patty/src/ax25.c

#include <stdio.h>
#include <string.h>
#include <errno.h>

#include <patty/ax25.h>

uint16_t fcs_table[256] = {
    0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
    0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
    0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
    0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
    0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
    0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
    0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
    0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
    0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
    0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
    0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
    0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
    0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
    0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
    0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
    0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
    0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
    0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
    0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
    0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
    0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
    0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
    0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
    0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
    0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
    0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
    0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
    0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
    0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
    0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
    0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
    0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
};

static inline uint16_t fcs_compute(uint8_t *data, size_t len) {
    uint16_t fcs = 0xffff;

    while (len--) {
        fcs = (fcs >> 8) ^ fcs_table[(fcs ^ ((uint16_t)*data)) & 0x00ff];
        data++;
    }

    return fcs;
}

static ssize_t frame_decode_address(patty_ax25_address *address, void *data, off_t offset) {
    int i, space = 0;
    uint8_t ssid = (((uint8_t *)data) + offset)[6];

    /*
     * First, unpack each callsign octet, ensuring all are within the 7 bit
     * ASCII space and do not have the extended bit set to 1.
     */
    for (i=0; i<6; i++) {
        uint8_t b = (((uint8_t *)data) + offset)[i];
        uint8_t c = b >> 1;

        if (!PATTY_AX25_ADDRESS_CHAR_VALID(c) || PATTY_AX25_ADDRESS_OCTET_LAST(b)) {
            errno = EINVAL;

            goto error;
        }

        if (c == ' ') {
            /*
             * Take note if we have reached the end of the call sign.
             */
            if (space == 0) {
                space = 1;
            }

            address->callsign[i] = '\0';
        } else {
            /*
             * If we have already reached the end of the callsign, and a
             * non-space character is encountered, then the address is
             * erroneous.
             */
            if (space) {
                errno = EINVAL;

                goto error;
            }

            address->callsign[i] = c;
        }
    }

    /*
     * Now, unpack the data from the SSID field.
     */
    address->repeated = PATTY_AX25_ADDRESS_SSID_REPEATED(ssid);
    address->ssid     = PATTY_AX25_ADDRESS_SSID_NUMBER(ssid);
    address->last     = PATTY_AX25_ADDRESS_OCTET_LAST(ssid);

    return 7;

error:
    return -1;
}

static ssize_t frame_decode_hops(patty_ax25_frame *frame, void *data, off_t start) {
    ssize_t decoded;
      off_t offset = start;

    int i;

    /*
     * Try to decode the AX.25-specified maximum number of hops in the current
     * frame.
     */
    for (i=0; i<PATTY_AX25_MAX_HOPS; i++) {
        if ((decoded = frame_decode_address(&frame->repeaters[i], data, offset)) < 0) {
            goto error;
        } else {
            offset += decoded;
        }

        frame->hops++;

        if (frame->repeaters[i].last) {
            frame->repeated = frame->repeaters[i].repeated;

            break;
        }
    }

    /*
     * If the last hop does not have the address extension bit set, then
     * that's a big problem.
     */
    if (frame->hops && !frame->repeaters[frame->hops-1].last) {
        errno = EINVAL;

        goto error;
    }

    return offset - start;

error:
    return -1;
}

static ssize_t frame_decode_payload(patty_ax25_frame *frame, void *data, off_t offset) {
    uint8_t control = ((uint8_t *)data)[0];
     size_t decoded = 0;

    /*
     * First, decode the control byte.
     */
    frame->control = control;

    decoded++;

    if (PATTY_AX25_CONTROL_INFO(control)) {
        frame->type  = PATTY_AX25_FRAME_INFO;
        frame->proto = ((uint8_t *)data)[1];

        decoded++;
    } else if (PATTY_AX25_CONTROL_SUPER(control)) {
        frame->type = PATTY_AX25_FRAME_SUPER;
    } else if (PATTY_AX25_CONTROL_UNNUMBERED(control)) {
        frame->type = PATTY_AX25_FRAME_UNNUMBERED;

        if (PATTY_AX25_CONTROL_UNNUMBERED_INFO(control)) {
            frame->proto = ((uint8_t *)data)[1];

            decoded++;
        }
    } else {
        frame->type = PATTY_AX25_FRAME_UNKNOWN;

        errno = EINVAL;

        goto error;
    }

    fprintf(stderr, "Decoded %lu bytes\n", decoded);

    return decoded;

error:
    return -1;
}

int patty_ax25_frame_decode(patty_ax25_frame *frame, void *data, size_t len) {
    ssize_t decoded;
      off_t offset = 0;

    memset(frame, '\0', sizeof(*frame));

    if ((decoded = frame_decode_address(&frame->dest, data, offset)) < 0) {
        goto error_decode;
    } else {
        offset += decoded;
    }

    /*
     * It would be considered erroneous if the destination address did have the
     * extension bit set to 1.
     */
    if ((decoded = frame_decode_address(&frame->src, data, offset)) < 0) {
        goto error_decode;
    } else {
        offset += decoded;
    }

    /*
     * Now, if the source address is not the final address in the frame, then
     * begin decoding repeater addresses.
     */
    if (!frame->src.last) {
        if ((decoded = frame_decode_hops(frame, data, offset)) < 0) {
            goto error_decode;
        } else {
            offset += decoded;
        }
    }

    fprintf(stderr, "Decoded %ld/%lu\n", offset, len);

    /*
     * Time to decode the rest of the payload, including control, PID, and
     * Frame Check Sequence.
     */
    if ((decoded = frame_decode_payload(frame, data, offset)) < 0) {
        goto error_decode;
    } else {
        offset += decoded;
    }

    return offset;

error_decode:
    return -1;
}