patty/src/ax25.c

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

#include <patty/ax25.h>

static ssize_t frame_decode_station(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->c    = PATTY_AX25_ADDRESS_SSID_C(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_station(&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_address(patty_ax25_frame *frame, void *data, off_t start) {
      off_t offset = start;
    ssize_t decoded;

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

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

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

    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 + offset)[0];
     size_t decoded = 0;

    int info = 0;

    frame->control = control;

    decoded++;

    if (PATTY_AX25_CONTROL_INFO(control)) {
        frame->type = PATTY_AX25_FRAME_INFO;

        info = 1;
    } 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)) {
            info = 1;
        }
    } else {
        frame->type = PATTY_AX25_FRAME_UNKNOWN;

        errno = EINVAL;

        goto error;
    }

    /*
     * If we've received either an Info or Unnumbered Info frame, then decode
     * the Protocol Identifier field, and calculate the location and size of
     * the payload as well.
     */
    if (info) {
        decoded++;

        frame->proto     = ((uint8_t *)data + offset)[1];
        frame->payload   = (void *)((uint8_t *)data + offset + decoded);
        frame->payloadsz = frame->size - offset - decoded;

        decoded += frame->payloadsz;
    }

    return decoded;

error:
    return -1;
}

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

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

    frame->size = size;

    /*
     * First, decode the variable-length Address field.
     */
    if ((decoded = frame_decode_address(frame, data, offset)) < 0) {
        goto error_decode;
    } else {
        offset += decoded;
    }

    /*
     * Now, decode the remaining Control Field, optional Protocol Identifier
     * field, and payload that may follow.
     */
    if ((decoded = frame_decode_payload(frame, data, offset)) < 0) {
        goto error_decode;
    } else {
        offset += decoded;
    }

    return offset;

error_decode:
    return -1;
}