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[minix.git] / minix / net / lwip / rttree.c

/* LWIP service - rttree.c - generic routing tree data structure */
/*
 * This module implements the Net/3 binary radix (Patricia) tree as described
 * in TCP/IP Illustrated Vol.2, with a few important changes.  First and
 * foremost, we make the assumption that all address masks are "normal", i.e.,
 * they can be expressed in terms of a "prefix length" or "bit count", meaning
 * that the first so many bits of the mask are set and the remaining bits are
 * all clear.  Based on this assumption, we store routing entries not just in
 * leaf nodes, but rather in a node at the bit count of the routing entry's
 * mask; this node may then also have children.  As a result, instead of "leaf"
 * and "internal" nodes, this module instead uses "data" and "link" nodes:
 *
 * - Data nodes are nodes with an associated routing entry.  The data node
 *   structure is always the first field of its corresponding routing entry
 *   structure.  Data nodes may have zero, one, or two children.  Its children
 *   are always a refinement of the address mask in the routing entry.
 * - Link nodes are nodes with no associated routing entry.  They always have
 *   exactly two children.  As with BSD's "internal" nodes: since the tree
 *   needs no more than one link node per routing entry, each routing entry
 *   structure contains a link node, which may be used anywhere in the tree.
 *
 * The result of this approach is that we do not use a linked list for each
 * leaf, since entries with the same address and different masks are not stored
 * as part of the same leaf node.  There is however still one case where a
 * linked list would be necessary: the coexistence of a full-mask network entry
 * and a host entry (net/32 vs host for IPv4, net/128 vs host for IPv6).  Since
 * this tree implementation is not used for ARP/ND6 (host) entries, the need to
 * support that case is not as high, and so it is currently not supported.  It
 * can be added later if needed.  In that case, the prototype of only
 * rttree_find_exact() will have to be changed, since rttree_add() already
 * supports the difference by passing a full mask vs passing no mask at all.
 *
 * There are other differences with the BSD implementation, and certainly also
 * more opportunities for improving performance.  For now, the implementation
 * should be good enough for its intended purpose.
 */

#include "lwip.h"
#include "rttree.h"

#define RTTREE_BITS_TO_BYTE(bits)       ((bits) >> 3)
#define RTTREE_BITS_TO_SHIFT(bits)      (7 - ((bits) & 7))
#define RTTREE_BITS_TO_BYTES(bits)      (RTTREE_BITS_TO_BYTE((bits) + 7))

/*
 * The given node is being added to the given routing tree, and just had its
 * bit count assigned.  Precompute any additional fields used for fast address
 * access on the node.
 */
static void
rttree_precompute(struct rttree * tree __unused, struct rttree_node * node)
{

        node->rtn_byte = RTTREE_BITS_TO_BYTE(node->rtn_bits);
        node->rtn_shift = RTTREE_BITS_TO_SHIFT(node->rtn_bits);
}

/*
 * For an operation on the routing tree 'tree', test whether the bit 'bit' is
 * set or clear in 'addr'.  Return 1 if the address has the bit set, 0 if it
 * does not.
 */
static unsigned int
rttree_test(const struct rttree * tree __unused, const void * addr,
        unsigned int bit)
{
        unsigned int byte, shift;

        byte = RTTREE_BITS_TO_BYTE(bit);
        shift = RTTREE_BITS_TO_SHIFT(bit);

        return (((const uint8_t *)addr)[byte] >> shift) & 1;
}

/*
 * For an operation on the routing tree 'tree', test whether a particular bit
 * as identified by the routing node 'node' is set or clear in 'address',
 * effectively computing the side (left or right) to take when descending down
 * the tree.  Return 1 if the address has the bit set, 0 if it does not.
 */
static inline unsigned int
rttree_side(const struct rttree * tree, const struct rttree_node * node,
        const void * addr)
{

        return (((const uint8_t *)addr)[node->rtn_byte] >>
            node->rtn_shift) & 1;
}

