You can find sources of the filter language in filter/
directory. File filter/config.Y contains filter grammar and basically translates
the source from user into a tree of f_inst structures. These trees are
later interpreted using code in filter/filter.c.
A filter is represented by a tree of f_inst structures, one structure per "instruction". Each f_inst contains code, aux value which is usually the data type this instruction operates on and two generic arguments (a1, a2). Some instructions contain pointer(s) to other instructions in their (a1, a2) fields.
Filters use a f_val structure for their data. Each f_val
contains type and value (types are constants prefixed with T_). Few
of the types are special; T_RETURN can be or-ed with a type to indicate
that return from a function or from the whole filter should be
forced. Important thing about f_val's is that they may be copied
with a simple =. That's fine for all currently defined types: strings
are read-only (and therefore okay), paths are copied for each
operation (okay too).
int val_compare (struct f_val v1, struct f_val v2) -- compare two values
first value
second value
Compares two values and returns -1, 0, 1 on <, =, > or CMP_ERROR on error. Tree module relies on this giving consistent results so that it can be used for building balanced trees.
int val_same (struct f_val v1, struct f_val v2) -- compare two values
first value
second value
Compares two values and returns 1 if they are same and 0 if not. Comparison of values of different types is valid and returns 0.
int
val_in_range
(struct f_val v1, struct f_val v2) -- implement ~ operator
element
set
Checks if v1 is element (~ operator) of v2.
struct f_val interpret (struct f_inst * what)
filter to interpret
Interpret given tree of filter instructions. This is core function of filter system and does all the hard work.
code (which is instruction code), aux (which is extension to instruction code, typically type), arg1 and arg2 - arguments. Depending on instruction, arguments are either integers, or pointers to instruction trees. Common instructions like +, that have two expressions as arguments use TWOARGS macro to get both of them evaluated.
f_val structures are copied around, so there are no problems with memory managment.
int f_run (struct filter * filter, struct rte ** rte, struct ea_list ** tmp_attrs, struct linpool * tmp_pool, int flags) -- run a filter for a route
filter to run
route being filtered, may be modified
temporary attributes, prepared by caller or generated by f_run()
all filter allocations go from this pool
flags
If filter needs to modify the route, there are several posibilities. rte might be read-only (with REF_COW flag), in that case rw copy is obtained by rte_cow() and rte is replaced. If rte is originally rw, it may be directly modified (and it is never copied).
The returned rte may reuse the (possibly cached, cloned) rta, or (if rta was modificied) contains a modified uncached rta, which uses parts allocated from tmp_pool and parts shared from original rta. There is one exception - if rte is rw but contains a cached rta and that is modified, rta in returned rte is also cached.
Ownership of cached rtas is consistent with rte, i.e. if a new rte is returned, it has its own clone of cached rta (and cached rta of read-only source rte is intact), if rte is modified in place, old cached rta is possibly freed.
int filter_same (struct filter * new, struct filter * old) -- compare two filters
first filter to be compared
second filter to be compared, notice that this filter is damaged while comparing.
Returns 1 in case filters are same, otherwise 0. If there are underlying bugs, it will rather say 0 on same filters than say 1 on different.
struct f_tree * find_tree (struct f_tree * t, struct f_val val)
tree to search in
value to find
Search for given value in the tree. I relies on fact that sorted tree is populated by f_val structures (that can be compared by val_compare()). In each node of tree, either single value (then t->from==t->to) or range is present.
Both set matching and switch() { } construction is implemented using this function,
thus both are as fast as they can be.
struct f_tree * build_tree (struct f_tree * from)
degenerated tree (linked by tree->left) to be transformed into form suitable for find_tree()
Transforms degenerated tree into balanced tree.
int same_tree (struct f_tree * t1, struct f_tree * t2)
first tree to be compared
second one
Compares two trees and returns 1 if they are same
We use a (compressed) trie to represent prefix sets. Every node in the trie represents one prefix (addr/plen) and plen also indicates the index of the bit in the address that is used to branch at the node. If we need to represent just a set of prefixes, it would be simple, but we have to represent a set of prefix patterns. Each prefix pattern consists of ppaddr/pplen and two integers: low and high, and a prefix paddr/plen matches that pattern if the first MIN(plen, pplen) bits of paddr and ppaddr are the same and low <= plen <= high.
We use a bitmask (accept) to represent accepted prefix lengths at a node. As there are 33 prefix lengths (0..32 for IPv4), but there is just one prefix of zero length in the whole trie so we have zero flag in f_trie (indicating whether the trie accepts prefix 0.0.0.0/0) as a special case, and accept bitmask represents accepted prefix lengths from 1 to 32.
There are two cases in prefix matching - a match when the length of the prefix is smaller that the length of the prefix pattern, (plen < pplen) and otherwise. The second case is simple - we just walk through the trie and look at every visited node whether that prefix accepts our prefix length (plen). The first case is tricky - we don't want to examine every descendant of a final node, so (when we create the trie) we have to propagate that information from nodes to their ascendants.
Suppose that we have two masks (M1 and M2) for a node. Mask M1 represents accepted prefix lengths by just the node and mask M2 represents accepted prefix lengths by the node or any of its descendants. Therefore M2 is a bitwise or of M1 and children's M2 and this is a maintained invariant during trie building. Basically, when we want to match a prefix, we walk through the trie, check mask M1 for our prefix length and when we came to final node, we check mask M2.
There are two differences in the real implementation. First, we use a compressed trie so there is a case that we skip our final node (if it is not in the trie) and we came to node that is either extension of our prefix, or completely out of path In the first case, we also have to check M2.
Second, we really need not to maintain two separate bitmasks. Checks for mask M1 are always larger than applen and we need just the first pplen bits of mask M2 (if trie compression hadn't been used it would suffice to know just $applen-th bit), so we have to store them together in accept mask - the first pplen bits of mask M2 and then mask M1.
There are four cases when we walk through a trie:
- we are in NULL - we are out of path (prefixes are inconsistent) - we are in the wanted (final) node (node length == plen) - we are beyond the end of path (node length > plen) - we are still on path and keep walking (node length < plen)
The walking code in trie_match_prefix() is structured according to these cases.
struct f_trie * f_new_trie (linpool * lp, uint node_size) -- allocates and returns a new empty trie
linear pool to allocate items from
node size to be used (f_trie_node and user data)
void * trie_add_prefix (struct f_trie * t, ip_addr px, int plen, int l, int h)
trie to add to
prefix address
prefix length
prefix lower bound
prefix upper bound
Adds prefix (prefix pattern) px/plen to trie t. l and h are lower and upper bounds on accepted prefix lengths, both inclusive. 0 <= l, h <= 32 (128 for IPv6).
Returns a pointer to the allocated node. The function can return a pointer to an existing node if px and plen are the same. If px/plen == 0/0 (or ::/0), a pointer to the root node is returned.
int trie_match_prefix (struct f_trie * t, ip_addr px, int plen)
trie
prefix address
prefix length
Tries to find a matching prefix pattern in the trie such that prefix px/plen matches that prefix pattern. Returns 1 if there is such prefix pattern in the trie.
int trie_same (struct f_trie * t1, struct f_trie * t2)
first trie to be compared
second one
Compares two tries and returns 1 if they are same
void trie_format (struct f_trie * t, buffer * buf)
trie to be formatted
destination buffer
Prints the trie to the supplied buffer.