dinitcheck: warn about non-absolute executable path

dinit's behavior depends on PATH environment if a service contains
command with non-absolute executable path. dinitcheck may not even find
correct executables in this case.

Such services may lead to security problems, systemd has been searching
executables only in compilation-time specified paths. As similar
features do not exist in dinit and aren't very meaningful, we just warn
about dangerous usage.

References: https://www.man7.org/linux/man-pages/man5/systemd.service.5.html#COMMAND_LINES
Signed-off-by: Yao Zi <[email protected]>

src/dinitcheck.cc

@@ -765,7 +765,10 @@ service_record *load_service(service_set_t &services, const std::string &name,
 
     auto check_command = [&](const char *setting_name, const char *command) {
         struct stat command_stat;
-        if (fstatat(dirfd, command, &command_stat, 0) == -1) {
+        if (command[0] != '/') {
+            report_service_description_err(name,
+                    std::string("executable '") + command + "' is not an absolute path");
+        } else if (fstatat(dirfd, command, &command_stat, 0) == -1) {
             report_service_description_err(name,
                     std::string("could not stat ") + setting_name + " executable '" + command
                     + "': " + strerror(errno));