/*
 * Check for the routing tree 'tree' whether the routing entry 'entry' matches
 * the address 'addr' exactly.  Return TRUE or FALSE depending on the outcome.
 * This function must be called only on entries that have already been
 * determined to span the full bit width.
 */
static inline int
rttree_equals(const struct rttree * tree, const struct rttree_entry * entry,
        const void * addr)
{
        unsigned int bits;

        bits = tree->rtt_bits;

        assert(bits == entry->rte_data.rtn_bits);

        return !memcmp(entry->rte_addr, addr, RTTREE_BITS_TO_BYTE(bits));
}

/*
 * Check for the routing tree 'tree' whether the routing entry 'entry' matches
 * the address 'addr'.  Return TRUE if the address is matched by the entry's
 * address and mask, or FALSE if not.
 */
static inline int
rttree_match(const struct rttree * tree, const struct rttree_entry * entry,
        const void * addr)
{
        const uint8_t *aptr, *aptr2, *mptr;
        unsigned int bits, bytes;

        if ((bits = entry->rte_data.rtn_bits) == 0)
                return TRUE;

        if ((mptr = (const uint8_t *)entry->rte_mask) == NULL)
                return rttree_equals(tree, entry, addr);

        aptr = (const uint8_t *)addr;
        aptr2 = (const uint8_t *)entry->rte_addr;

        for (bytes = RTTREE_BITS_TO_BYTES(bits); bytes > 0; bytes--) {
                if ((*aptr & *mptr) != *aptr2)
                        return FALSE;

                aptr++;
                aptr2++;
                mptr++;
        }

        return TRUE;
}

/*
 * Find the first bit that differs between the two given addresses.  Return the
 * bit number if found, or the full bit width if the addresses are equal.
 */
static unsigned int
rttree_diff(const struct rttree * tree, const void * addr, const void * addr2)
{
        const uint8_t *aptr, *aptr2;
        unsigned int bit, i;
        uint8_t b;

        aptr = (const uint8_t *)addr;
        aptr2 = (const uint8_t *)addr2;

        for (bit = 0; bit < tree->rtt_bits; bit += NBBY, aptr++, aptr2++) {
                if ((b = *aptr ^ *aptr2) != 0) {
                        for (i = 0; i < NBBY; i++)
                                if (b & (1 << (NBBY - i - 1)))
                                        break;
                        return bit + i;
                }
        }

        return bit;
}

/*
 * Add a link node to the free list of the given routing tree, marking it as
 * free in the process.
 */
static void
rttree_add_free(struct rttree * tree, struct rttree_node * node)
{

        node->rtn_child[0] = NULL;
        if ((node->rtn_child[1] = tree->rtt_free) != NULL)
                node->rtn_child[1]->rtn_child[0] = node;
        tree->rtt_free = node;
        node->rtn_parent = NULL;
        node->rtn_type = RTNT_FREE;
}

/*
 * Remove the given free link node from the free list.  The caller must already
 * have verified that the node is on the free list, and has to change the node
 * type as appropriate afterward.
 */
static void
rttree_del_free(struct rttree * tree, struct rttree_node * node)
{

        assert(node->rtn_type == RTNT_FREE);

        if (node->rtn_child[0] != NULL)
                node->rtn_child[0]->rtn_child[1] = node->rtn_child[1];
        else
                tree->rtt_free = node->rtn_child[1];
        if (node->rtn_child[1] != NULL)
                node->rtn_child[1]->rtn_child[0] = node->rtn_child[0];
}

/*
 * Obtain, remove, and return a free link node from the free list.  This
 * function must be called only when it is already known that the free list is
 * not empty.  The caller has to change the node type as appropriate afterward.
 */
static struct rttree_node *
rttree_get_free(struct rttree * tree)
{
        struct rttree_node * node;

        node = tree->rtt_free;
        assert(node != NULL);
        assert(node->rtn_type == RTNT_FREE);

        rttree_del_free(tree, node);

        return node;
}

/*
 * Initialize the given routing tree, with the given address bit width.
 */
void
rttree_init(struct rttree * tree, unsigned int bits)
{

        tree->rtt_root = NULL;
        tree->rtt_free = NULL;
        tree->rtt_bits = bits;
}

/*
 * Look up the most narrow routing tree entry that matches the given address.
 * Return the entry on success, or NULL if no matching entry is found.
 */
struct rttree_entry *
rttree_lookup_match(struct rttree * tree, const void * addr)
{
        struct rttree_entry *entry, *best;
        struct rttree_node *node;
        unsigned int side;

