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doc: Update documentation

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Signed-off-by: Donald Lee <dlqs@gmx.com>
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Donald Lee 2021-07-09 01:42:23 +08:00
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doc/developer/scripting.rst
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@ -14,8 +14,8 @@ is implemented using the standard Lua C bindings. The supported version of Lua
is 5.3.
C objects may be passed into Lua and Lua objects may be retrieved by C code via
a marshalling system. In this way, arbitrary data from FRR may be passed to
scripts. It is possible to pass C functions as well.
a encoding/decoding system. In this way, arbitrary data from FRR may be passed to
scripts.
The Lua environment is isolated from the C environment; user scripts cannot
access FRR's address space unless explicitly allowed by FRR.
@ -53,150 +53,290 @@ Reasons against supporting multiple scripting languages:
with which a given script can be shared
General
^^^^^^^
-------
FRR's concept of a script is somewhat abstracted away from the fact that it is
Lua underneath. A script in has two things:
- name
- state
In code:
FRR's scripting functionality is provided in the form of Lua functions in Lua
scripts (``.lua`` files). One Lua script may contain many Lua functions. These
are respectively encapsulated in the following structures:
.. code-block:: c
struct frrscript {
/* Script name */
char *name;
/* Lua file name */
char *name;
/* Lua state */
struct lua_State *L;
/* hash of lua_function_states */
struct hash *lua_function_hash;
};
struct lua_function_state {
/* Lua function name */
char *name;
lua_State *L;
};
``name`` is simply a string. Everything else is in ``state``, which is itself a
Lua library object (``lua_State``). This is an opaque struct that is
manipulated using ``lua_*`` functions. The basic ones are imported from
``lua.h`` and the rest are implemented within FRR to fill our use cases. The
thing to remember is that all operations beyond the initial loading the script
take place on this opaque state object.
`struct frrscript`: Since all Lua functions are contained within scripts, the
following APIs manipulates this structure. ``name`` contains the
Lua script name and a hash of Lua functions to their function names.
There are four basic actions that can be done on a script:
`struct lua_function_state` is an internal structure, but it essentially contains
the name of the Lua function and its state (a stack), which is run using Lua
library functions.
- load
- execute
- query state
- unload
In general, to run a Lua function, these steps must take place:
They are typically done in this order.
- Initialization
- Load
- Call
- Delete
Loading
^^^^^^^
A snippet of Lua code is referred to as a "chunk". These are simply text. FRR
presently assumes chunks are located in individual files specific to one task.
These files are stored in the scripts directory and must end in ``.lua``.
A script object is created by loading a script. This is done with
``frrscript_load()``. This function takes the name of the script and an
optional callback function. The string ".lua" is appended to the script name,
and the resultant filename is looked for in the scripts directory.
For example, to load ``/etc/frr/scripts/bingus.lua``:
.. code-block:: c
struct frrscript *fs = frrscript_load("bingus", NULL);
During loading the script is validated for syntax and its initial environment
is setup. By default this does not include the Lua standard library; there are
security issues to consider, though for practical purposes untrusted users
should not be able to write the scripts directory anyway. If desired the Lua
standard library may be added to the script environment using
``luaL_openlibs(fs->L)`` after loading the script. Further information on
setting up the script environment is in the Lua manual.
Executing
^^^^^^^^^
After loading, scripts may be executed. A script may take input in the form of
variable bindings set in its environment prior to being run, and may provide
results by setting the value of variables. Arbitrary C values may be
transferred into the script environment, including functions.
A typical execution call looks something like this:
.. code-block:: c
struct frrscript *fs = frrscript_load(...);
int status_ok = 0, status_fail = 1;
struct prefix p = ...;
int result = frrscript_call(fs,
("STATUS_FAIL", &status_fail),
("STATUS_OK", &status_ok),
("prefix", &p));
To execute a loaded script, we need to define the inputs. These inputs are
passed in by binding values to variable names that will be accessible within the
Lua environment. Basically, all communication with the script takes place via
global variables within the script, and to provide inputs we predefine globals
before the script runs. This is done by passing ``frrscript_call()`` a list of
parenthesized pairs, where the first and second fields identify, respectively,
the name of the global variable within the script environment and the value it
is bound to.
The script is then executed and returns a general status code. In the success
case this will be 0, otherwise it will be nonzero. The script itself does not
determine this code, it is provided by the Lua interpreter.
