/*
 * lib/textsearch.c     Generic text search interface
 *
 *              This program is free software; you can redistribute it and/or
 *              modify it under the terms of the GNU General Public License
 *              as published by the Free Software Foundation; either version
 *              2 of the License, or (at your option) any later version.
 *
 * Authors:     Thomas Graf <tgraf@suug.ch>
 *              Pablo Neira Ayuso <pablo@netfilter.org>
 *
 * ==========================================================================
 *
 * INTRODUCTION
 *
 *   The textsearch infrastructure provides text searching facilities for
 *   both linear and non-linear data. Individual search algorithms are
 *   implemented in modules and chosen by the user.
 *
 * ARCHITECTURE
 *
 *      User
 *     +----------------+
 *     |        finish()|<--------------(6)-----------------+
 *     |get_next_block()|<--------------(5)---------------+ |
 *     |                |                     Algorithm   | |
 *     |                |                    +------------------------------+
 *     |                |                    |  init()   find()   destroy() |
 *     |                |                    +------------------------------+
 *     |                |       Core API           ^       ^          ^
 *     |                |      +---------------+  (2)     (4)        (8)
 *     |             (1)|----->| prepare()     |---+       |          |
 *     |             (3)|----->| find()/next() |-----------+          |
 *     |             (7)|----->| destroy()     |----------------------+
 *     +----------------+      +---------------+
 *  
 *   (1) User configures a search by calling _prepare() specifying the
 *       search parameters such as the pattern and algorithm name.
 *   (2) Core requests the algorithm to allocate and initialize a search
 *       configuration according to the specified parameters.
 *   (3) User starts the search(es) by calling _find() or _next() to
 *       fetch subsequent occurrences. A state variable is provided
 *       to the algorithm to store persistent variables.
 *   (4) Core eventually resets the search offset and forwards the find()
 *       request to the algorithm.
 *   (5) Algorithm calls get_next_block() provided by the user continuously
 *       to fetch the data to be searched in block by block.
 *   (6) Algorithm invokes finish() after the last call to get_next_block
 *       to clean up any leftovers from get_next_block. (Optional)
 *   (7) User destroys the configuration by calling _destroy().
 *   (8) Core notifies the algorithm to destroy algorithm specific
 *       allocations. (Optional)
 *
 * USAGE
 *
 *   Before a search can be performed, a configuration must be created
 *   by calling textsearch_prepare() specifying the searching algorithm,
 *   the pattern to look for and flags. As a flag, you can set TS_IGNORECASE
 *   to perform case insensitive matching. But it might slow down
 *   performance of algorithm, so you should use it at own your risk.
 *   The returned configuration may then be used for an arbitrary
 *   amount of times and even in parallel as long as a separate struct
 *   ts_state variable is provided to every instance.
 *
 *   The actual search is performed by either calling textsearch_find_-
 *   continuous() for linear data or by providing an own get_next_block()
 *   implementation and calling textsearch_find(). Both functions return
 *   the position of the first occurrence of the pattern or UINT_MAX if
 *   no match was found. Subsequent occurrences can be found by calling
 *   textsearch_next() regardless of the linearity of the data.
 *
 *   Once you're done using a configuration it must be given back via
 *   textsearch_destroy.
 *
 * EXAMPLE
 *
 *   int pos;
 *   struct ts_config *conf;
 *   struct ts_state state;
 *   const char *pattern = "chicken";
 *   const char *example = "We dance the funky chicken";
 *
 *   conf = textsearch_prepare("kmp", pattern, strlen(pattern),
 *                             GFP_KERNEL, TS_AUTOLOAD);
 *   if (IS_ERR(conf)) {
 *       err = PTR_ERR(conf);
 *       goto errout;
 *   }
 *
 *   pos = textsearch_find_continuous(conf, &state, example, strlen(example));
 *   if (pos != UINT_MAX)
 *       panic("Oh my god, dancing chickens at %d\n", pos);
 *
 *   textsearch_destroy(conf);
 * ==========================================================================
 */

#include <linux/module.h>
#include <linux/types.h>
#include <linux/string.h>
#include <linux/init.h>
#include <linux/rculist.h>
#include <linux/rcupdate.h>
#include <linux/err.h>
#include <linux/textsearch.h>
#include <linux/slab.h>

static LIST_HEAD(ts_ops);
static DEFINE_SPINLOCK(ts_mod_lock);

static inline struct ts_ops *lookup_ts_algo(const char *name)
{
        struct ts_ops *o;

        rcu_read_lock();
        list_for_each_entry_rcu(o, &ts_ops, list) {
                if (!strcmp(name, o->name)) {
                        if (!try_module_get(o->owner))
                                o = NULL;
                        rcu_read_unlock();
                        return o;
                }
        }
        rcu_read_unlock();

        return NULL;
}

/**
 * textsearch_register - register a textsearch module
 * @ops: operations lookup table
 *
 * This function must be called by textsearch modules to announce
 * their presence. The specified &@ops must have %name set to a
 * unique identifier and the callbacks find(), init(), get_pattern(),
 * and get_pattern_len() must be implemented.
 *
 * Returns 0 or -EEXISTS if another module has already registered
 * with same name.
 */
int textsearch_register(struct ts_ops *ops)
{
        int err = -EEXIST;
        struct ts_ops *o;

        if (ops->name == NULL || ops->find == NULL || ops->init == NULL ||
            ops->get_pattern == NULL || ops->get_pattern_len == NULL)
                return -EINVAL;

        spin_lock(&ts_mod_lock);
        list_for_each_entry(o, &ts_ops, list) {
                if (!strcmp(ops->name, o->name))
                        goto errout;
        }

        list_add_tail_rcu(&ops->list, &ts_ops);
        err = 0;
errout:
        spin_unlock(&ts_mod_lock);
        return err;
}