        /*
         * The current implementation is "forward-tracking", testing all
         * potentially matching entries while descending into the tree and
         * remembering the "best" (narrowest matching) entry.  The assumption
         * here is that most lookups will end up returning the default route or
         * another broad route, and thus quickly fail a narrower match and bail
         * out early.  This assumption is in part motivated by the fact that
         * our routing trees do not store link-layer (ARP/ND6) entries.  If
         * desired, the implementation can easily be rewritten to do
         * backtracking instead.
         */
        best = NULL;

        for (node = tree->rtt_root; node != NULL;
            node = node->rtn_child[side]) {
                if (node->rtn_type == RTNT_DATA) {
                        entry = (struct rttree_entry *)node;

                        if (!rttree_match(tree, entry, addr))
                                break;

                        best = entry;
                }

                side = rttree_side(tree, node, addr);
        }

        return best;
}

/*
 * Look up a routing entry that is an exact match for the given (full) address.
 * Return the entry if it was found, or NULL otherwise.
 */
struct rttree_entry *
rttree_lookup_host(struct rttree * tree, const void * addr)
{
        struct rttree_entry *entry;
        struct rttree_node *node;
        unsigned int side;

        for (node = tree->rtt_root; node != NULL;
            node = node->rtn_child[side]) {
                if (node->rtn_type == RTNT_DATA &&
                    node->rtn_bits == tree->rtt_bits) {
                        entry = (struct rttree_entry *)node;

                        if (rttree_equals(tree, entry, addr))
                                return entry;

                        break;
                }

                side = rttree_side(tree, node, addr);
        }

        return NULL;
}

/*
 * Look up a routing entry that is an exact match for the given address and
 * prefix length.  Return the entry if found, or NULL otherwise.
 */
struct rttree_entry *
rttree_lookup_exact(struct rttree * tree, const void * addr,
        unsigned int prefix)
{
        struct rttree_entry *entry;
        struct rttree_node *node;
        unsigned int side;

        for (node = tree->rtt_root; node != NULL && node->rtn_bits <= prefix;
            node = node->rtn_child[side]) {
                if (node->rtn_type == RTNT_DATA) {
                        entry = (struct rttree_entry *)node;

                        if (!rttree_match(tree, entry, addr))
                                return NULL;

                        if (node->rtn_bits == prefix)
                                return entry;
                }

                side = rttree_side(tree, node, addr);
        }

        return NULL;
}

/*
 * Enumerate entries in the routing tree.  If 'last' is NULL, return the first
 * entry.  Otherwise, return the next entry starting from 'last'.  In both
 * cases, if no (more) entries are present in the tree, return NULL.  The order
 * of the returned entries is stable across tree modifications and the function
 * may be called multiple times on the same entry.  More specifically, it is
 * safe to continue enumeration from a previous entry after deleting its
 * successor from the tree.
 */
struct rttree_entry *
rttree_enum(struct rttree * tree, struct rttree_entry * last)
{
        struct rttree_node *node, *parent;

        /*
         * For the first query, we may have to return the tree root right away.
         * For subsequent queries, we have to move ahead by at least one node.
         */
        if (last == NULL) {
                if ((node = tree->rtt_root) == NULL)
                        return NULL;

                if (node->rtn_type == RTNT_DATA)
                        return (struct rttree_entry *)node;
        } else
                node = &last->rte_data;

        /* A basic iterative pre-order binary-tree depth-first search. */
        do {
                assert(node != NULL);

                /* Can we descend further, either left or right? */
                if (node->rtn_child[0] != NULL)
                        node = node->rtn_child[0];
                else if (node->rtn_child[1] != NULL)
                        node = node->rtn_child[1];
                else {
                        /*
                         * No.  Go back up the tree, until we can go right
                         * where we went left before.. or run out of tree.
                         */
                        for (;; node = parent) {
                                if ((parent = node->rtn_parent) == NULL)
                                        return NULL;

                                if (parent->rtn_child[0] == node &&
                                    parent->rtn_child[1] != NULL) {
                                        node = parent->rtn_child[1];

                                        break;
                                }
                        }
                }

