From e2bbf175a2184bd76f6c54ccf8456babeb1a46fc Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Tue, 9 Apr 2024 15:16:35 +0200 Subject: Adding upstream version 9.1. Signed-off-by: Daniel Baumann --- doc/developer/modules.rst | 142 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 142 insertions(+) create mode 100644 doc/developer/modules.rst (limited to 'doc/developer/modules.rst') diff --git a/doc/developer/modules.rst b/doc/developer/modules.rst new file mode 100644 index 0000000..0feac8e --- /dev/null +++ b/doc/developer/modules.rst @@ -0,0 +1,142 @@ +.. _modules: + +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" + #include "libfrr.h" + #include "frrevent.h" + + static int module_late_init(struct event_loop *master) + { + /* Do initialization stuff here */ + return 0; + } + + 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. -- cgit v1.2.3