/**
 * textsearch_unregister - unregister a textsearch module
 * @ops: operations lookup table
 *
 * This function must be called by textsearch modules to announce
 * their disappearance for examples when the module gets unloaded.
 * The &ops parameter must be the same as the one during the
 * registration.
 *
 * Returns 0 on success or -ENOENT if no matching textsearch
 * registration was found.
 */
int textsearch_unregister(struct ts_ops *ops)
{
        int err = 0;
        struct ts_ops *o;

        spin_lock(&ts_mod_lock);
        list_for_each_entry(o, &ts_ops, list) {
                if (o == ops) {
                        list_del_rcu(&o->list);
                        goto out;
                }
        }

        err = -ENOENT;
out:
        spin_unlock(&ts_mod_lock);
        return err;
}

struct ts_linear_state
{
        unsigned int    len;
        const void      *data;
};

static unsigned int get_linear_data(unsigned int consumed, const u8 **dst,
                                    struct ts_config *conf,
                                    struct ts_state *state)
{
        struct ts_linear_state *st = (struct ts_linear_state *) state->cb;

        if (likely(consumed < st->len)) {
                *dst = st->data + consumed;
                return st->len - consumed;
        }

        return 0;
}

/**
 * textsearch_find_continuous - search a pattern in continuous/linear data
 * @conf: search configuration
 * @state: search state
 * @data: data to search in
 * @len: length of data
 *
 * A simplified version of textsearch_find() for continuous/linear data.
 * Call textsearch_next() to retrieve subsequent matches.
 *
 * Returns the position of first occurrence of the pattern or
 * %UINT_MAX if no occurrence was found.
 */ 
unsigned int textsearch_find_continuous(struct ts_config *conf,
                                        struct ts_state *state,
                                        const void *data, unsigned int len)
{
        struct ts_linear_state *st = (struct ts_linear_state *) state->cb;

        conf->get_next_block = get_linear_data;
        st->data = data;
        st->len = len;

        return textsearch_find(conf, state);
}

/**
 * textsearch_prepare - Prepare a search
 * @algo: name of search algorithm
 * @pattern: pattern data
 * @len: length of pattern
 * @gfp_mask: allocation mask
 * @flags: search flags
 *
 * Looks up the search algorithm module and creates a new textsearch
 * configuration for the specified pattern. Upon completion all
 * necessary refcnts are held and the configuration must be put back
 * using textsearch_put() after usage.
 *
 * Note: The format of the pattern may not be compatible between
 *       the various search algorithms.
 *
 * Returns a new textsearch configuration according to the specified
 * parameters or a ERR_PTR(). If a zero length pattern is passed, this
 * function returns EINVAL.
 */
struct ts_config *textsearch_prepare(const char *algo, const void *pattern,
                                     unsigned int len, gfp_t gfp_mask, int flags)
{
        int err = -ENOENT;
        struct ts_config *conf;
        struct ts_ops *ops;
        
        if (len == 0)
                return ERR_PTR(-EINVAL);

        ops = lookup_ts_algo(algo);
#ifdef CONFIG_MODULES
        /*
         * Why not always autoload you may ask. Some users are
         * in a situation where requesting a module may deadlock,
         * especially when the module is located on a NFS mount.
         */
        if (ops == NULL && flags & TS_AUTOLOAD) {
                request_module("ts_%s", algo);
                ops = lookup_ts_algo(algo);
        }
#endif

        if (ops == NULL)
                goto errout;

        conf = ops->init(pattern, len, gfp_mask, flags);
        if (IS_ERR(conf)) {
                err = PTR_ERR(conf);
                goto errout;
        }

        conf->ops = ops;
        return conf;

errout:
        if (ops)
                module_put(ops->owner);
                
        return ERR_PTR(err);
}

/**
 * textsearch_destroy - destroy a search configuration
 * @conf: search configuration
 *
 * Releases all references of the configuration and frees
 * up the memory.
 */
void textsearch_destroy(struct ts_config *conf)
{
        if (conf->ops) {
                if (conf->ops->destroy)
                        conf->ops->destroy(conf);
                module_put(conf->ops->owner);
        }

        kfree(conf);
}

EXPORT_SYMBOL(textsearch_register);
EXPORT_SYMBOL(textsearch_unregister);
EXPORT_SYMBOL(textsearch_prepare);
EXPORT_SYMBOL(textsearch_find_continuous);
EXPORT_SYMBOL(textsearch_destroy);