                /* Skip link nodes. */
        } while (node->rtn_type != RTNT_DATA);

        return (struct rttree_entry *)node;
}

/*
 * Set the node 'node' to be part of tree 'tree', with type 'type' (either
 * RTNT_DATA or RTNT_LINK) and a bit count of 'prefix'.  The node is set to be
 * a child of 'parent' on side 'side', unless 'parent' is NULL in which case
 * the node is set to be the topmost node in the tree (and 'side' is ignored).
 * The node's children are set to 'left' and 'right'; for each, if not NULL,
 * its parent is set to 'node'.
 */
static void
rttree_set(struct rttree * tree, struct rttree_node * node, int type,
        unsigned int prefix, struct rttree_node * parent, int side,
        struct rttree_node * left, struct rttree_node * right)
{

        assert(type == RTNT_DATA || type == RTNT_LINK);
        assert(prefix <= tree->rtt_bits);
        assert(side == 0 || side == 1);

        node->rtn_type = type;
        node->rtn_bits = prefix;

        /* With rtn_bits assigned, precompute any derived fields. */
        rttree_precompute(tree, node);

        if ((node->rtn_parent = parent) != NULL)
                parent->rtn_child[side] = node;
        else
                tree->rtt_root = node;

        if ((node->rtn_child[0] = left) != NULL)
                left->rtn_parent = node;
        if ((node->rtn_child[1] = right) != NULL)
                right->rtn_parent = node;
}

/*
 * In the routing tree 'tree', replace old node 'onode' with new node 'node',
 * setting the type of the latter to 'type'.  The tree is updated accordingly,
 * but it is left up to the caller to deal with the old node as appropriate.
 */
static void
rttree_replace(struct rttree * tree, struct rttree_node * onode,
        struct rttree_node * node, int type)
{
        struct rttree_node *parent;
        unsigned int side;

        /*
         * Replacing one data node with another data node is not something that
         * is currently being done, even if it would work.
         */
        assert(onode->rtn_type != RTNT_DATA || node->rtn_type != RTNT_DATA);
        assert(onode->rtn_child[0] != NULL);
        assert(onode->rtn_child[1] != NULL);

        parent = onode->rtn_parent;

        side = (parent != NULL && parent->rtn_child[1] == onode);

        rttree_set(tree, node, type, onode->rtn_bits, parent, side,
            onode->rtn_child[0], onode->rtn_child[1]);
}

/*
 * Add a new routing entry 'entry' to the routing tree 'tree'.  The entry
 * object will be initialized as a result.  The address to add is given as
 * 'addr', and the address mask as 'mask'.  Both those pointers must be point
 * to memory that is as long-lived as the routing entry; this is typically
 * accomplished by storing them in a larger object that embeds 'entry'.
 * However, 'mask' may be NULL, signifying a host type entry with an implied
 * full mask.  If not NULL, the given mask must be normalized, i.e., it must
 * consist of a run of zero or more 1-bits followed by a remainder of only
 * 0-bits.  The number of 1-bits must also be given as a bit count 'prefix',
 * even if 'mask' is NULL.  The address must be normalized to its mask: no bits
 * starting from bit 'prefix' must be set in 'addr'.  Return OK if adding the
 * routing entry succeeded, or EEXIST if an entry already exists for the
 * combination of that address and mask.  If the caller has already verified
 * with rttree_lookup_exact() that no such entry exists, the call will succeed.
 */
int
rttree_add(struct rttree * tree, struct rttree_entry * entry,
        const void * addr, const void * mask, unsigned int prefix)
{
        struct rttree_node *node, *parent, *link;
        struct rttree_entry *other_entry;
        unsigned int bit = 0 /*gcc*/, side, side2;
        int match;

        assert(mask != NULL || prefix == tree->rtt_bits);

