"%s=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
+		"%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
+		"%s=\"9418\" "
+		"%s=\"80:443\" "
+		"%s bash -i\n\n",
+		ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
+		ENV_TCP_CONNECT_NAME, prog);
+	fprintf(stderr,
+		"This sandboxer can use Landlock features "
+		"up to ABI version %d.\n",
+		LANDLOCK_ABI_LAST);
+}
+
 int main(const int argc, char *const argv[], char *const *const envp)
 {
 	const char *cmd_path;
@@ -237,44 +284,7 @@ int main(const int argc, char *const argv[], char *const *const envp)
 	};
 
 	if (argc < 2) {
-		fprintf(stderr,
-			"usage: %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\"%s "
-			"<cmd> [args]...\n\n",
-			ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
-			ENV_TCP_CONNECT_NAME, argv[0]);
-		fprintf(stderr,
-			"Execute a command in a restricted environment.\n\n");
-		fprintf(stderr,
-			"Environment variables containing paths and ports "
-			"each separated by a colon:\n");
-		fprintf(stderr,
-			"* %s: list of paths allowed to be used in a read-only way.\n",
-			ENV_FS_RO_NAME);
-		fprintf(stderr,
-			"* %s: list of paths allowed to be used in a read-write way.\n\n",
-			ENV_FS_RW_NAME);
-		fprintf(stderr,
-			"Environment variables containing ports are optional "
-			"and could be skipped.\n");
-		fprintf(stderr,
-			"* %s: list of ports allowed to bind (server).\n",
-			ENV_TCP_BIND_NAME);
-		fprintf(stderr,
-			"* %s: list of ports allowed to connect (client).\n",
-			ENV_TCP_CONNECT_NAME);
-		fprintf(stderr,
-			"\nexample:\n"
-			"%s=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
-			"%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
-			"%s=\"9418\" "
-			"%s=\"80:443\" "
-			"%s bash -i\n\n",
-			ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
-			ENV_TCP_CONNECT_NAME, argv[0]);
-		fprintf(stderr,
-			"This sandboxer can use Landlock features "
-			"up to ABI version %d.\n",
-			LANDLOCK_ABI_LAST);
+		print_help(argv[0]);
 		return 1;
 	}
 
-- 
2.39.5