Querying State
Initialization
^^^^^^^^^^^^^^
.. todo::
The ``frrscript`` object encapsulates the Lua function state(s) from
one Lua script file. To create, use ``frrscript_new()`` which takes the
name of the Lua script.
The string ".lua" is appended to the script name, and the resultant filename
will be used to look for the script when we want to load a Lua function from it.
This section will be updated once ``frrscript_get_result`` has been
updated to work with the new ``frrscript_call`` and the rest of the new API.
Unloading
^^^^^^^^^
To destroy a script and its associated state:
For example, to create ``frrscript`` for ``/etc/frr/scripts/bingus.lua``:
.. code-block:: c
frrscript_unload(fs);
struct frrscript *fs = frrscript_new("bingus");
.. _marshalling:
The script is *not* read at this stage.
This function cannot be used to test for a script's presence.
Marshalling
^^^^^^^^^^^
Load
^^^^
The function to be called must first be loaded. Use ``frrscript_load()``
which takes a ``frrscript`` object, the name of the Lua function
and a callback function.
For example, to load the Lua function ``on_foo``
in ``/etc/frr/scripts/bingus.lua``:
.. code-block:: c
int ret = frrscript_load(fs, "on_foo", NULL);
This function returns 0 if and only if the Lua function was successfully loaded.
A non-zero return could indicate either a missing Lua script, a missing
Lua function, or an error when loading the function.
During loading the script is validated for syntax and its environment
is set up. By default this does not include the Lua standard library; there are
security issues to consider, though for practical purposes untrusted users
should not be able to write the scripts directory anyway.
Call
^^^^
After loading, Lua functions may be called.
Input
"""""
Inputs to the Lua script should be given by providing a list of parenthesized
pairs,
where the first and second field identify the name of the variable and the
value it is bound to, respectively.
The types of the values must have registered encoders (more below); the compiler
will warn you otherwise.
These variables are first encoded in-order, then provided as arguments
to the Lua function. In the example, note that ``c`` is passed in as a value
while ``a`` and ``b`` are passed in as pointers.
.. code-block:: c
int a = 100, b = 200, c = 300;
frrscript_call(fs, "on_foo", ("a", &a), ("b", &b), ("c", c));
.. code-block:: lua
function on_foo(a, b, c)
-- a is 100, b is 200, c is 300
...
Output
""""""
.. code-block:: c
int a = 100, b = 200, c = 300;
frrscript_call(fs, "on_foo", ("a", &a), ("b", &b), ("c", c));
// a is 500, b is 200, c is 300
int* d = frrscript_get_result(fs, "on_foo", "d", lua_tointegerp);
// d is 800
.. code-block:: lua
function on_foo(a, b, c)
b = 600
return { ["a"] = 500, ["c"] = 700, ["d"] = 800 }
end
**Lua functions being called must return a single table of string names to
values.**
(Lua functions should return an empty table if there is no output.)
The keys of the table are mapped back to names of variables in C. Note that
the values in the table can also be tables. Since tables are Lua's primary
data structure, this design lets us return any Lua value.
After the Lua function returns, the names of variables to ``frrscript_call()``
are matched against keys of the returned table, and then decoded. The types
being decoded must have registered decoders (more below); the compiler will
warn you otherwise.
In the example, since ``a`` was in the returned table and ``b`` was not,
``a`` was decoded and its value modified, while ``b`` was not decoded.
``c`` was decoded as well, but its decoder is a noop.
What modifications happen given a variable depends whether its name was
in the returned table and the decoder's implementation.
.. warning::
Always keep in mind that non const-qualified pointers in
``frrscript_call()`` may be modified - this may be a source of bugs.
On the other hand, const-qualified pointers and other values cannot
be modified.
.. tip::
You can make a copy of a data structure and pass that in instead,
so that modifications only happen to that copy.
``frrscript_call()`` returns 0 if and only if the Lua function was successfully
called. A non-zero return could indicate either a missing Lua script, a missing
Lua function, or an error from the Lua interpreter.
In the above example, ``d`` was not an input to ``frrscript_call()``, so its
value must be explicitly retrieved with ``frrscript_get_result``.
``frrscript_get_result()`` takes a
decoder and string name which is used as a key to search the returned table.
Returns the pointer to the decoded value, or NULL if it was not found.