        /*
         * We start by determining the path, bit count, and method of the
         * addition.  We do this with a lookup on the address, for the full
         * address width--that is, not limited to the given prefix length.  As
         * a result, at some point we will find either a NULL pointer, or a
         * data node with a width that is at least as large as the given prefix
         * length.  The NULL case is easy: we EXTEND the tree with our new
         * entry wherever we ran into the NULL pointer.
         *
         * If instead we find a sufficiently wide data node, then we see if it
         * is a match for the new address.  If so, our new data node should
         * either be INSERTed between two nodes along the path taken so far, or
         * REPLACE a link node along that path with the new data node.  If it
         * it is not a match, then the action to take depends on whether the
         * first differing bit falls within the given prefix length: if so, we
         * have to BRANCH along the path, using a link node allocated for that
         * differing bit; if not, we should use INSERT or REPLACE after all.
         *
         * As the only exceptional case, we might in fact find an entry for the
         * exact same address and prefix length as what is being added.  In the
         * current design of the routing tree, this is always a failure case.
         */
        parent = NULL;
        side = 0;
        other_entry = NULL;

        for (node = tree->rtt_root; node != NULL;
            node = node->rtn_child[side]) {
                if (node->rtn_type == RTNT_DATA) {
                        other_entry = (struct rttree_entry *)node;

                        bit = rttree_diff(tree, other_entry->rte_addr, addr);

                        match = (bit >= node->rtn_bits);

                        /* Test whether the exact entry already exists. */
                        if (match && node->rtn_bits == prefix)
                                return EEXIST;

                        /*
                         * Test the INSERT/REPLACE and BRANCH cases.  Note that
                         * this condition is in a terse, optimized form that
                         * does not map directly to the two different cases.
                         */
                        if (!match || node->rtn_bits > prefix) {
                                if (bit > prefix)
                                        bit = prefix;
                                break;
                        }
                }

                parent = node;
                side = rttree_side(tree, node, addr);
        }

        /*
         * At this point, addition is going to succeed no matter what.  Start
         * by initializing part of 'entry'.  In particular, add the given
         * entry's link node to the list of free link nodes, because the common
         * case is that we end up not using it.  If we do, we will just take it
         * off again right away.  The entry's data node will be initialized as
         * part of the addition process below.
         */
        entry->rte_addr = addr;
        entry->rte_mask = mask;

        rttree_add_free(tree, &entry->rte_link);

        /*
         * First deal with the EXTEND case.  In that case we already know the
         * intended parent and the side (left/right) for the addition.
         */
        if (node == NULL) {
                assert(parent == NULL || parent->rtn_bits < prefix);
                assert(parent == NULL || parent->rtn_child[side] == NULL);

                rttree_set(tree, &entry->rte_data, RTNT_DATA, prefix, parent,
                    side, NULL /*left*/, NULL /*right*/);

                return OK;
        }

        /*
         * For the other three cases, we now have to walk back along the path
         * we have taken so far in order to find the correct insertion point.
         */
        while (parent != NULL && parent->rtn_bits >= bit) {
                node = parent;

                parent = node->rtn_parent;
        }

        if (bit == prefix && node->rtn_bits == bit) {
                /*
                 * The REPLACE case.  Replace the link node 'node' with our new
                 * entry.  Afterwards, mark the link node as free.
                 */
                assert(node->rtn_type != RTNT_DATA);

                rttree_replace(tree, node, &entry->rte_data, RTNT_DATA);

                rttree_add_free(tree, node);
        } else if (bit == prefix) {
                /*
                 * The INSERT case.  Insert the data node between 'parent' and
                 * 'node'.  Note that 'parent' may be NULL.  We need to use the
                 * address we found earlier, as 'other_entry', to determine
                 * whether we should add 'node' to the left or right of the
                 * inserted data node.
                 */
                assert(node->rtn_bits > bit);
                assert(parent == NULL || parent->rtn_bits < bit);
                assert(other_entry != NULL);

                side = (parent != NULL && parent->rtn_child[1] == node);

                side2 = rttree_test(tree, other_entry->rte_addr, bit);

                rttree_set(tree, &entry->rte_data, RTNT_DATA, prefix, parent,
                    side, (!side2) ? node : NULL, (side2) ? node : NULL);
        } else {
                /*
                 * The BRANCH case.  In this case, it is impossible that we
                 * find a link node with a bit count equal to the first
                 * differing bit between the address we found and the address
                 * we want to insert: if such a node existed, we would have
                 * descended down its other child during the initial lookup.
                 *
                 * Interpose a link node between 'parent' and 'current' for bit
                 * 'bit', with its other child set to point to 'entry'.  Again,
                 * we need to perform an additional bit test here, because even
                 * though we know that the address we found during the lookup
                 * differs from the given address at bit 'bit', we do not know
                 * the value of either bit yet.
                 */
                assert(bit < prefix);
                assert(node->rtn_bits > bit);
                assert(parent == NULL || parent->rtn_bits < bit);

                link = rttree_get_free(tree);

                side = (parent != NULL && parent->rtn_child[1] == node);

                side2 = rttree_test(tree, addr, bit);

