xz: Use stricter pledge(2) and Landlock sandbox.

This makes these sandboxing methods stricter when no files are
created or deleted. That is, it's a middle ground between the
initial sandbox and the strictest single-file-to-stdout sandbox:
this allows opening files for reading but output has to go to stdout.

src/xz/main.c

@@ -223,21 +223,41 @@ main(int argc, char **argv)
 		signals_init();
 
 #ifdef ENABLE_SANDBOX
-	// Set a flag that strict sandboxing is allowed if all these are true:
-	//   - --files or --files0 wasn't used.
-	//   - There is exactly one input file or we are reading from stdin.
-	//   - We won't create any files: output goes to stdout or --test
-	//     or --list was used. Note that --test implies opt_stdout = true
-	//     but --list doesn't.
+	// Read-only sandbox can be enabled if we won't create or delete
+	// any files:
 	//
-	// This is obviously not ideal but it was easy to implement and
-	// it covers the most common use cases.
+	//   - --stdout, --test, or --list was used. Note that --test
+	//     implies opt_stdout = true but --list doesn't.
 	//
-	// TODO: Make sandboxing work for other situations too.
-	if (args.files_name == NULL && args.arg_count == 1
-			&& (opt_stdout || strcmp("-", args.arg_names[0]) == 0
-				|| opt_mode == MODE_LIST))
+	//   - Output goes to stdout because --files or --files0 wasn't used
+	//     and no arguments were given on the command line or the
+	//     arguments are all "-" (indicating standard input).
+	bool to_stdout_only = opt_stdout || opt_mode == MODE_LIST;
+	if (!to_stdout_only && args.files_name == NULL) {
+		// If all of the filenames provided are "-" (more than one
+		// "-" could be specified), then we are only going to be
+		// writing to standard output. Note that if no filename args
+		// were provided, args.c puts a single "-" in arg_names[0].
+		to_stdout_only = true;
+
+		for (unsigned i = 0; i < args.arg_count; ++i) {
+			if (strcmp("-", args.arg_names[i]) != 0) {
+				to_stdout_only = false;
+				break;
+			}
+		}
+	}
+
+	if (to_stdout_only) {
+		sandbox_enable_read_only();
+
+		// Allow strict sandboxing if we are processing exactly one
+		// file to standard output. This requires that --files or
+		// --files0 wasn't specified (an unknown number of filenames
+		// could be provided that way).
+		if (args.files_name == NULL && args.arg_count == 1)
 			sandbox_allow_strict();
+	}
 #endif
 
 	// coder_run() handles compression, decompression, and testing.

src/xz/sandbox.c

@@ -81,6 +81,18 @@ sandbox_init(void)
 }
 
 
+extern void
+sandbox_enable_read_only(void)
+{
+	// We will be opening files for reading but
+	// won't create or remove any files.
+	if (pledge("stdio rpath", ""))
+		message_fatal(_("Failed to enable the sandbox"));
+
+	return;
+}
+
+
 extern void
 sandbox_enable_strict_if_allowed(int src_fd lzma_attribute((__unused__)),
 		int pipe_event_fd lzma_attribute((__unused__)),
@@ -89,6 +101,7 @@ sandbox_enable_strict_if_allowed(int src_fd lzma_attribute((__unused__)),
 	if (!prepare_for_strict_sandbox())
 		return;
 
+	// All files that need to be opened have already been opened.
 	if (pledge("stdio", ""))
 		message_fatal(_("Failed to enable the sandbox"));
 
@@ -222,6 +235,17 @@ sandbox_init(void)
 }
 
 
+extern void
+sandbox_enable_read_only(void)
+{
+	// We will be opening files for reading but
+	// won't create or remove any files.
+	const uint64_t required_rights = LANDLOCK_ACCESS_FS_READ_FILE;
+	enable_landlock(required_rights);
+	return;
+}
+
+
 extern void
 sandbox_enable_strict_if_allowed(int src_fd lzma_attribute((__unused__)),
 		int pipe_event_fd lzma_attribute((__unused__)),
@@ -254,6 +278,14 @@ sandbox_init(void)
 }
 
 
+extern void
+sandbox_enable_read_only(void)
+{
+	// Nothing to do.
+	return;
+}
+
+
 extern void
 sandbox_enable_strict_if_allowed(
 		int src_fd, int pipe_event_fd, int pipe_write_fd)

src/xz/sandbox.h

@@ -21,6 +21,10 @@
 extern void sandbox_init(void);
 
 
+/// \brief      Enable sandboxing that only allows opening files for reading
+extern void sandbox_enable_read_only(void);
+
+
 /// \brief      Tell sandboxing code that strict sandboxing can be used
 ///
 /// This function only sets a flag which will be read by