fuzz-systemctl-parse-argv: a new fuzzer

Does what the name suggests. Obviously inspired by sudoers, but note that
our tools are not supposed to be installed suid, so there is no privilege
boundary to cross here.

src/systemctl/fuzz-systemctl-parse-argv.c

@@ -0,0 +1,58 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <stdio.h>
+#include <unistd.h>
+
+#include "env-util.h"
+#include "fd-util.h"
+#include "fuzz.h"
+#include "stdio-util.h"
+#include "strv.h"
+#include "systemctl.h"
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+        _cleanup_strv_free_ char **argv = NULL;
+        _cleanup_close_ int orig_stdout_fd = -1;
+        int r;
+
+        /* We don't want to fill the logs with messages about parse errors.
+         * Disable most logging if not running standalone */
+        if (!getenv("SYSTEMD_LOG_LEVEL"))
+                log_set_max_level(LOG_CRIT);
+
+        arg_pager_flags = PAGER_DISABLE; /* We shouldn't execute the pager */
+
+        argv = strv_parse_nulstr((const char *)data, size);
+        if (!argv)
+                return log_oom();
+
+        if (!argv[0])
+                return 0; /* argv[0] should always be present, but may be zero-length. */
+
+        if (getenv_bool("SYSTEMD_FUZZ_OUTPUT") <= 0) {
+                orig_stdout_fd = fcntl(fileno(stdout), F_DUPFD_CLOEXEC, 3);
+                if (orig_stdout_fd < 0)
+                        log_warning_errno(orig_stdout_fd, "Failed to duplicate fd 1: %m");
+                else
+                        assert_se(freopen("/dev/null", "w", stdout));
+
+                opterr = 0; /* do not print errors */
+        }
+
+        optind = 0; /* this tells the getopt machinery to reinitialize */
+
+        r = systemctl_dispatch_parse_argv(strv_length(argv), argv);
+        if (r < 0)
+                log_error_errno(r, "Failed to parse args: %m");
+        else
+                log_info(r == 0 ? "Done!" : "Action!");
+
+        if (orig_stdout_fd >= 0) {
+                char path[STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int)];
+
+                xsprintf(path, "/proc/self/fd/%d", orig_stdout_fd);
+                assert_se(freopen(path, "w", stdout));
+        }
+
+        return 0;
+}