In the example, ``d`` is a "new" value in C space,
so memory allocation might take place. Hence the caller is
responsible for memory deallocation.
Delete
^^^^^^
To delete a script and the all Lua states associated with it:
.. code-block:: c
frrscript_delete(fs);
A complete example
""""""""""""""""""
So, a typical exection call, with error checking, looks something like this:
.. code-block:: c
struct frrscript *fs = frrscript_new("my_script"); // name *without* .lua
int ret = frrscript_load(fs, "on_foo", NULL);
if (ret != 0)
goto DONE; // Lua script or function might have not been found
int a = 100, b = 200, c = 300;
ret = frrscript_call(fs, "on_foo", ("a", &a), ("b", &b), ("c", c));
if (ret != 0)
goto DONE; // Lua function might have not successfully run
// a and b might be modified
assert(a == 500);
assert(b == 200);
// c could not have been modified
assert(c == 300);
// d is new
int* d = frrscript_get_result(fs, "on_foo", "d", lua_tointegerp);
if (!d)
goto DONE; // "d" might not have been in returned table
assert(*d == 800);
XFREE(MTYPE_TMP, d); // caller responsible for free
DONE:
frrscript_delete(fs);
.. code-block:: lua
function on_foo(a, b, c)
b = 600
return { a = 500, c = 700, d = 800 }
end
Note that ``{ a = ...`` is same as ``{ ["a"] = ...``; it is Lua shorthand to
use the variable name as the key in a table.
Encoding and Decoding
^^^^^^^^^^^^^^^^^^^^^
Earlier sections glossed over the types of values that can be passed into
``frrscript_call`` and how data is passed between C and Lua. Lua, as a dynamically
typed, garbage collected language, cannot directly use C values without some
kind of marshalling / unmarshalling system to translate types between the two
runtimes.
``frrscript_call()`` and how data is passed between C and Lua. Lua, as a
dynamically typed, garbage collected language, cannot directly use C values
without some kind of encoding / decoding system to
translate types between the two runtimes.
Lua communicates with C code using a stack. C code wishing to provide data to
Lua scripts must provide a function that marshalls the C data into a Lua
Lua scripts must provide a function that encodes the C data into a Lua
representation and pushes it on the stack. C code wishing to retrieve data from
Lua must provide a corresponding unmarshalling function that retrieves a Lua
value from the stack and converts it to the corresponding C type. These
functions are known as encoders and decoders in FRR.
Lua must provide a corresponding decoder function that retrieves a Lua
value from the stack and converts it to the corresponding C type.
An encoder is a function that takes a ``lua_State *`` and a C type and pushes
onto the Lua stack a value representing the C type. For C structs, the usual
case, this will typically be a Lua table (tables are the only datastructure Lua
has). For example, here is the encoder function for ``struct prefix``:
Encoders and decoders are provided for common data types.
Developers wishing to pass their own data structures between C and Lua need to
create encoders and decoders for that data type.
We try to keep them named consistently.
There are three kinds of encoders and decoders:
1. lua_push*: encodes a value onto the Lua stack.
Required for ``frrscript_call``.
2. lua_decode*: decodes a value from the Lua stack.
Required for ``frrscript_call``.
Only non const-qualified pointers may be actually decoded (more below).
3. lua_to*: allocates memory and decodes a value from the Lua stack.
Required for ``frrscript_get_result``.
This design allows us to combine typesafe *modification* of C values as well as
*allocation* of new C values.
In the following sections, we will use the encoders/decoders for ``struct prefix`` as an example.
Encoding
""""""""
An encoder function takes a ``lua_State *``, a C type and pushes that value onto
the Lua state (a stack).
For C structs, the usual case,
this will typically be encoded to a Lua table, then pushed onto the Lua stack.
Here is the encoder function for ``struct prefix``:
.. code-block:: c
@ -204,8 +344,6 @@ has). For example, here is the encoder function for ``struct prefix``:
{
char buffer[PREFIX_STRLEN];
zlog_debug("frrlua: pushing prefix table");
lua_newtable(L);
lua_pushstring(L, prefix2str(prefix, buffer, PREFIX_STRLEN));
lua_setfield(L, -2, "network");
@ -215,7 +353,7 @@ has). For example, here is the encoder function for ``struct prefix``:
lua_setfield(L, -2, "family");
}
This function pushes a single value onto the Lua stack. It is a table whose
This function pushes a single value, a table, onto the Lua stack, whose
equivalent in Lua is:
.. code-block:: c
@ -223,16 +361,23 @@ equivalent in Lua is:
{ ["network"] = "1.2.3.4/24", ["prefixlen"] = 24, ["family"] = 2 }
Decoding
""""""""
Decoders are a bit more involved. They do the reverse; a decoder function takes
a ``lua_State *``, pops a value off the Lua stack and converts it back into its
C type.
