/* Copyright (C) 2007-2019 Open Information Security Foundation

 *

 * You can copy, redistribute or modify this Program under the terms of

 * the GNU General Public License version 2 as published by the Free

 * Software Foundation.

 *

 * This program is distributed in the hope that it will be useful,

 * but WITHOUT ANY WARRANTY; without even the implied warranty of

 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

 * GNU General Public License for more details.

 *

 * You should have received a copy of the GNU General Public License

 * version 2 along with this program; if not, write to the Free Software

 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA

 * 02110-1301, USA.

 */

/**

 * \file

 *

 * \author Victor Julien <victor@inliniac.net>

 * \author Anoop Saldanha <anoopsaldanha@gmail.com>

 *

 * Implements the offset keyword.

 */

#include "suricata-common.h"

#include "decode.h"

#include "detect.h"

#include "detect-parse.h"

#include "detect-content.h"

#include "detect-uricontent.h"

#include "detect-byte.h"

#include "detect-byte-extract.h"

#include "detect-offset.h"

#include "flow-var.h"

#include "util-byte.h"

#include "util-debug.h"

static int DetectOffsetSetup(DetectEngineCtx *, Signature *, const char *);

void DetectOffsetRegister (void)

{

    sigmatch_table[DETECT_OFFSET].name = "offset";

    sigmatch_table[DETECT_OFFSET].desc = "designate from which byte in the payload will be checked to find a match";

    sigmatch_table[DETECT_OFFSET].url = "/rules/payload-keywords.html#offset";

    sigmatch_table[DETECT_OFFSET].Setup = DetectOffsetSetup;

}

int DetectOffsetSetup (DetectEngineCtx *de_ctx, Signature *s, const char *offsetstr)

{

    const char *str = offsetstr;

    /* retrieve the sm to apply the offset against */

    SigMatch *pm = DetectGetLastSMFromLists(s, DETECT_CONTENT, -1);

    if (pm == NULL) {

        SCLogError("offset needs preceding content option.");

        return -1;

    }

    /* verify other conditions */

    DetectContentData *cd = (DetectContentData *)pm->ctx;

    if (cd->flags & DETECT_CONTENT_STARTS_WITH) {

        SCLogError("can't use offset with startswith.");

        return -1;

    }

    if (cd->flags & DETECT_CONTENT_OFFSET) {

        SCLogError("can't use multiple offsets for the same content.");

        return -1;

    }

    if (cd->flags & (DETECT_CONTENT_WITHIN | DETECT_CONTENT_DISTANCE)) {

        SCLogError("can't use a relative "

                   "keyword like within/distance with a absolute "

                   "relative keyword like depth/offset for the same "

                   "content.");

        return -1;

    }

    if (cd->flags & DETECT_CONTENT_NEGATED && cd->flags & DETECT_CONTENT_FAST_PATTERN) {

        SCLogError("can't have a relative "

                   "negated keyword set along with 'fast_pattern'.");

        return -1;

    }

    if (cd->flags & DETECT_CONTENT_FAST_PATTERN_ONLY) {

        SCLogError("can't have a relative "

                   "keyword set along with 'fast_pattern:only;'.");

        return -1;

    }

    if (str[0] != '-' && isalpha((unsigned char)str[0])) {

        DetectByteIndexType index;

        if (!DetectByteRetrieveSMVar(str, s, &index)) {

            SCLogError("unknown byte_ keyword var "

                       "seen in offset - %s.",

                    str);

            return -1;

        }

        cd->offset = index;

        cd->flags |= DETECT_CONTENT_OFFSET_VAR;

    } else {

        if (StringParseUint16(&cd->offset, 0, 0, str) < 0)

        {

            SCLogError("invalid value for offset: %s.", str);

            return -1;

        }

        if (cd->depth != 0) {

            if (cd->depth < cd->content_len) {

                SCLogDebug("depth increased to %"PRIu32" to match pattern len",

                           cd->content_len);

                cd->depth = cd->content_len;

            }

            /* Updating the depth as is relative to the offset */

            cd->depth += cd->offset;

        }

    }

    cd->flags |= DETECT_CONTENT_OFFSET;

    return 0;

}