update(engine): allow using -p to pass a format to plugin events

userspace/engine/rule_loader_compiler.cpp

@@ -509,7 +509,12 @@ void rule_loader::compiler::compile_rule_infos(
 
 		// build rule output message
 		rule.output = r.output;
-		if (r.source == falco_common::syscall_source)
+
+		// plugins sources do not have any container info and so we won't apply -pk, -pc, etc.
+		// on the other hand, when using plugins you might want to append custom output based on the plugin
+		// TODO: this is not flexible enough (esp. if you mix plugin with syscalls),
+		// it would be better to add configuration options to control the output.
+		if (!cfg.replace_output_container_info || r.source == falco_common::syscall_source)
 		{
 			apply_output_substitutions(cfg, rule.output);
 		}

userspace/falco/app/options.cpp

@@ -172,7 +172,7 @@ void options::define(cxxopts::Options& opts)
 		("N",                             "Only print field names when used in conjunction with the --list option. It has no effect when used with other options.", cxxopts::value(names_only)->default_value("false"))
 		("o,option",                      "Set the value of option <opt> to <val>. Overrides values in the configuration file. <opt> can be identified using its location in the configuration file using dot notation. Elements of list entries can be accessed via square brackets [].\n    E.g. base.id = val\n         base.subvalue.subvalue2 = val\n         base.list[1]=val", cxxopts::value(cmdline_config_options), "<opt>=<val>")
 		("plugin-info",                   "Print info for the plugin specified by <plugin_name> and exit.\nThis includes all descriptive information like name and author, along with the\nschema format for the init configuration and a list of suggested open parameters.\n<plugin_name> can be the plugin's name or its configured 'library_path'.", cxxopts::value(print_plugin_info), "<plugin_name>")
-		("p,print",                       "Print (or replace) additional information in the rule's output for syscall events.\nUse -pc or -pcontainer to append container details.\nUse -pk or -pkubernetes to add both container and Kubernetes details.\nIf using gVisor, choose -pcg or -pkg variants (or -pcontainer-gvisor and -pkubernetes-gvisor, respectively).\nIf a rule's output contains %container.info, it will be replaced with the corresponding details. Otherwise, these details will be directly appended to the rule's output.\nAlternatively, use -p <output_format> for a custom format. In this case, the given <output_format> will be appended to the rule's output without any replacement. This option does not affect plugin events.", cxxopts::value(print_additional), "<output_format>")
+		("p,print",                       "Print (or replace) additional information in the rule's output.\nUse -pc or -pcontainer to append container details to syscall events.\nUse -pk or -pkubernetes to add both container and Kubernetes details to syscall events.\nIf using gVisor, choose -pcg or -pkg variants (or -pcontainer-gvisor and -pkubernetes-gvisor, respectively).\nIf a syscall rule's output contains %container.info, it will be replaced with the corresponding details. Otherwise, these details will be directly appended to the rule's output.\nAlternatively, use -p <output_format> for a custom format. In this case, the given <output_format> will be appended to the rule's output without any replacement to all events, including plugin events.", cxxopts::value(print_additional), "<output_format>")
 		("P,pidfile",                     "Write PID to specified <pid_file> path. By default, no PID file is created.", cxxopts::value(pidfilename)->default_value(""), "<pid_file>")
 		("r",                             "Rules file or directory to be loaded. This option can be passed multiple times. Falco defaults to the values in the configuration file when this option is not specified.", cxxopts::value<std::vector<std::string>>(), "<rules_file>")
 		("S,snaplen",                     "Collect only the first <len> bytes of each I/O buffer for 'syscall' events. By default, the first 80 bytes are collected by the driver and sent to the user space for processing. Use this option with caution since it can have a strong performance impact.", cxxopts::value(snaplen)->default_value("0"), "<len>")