However, since Lua programs have the ability to directly modify their inputs
(i.e. values passed in via ``frrscript_call``), we need two separate decoder
functions, called ``lua_decode_*`` and ``lua_to*``.
A ``lua_decode_*`` function takes a ``lua_State*``, an index, and a C type, and
unmarshalls a Lua value into that C type.
Again, for ``struct prefix``:
There are two: ``lua_decode*`` and ``lua_to*``. The former does no mememory
allocation and is needed for ``frrscript_call``.
The latter performs allocation and is optional.
A ``lua_decode_*`` function takes a ``lua_State*``, an index, and a pointer
to a C data structure, and directly modifies the structure with values from the
Lua stack. Note that only non const-qualified pointers may be modified;
``lua_decode_*`` for other types will be noops.
Again, for ``struct prefix *``:
.. code-block:: c
@ -240,22 +385,45 @@ Again, for ``struct prefix``:
{
lua_getfield(L, idx, "network");
(void)str2prefix(lua_tostring(L, -1), prefix);
/* pop the netork string */
lua_pop(L, 1);
/* pop the table */
/* pop the prefix table */
lua_pop(L, 1);
}
Note:
- Before ``lua_decode*`` is run, the "prefix" table is already on the top of
the stack. ``frrscript_call`` does this for us.
- However, at the end of ``lua_decode*``, the "prefix" table should be popped.
- The other two fields in the "network" table are disregarded, meaning that any
modification to them is discarded in C space. In this case, this is desired
behavior.
.. warning::
``lua_decode_prefix`` functions should leave the Lua stack completely empty
when they return.
For decoders that unmarshall fields from tables, remember to pop the table
at the end.
``lua_decode*`` functions should pop all values that ``lua_to*`` pushed onto
the Lua stack.
For encoders that pushed a table, its decoder should pop the table at the end.
The above is an example.
A ``lua_to*`` function perform a similar role except that it first allocates
memory for the new C type before decoding the value from the Lua stack, then
returns a pointer to the newly allocated C type.
``int`` is not a non const-qualified pointer, so for ``int``:
.. code-block:: c
void lua_decode_int_noop(lua_State *L, int idx, int i)
{ //noop
}
A ``lua_to*`` function provides identical functionality except that it first
allocates memory for the new C type before decoding the value from the Lua stack,
then returns a pointer to the newly allocated C type. You only need to implement
this function to use with ``frrscript_get_result`` to retrieve a result of
this type.
This function can and should be implemented using ``lua_decode_*``:
.. code-block:: c
@ -274,18 +442,11 @@ allocated with ``MTYPE_TMP``. This way it is possible to unload the script
(destroy the state) without invalidating any references to values stored in it.
Note that it is the caller's responsibility to free the data.
For consistency, we should always name functions of the first type
``lua_decode_*``.
Functions of the second type should be named ``lua_to*``, as this is the
naming convention used by the Lua C library for the basic types e.g.
``lua_tointeger`` and ``lua_tostring``.
This two-function design allows the compiler to warn if a value passed into
``frrscript_call`` does not have a encoder and decoder for that type.
The ``lua_to*`` functions enable us to easily create decoders for nested
structures.
Registering encoders and decoders for frrscript_call
""""""""""""""""""""""""""""""""""""""""""""""""""""
To register a new type with its corresponding encoding and decoding functions,
To register a new type with its ``lua_push*`` and ``lua_decode*`` functions,
add the mapping in the following macros in ``frrscript.h``:
.. code-block:: diff
@ -331,11 +492,12 @@ For that, use ``lua_decode_noop``:
.. note::
Marshalled types are not restricted to simple values like integers, strings
and tables. It is possible to marshall a type such that the resultant object
in Lua is an actual object-oriented object, complete with methods that call
back into defined C functions. See the Lua manual for how to do this; for a
code example, look at how zlog is exported into the script environment.
