Modules

FRR has facilities to load DSOs at startup via dlopen(). These are used to implement modules, such as SNMP and FPM.

Limitations

  • can’t load, unload, or reload during runtime. This just needs some work and can probably be done in the future.
  • doesn’t fix any of the “things need to be changed in the code in the library” issues. Most prominently, you can’t add a CLI node because CLI nodes are listed in the library…
  • if your module crashes, the daemon crashes. Should be obvious.
  • does not provide a stable API or ABI. Your module must match a version of FRR and you may have to update it frequently to match changes.
  • does not create a license boundary. Your module will need to link libzebra and include header files from the daemons, meaning it will be GPL-encumbered.

Installation

Look for moduledir in configure.ac, default is normally /usr/lib64/frr/modules but depends on --libdir / --prefix.

The daemon’s name is prepended when looking for a module, e.g. “snmp” tries to find “zebra_snmp” first when used in zebra. This is just to make it nicer for the user, with the snmp module having the same name everywhere.

Modules can be packaged separately from FRR. The SNMP and FPM modules are good candidates for this because they have dependencies (net-snmp / protobuf) that are not FRR dependencies. However, any distro packages should have an “exact-match” dependency onto the FRR package. Using a module from a different FRR version will probably blow up nicely.

For snapcraft (and during development), modules can be loaded with full path (e.g. -M $SNAP/lib/frr/modules/zebra_snmp.so). Note that libtool puts output files in the .libs directory, so during development you have to use ./zebra -M .libs/zebra_snmp.so.

Creating a module

… best to look at the existing SNMP or FPM modules.

Basic boilerplate:

#include "hook.h"
#include "module.h"

static int
module_init (void)
{
  hook_register(frr_late_init, module_late_init);
  return 0;
}

FRR_MODULE_SETUP(
    .name = "my module",
    .version = "0.0",
    .description = "my module",
    .init = module_init,
)

The frr_late_init hook will be called after the daemon has finished its other startup and is about to enter the main event loop; this is the best place for most initialisation.

Compiler & Linker magic

There’s a THIS_MODULE (like in the Linux kernel), which uses visibility attributes to restrict it to the current module. If you get a linker error with _frrmod_this_module, there is some linker SNAFU. This shouldn’t be possible, though one way to get it would be to not include libzebra (which provides a fallback definition for the symbol).

libzebra and the daemons each have their own THIS_MODULE, as do all loadable modules. In any other libraries (e.g. libfrrsnmp), THIS_MODULE will use the definition in libzebra; same applies if the main executable doesn’t use FRR_DAEMON_INFO (e.g. all testcases).

The deciding factor here is “what dynamic linker unit are you using the symbol from.” If you’re in a library function and want to know who called you, you can’t use THIS_MODULE (because that’ll just tell you you’re in the library). Put a macro around your function that adds THIS_MODULE in the caller’s code calling your function.

The idea is to use this in the future for module unloading. Hooks already remember which module they were installed by, as groundwork for a function that removes all of a module’s installed hooks.

There’s also the frr_module symbol in modules, pretty much a standard entry point for loadable modules.

Command line parameters

Command line parameters can be passed directly to a module by appending a colon to the module name when loading it, e.g. -M mymodule:myparameter. The text after the colon will be accessible in the module’s code through THIS_MODULE->load_args. For example, see how the format parameter is configured in the zfpm_init() function inside zebra_fpm.c.

Hooks

Hooks are just points in the code where you can register your callback to be called. The parameter list is specific to the hook point. Since there is no stable API, the hook code has some extra type safety checks making sure you get a compiler warning when the hook parameter list doesn’t match your callback. Don’t ignore these warnings.

Relation to MTYPE macros

The MTYPE macros, while primarily designed to decouple MTYPEs from the library and beautify the code, also work very nicely with loadable modules – both constructors and destructors are executed when loading/unloading modules.

This means there is absolutely no change required to MTYPEs, you can just use them in a module and they will even clean up themselves when we implement module unloading and an unload happens. In fact, it’s impossible to create a bug where unloading fails to de-register a MTYPE.