                /* Use NULL for the data node we are about to add. */
                rttree_set(tree, link, RTNT_LINK, bit, parent, side,
                    (side2) ? node : NULL, (!side2) ? node : NULL);

                /* This addition will replace the NULL pointer again. */
                rttree_set(tree, &entry->rte_data, RTNT_DATA, prefix, link,
                    side2, NULL /*left*/, NULL /*right*/);
        }

        return OK;
}

/*
 * Remove a particular node 'node' from the routing tree 'tree'.  The given
 * node must have zero or one children.  As integrity check only, if 'nonempty'
 * is set, the node must have one child.  If the node has one child, that child
 * will be linked to the node's parent (or the tree root), thus cutting the
 * node itself out of the tree.  If the node has zero children, the
 * corresponding slot in its parent (or the tree root) will be cleared.  The
 * function will return a pointer to the parent node if it too qualifies for
 * removal afterwards, or NULL if no further removal action needs to be taken.
 */
static struct rttree_node *
rttree_remove(struct rttree * tree, struct rttree_node * node,
        int nonempty __unused)
{
        struct rttree_node *parent, *child;
        unsigned int side;

        if ((child = node->rtn_child[0]) == NULL)
                child = node->rtn_child[1];

        assert(child != NULL || !nonempty);

        if ((parent = node->rtn_parent) != NULL) {
                side = (parent->rtn_child[1] == node);

                parent->rtn_child[side] = child;

                if (child != NULL)
                        child->rtn_parent = parent;
                else if (parent->rtn_type == RTNT_LINK)
                        return parent;
        } else {
                tree->rtt_root = child;

                if (child != NULL)
                        child->rtn_parent = NULL;
        }

        return NULL;
}

/*
 * Delete the routing entry 'entry' from the routing tree 'tree'.  The entry
 * must have been added before.  This function always succeeds.
 */
void
rttree_delete(struct rttree * tree, struct rttree_entry * entry)
{
        struct rttree_node *node, *link;

        /*
         * Remove the data node from the tree.  If the data node also has two
         * children, we have to replace it with a link node.  Otherwise, we
         * have to remove it and, if it has no children at all, possibly remove
         * its parent as well.
         */
        node = &entry->rte_data;

        assert(node->rtn_type == RTNT_DATA);

        if (node->rtn_child[0] != NULL && node->rtn_child[1] != NULL) {
                /*
                 * The link node we allocate here may actually be the entry's
                 * own link node.  We do not make an exception for that case
                 * here, as we have to deal with the entry's link node being in
                 * use a bit further down anyway.
                 */
                link = rttree_get_free(tree);

                rttree_replace(tree, node, link, RTNT_LINK);
        } else {
                /*
                 * Remove the data node from the tree.  If the node has no
                 * children, its removal may leave a link node with one child.
                 * That would be its original parent.  That node must then also
                 * be removed from the tree, and freed up.
                 */
                link = rttree_remove(tree, node, FALSE /*nonempty*/);

                if (link != NULL) {
                        (void)rttree_remove(tree, link, TRUE /*nonempty*/);

                        rttree_add_free(tree, link);
                }
        }

        /*
         * Remove the entry's link node from either the tree or the free list,
         * depending on the type currently assigned to it.  If it has to be
         * removed from the tree, it must be replaced with another link node.
         * There will always be enough link nodes available for this to work.
         */
        node = &entry->rte_link;

        if (node->rtn_type == RTNT_LINK) {
                link = rttree_get_free(tree);

                rttree_replace(tree, node, link, RTNT_LINK);
        } else {
                assert(node->rtn_type == RTNT_FREE);

                rttree_del_free(tree, node);
        }
}