Encodable/decodable types are not restricted to simple values like integers,
strings and tables.
It is possible to encode a type such that the resultant object in Lua
is an actual object-oriented object, complete with methods that call
back into defined C functions. See the Lua manual for how to do this;
for a code example, look at how zlog is exported into the script environment.
Script Environment
@ -364,10 +526,11 @@ Examples
For a complete code example involving passing custom types, retrieving results,
and doing complex calculations in Lua, look at the implementation of the
``match script SCRIPT`` command for BGP routemaps. This example calls into a
script with a route prefix and attributes received from a peer and expects the
script to return a match / no match / match and update result.
script with a function named ``route_match``,
provides route prefix and attributes received from a peer and expects the
function to return a match / no match / match and update result.
An example script to use with this follows. This script matches, does not match
An example script to use with this follows. This function matches, does not match
or updates a route depending on how many BGP UPDATE messages the peer has
received when the script is called, simply as a demonstration of what can be
accomplished with scripting.
@ -378,64 +541,75 @@ accomplished with scripting.
-- Example route map matching
-- author: qlyoung
--
-- The following variables are available to us:
-- The following variables are available in the global environment:
-- log
-- logging library, with the usual functions
-- prefix
--
-- route_match arguments:
-- table prefix
-- the route under consideration
-- attributes
-- table attributes
-- the route's attributes
-- peer
-- table peer
-- the peer which received this route
-- RM_FAILURE
-- integer RM_FAILURE
-- status code in case of failure
-- RM_NOMATCH
-- integer RM_NOMATCH
-- status code for no match
-- RM_MATCH
-- integer RM_MATCH
-- status code for match
-- RM_MATCH_AND_CHANGE
-- integer RM_MATCH_AND_CHANGE
-- status code for match-and-set
--
-- We need to set the following out values:
-- action
-- Set to the appropriate status code to indicate what we did
-- attributes
-- Setting fields on here will propagate them back up to the caller if
-- 'action' is set to RM_MATCH_AND_CHANGE.
-- route_match returns table with following keys:
-- integer action, required
-- resultant status code. Should be one of RM_*
-- table attributes, optional
-- updated route attributes
--
function route_match(prefix, attributes, peer,
RM_FAILURE, RM_NOMATCH, RM_MATCH, RM_MATCH_AND_CHANGE)
log.info("Evaluating route " .. prefix.network .. " from peer " .. peer.remote_id.string)
function on_match (prefix, attributes)
log.info("Match")
return {
attributes = RM_MATCH
}
end
function on_nomatch (prefix, attributes)
log.info("No match")
return {
action = RM_NOMATCH
}
end
function on_match_and_change (prefix, attributes)
log.info("Match and change")
attributes["metric"] = attributes["metric"] + 7
return {
action = RM_MATCH_AND_CHANGE,
attributes = attributes
}
end
special_routes = {
["172.16.10.4/24"] = on_match,
["172.16.13.1/8"] = on_nomatch,
["192.168.0.24/8"] = on_match_and_change,
}
log.info("Evaluating route " .. prefix.network .. " from peer " .. peer.remote_id.string)
function on_match (prefix, attrs)
log.info("Match")
action = RM_MATCH
end
function on_nomatch (prefix, attrs)
log.info("No match")
action = RM_NOMATCH
end
function on_match_and_change (prefix, attrs)
action = RM_MATCH_AND_CHANGE
log.info("Match and change")
attrs["metric"] = attrs["metric"] + 7
end
special_routes = {
["172.16.10.4/24"] = on_match,
["172.16.13.1/8"] = on_nomatch,
["192.168.0.24/8"] = on_match_and_change,
}
if special_routes[prefix.network] then
special_routes[prefix.network](prefix, attributes)
elseif peer.stats.update_in % 3 == 0 then
on_match(prefix, attributes)
elseif peer.stats.update_in % 2 == 0 then
on_nomatch(prefix, attributes)
else
on_match_and_change(prefix, attributes)
end
if special_routes[prefix.network] then
return special_routes[prefix.network](prefix, attributes)
elseif peer.stats.update_in % 3 == 0 then
return on_match(prefix, attributes)
elseif peer.stats.update_in % 2 == 0 then
return on_nomatch(prefix, attributes)
else
return on_match_and_change(prefix, attributes)
end
end