patty/src/frame.c

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

#include <patty/ax25.h>

static ssize_t decode_station(patty_ax25_addr *addr,
                              void *data,
                              off_t offset) {
    int i, space = 0;

    for (i=0; i<PATTY_AX25_CALLSIGN_LEN; 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 == ' ' && !space) {
            space = 1;
        } else if (c != ' ' && space) {
            errno = EINVAL;

            goto error;
        }
    }

    memcpy(addr, ((uint8_t *)data) + offset, sizeof(*addr));

    return PATTY_AX25_ADDRESS_SIZE;

error:
    return -1;
}

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

    int i;

    patty_ax25_addr *addr = NULL;

    /*
     * Try to count the AX.25-specified maximum number of hops in the current
     * frame.
     */
    for (i=0; i<PATTY_AX25_MAX_HOPS; i++) {
        addr = (patty_ax25_addr *)((uint8_t *)data + offset);

        if ((decoded = decode_station(&frame->repeaters[i], data, offset)) < 0) {
            goto error;
        } else {
            offset += decoded;
        }

        frame->hops++;

        if (PATTY_AX25_ADDRESS_OCTET_LAST(addr->ssid)) {
            break;
        }
    }

    /*
     * If the last hop does not have the address extension bit set, then
     * that's a big problem.
     */
    if (addr && !PATTY_AX25_ADDRESS_OCTET_LAST(addr->ssid)) {
        errno = EINVAL;

        goto error;
    }

    return offset - start;

error:
    return -1;
}

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

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

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

    /*
     * Determine whether the current frame is a Command or Response, and which
     * version of AX.25 this frame adheres to.
     */
    if (PATTY_AX25_ADDRESS_SSID_C(frame->src.ssid)) {
        frame->cr      = PATTY_AX25_FRAME_RESPONSE;
        frame->version = PATTY_AX25_ADDRESS_SSID_C(frame->dest.ssid)?
            PATTY_AX25_2_0: PATTY_AX25_OLD;
    } else {
        frame->cr      = PATTY_AX25_FRAME_COMMAND;
        frame->version = PATTY_AX25_ADDRESS_SSID_C(frame->src.ssid)?
            PATTY_AX25_OLD: PATTY_AX25_2_0;
    }

    /*
     * If the source address is not the final address in the frame, begin
     * decoding repeater addresses.
     */
    if (!PATTY_AX25_ADDRESS_OCTET_LAST(frame->src.ssid)) {
        if ((decoded = decode_hops(frame, data, offset)) < 0) {
            goto error;
        } else {
            offset += decoded;
        }
    }

    return offset - start;

error:
    return -1;
}

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

    int info = 0;

    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;
    }

    frame->control = control;

    decoded++;

    /*
     * 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->info  = (void *)((uint8_t *)data + offset + decoded);
        frame->len   = size - offset - decoded;

        decoded += frame->len;
    }

    return decoded;

error:
    return -1;
}

ssize_t 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));

    /*
     * First, decode the variable-length Address field.
     */
    if ((decoded = decode_address(frame, data, offset)) < 0) {
        errno = EIO;

        goto error_decode;
    } else {
        offset += decoded;
    }

    /*
     * Next, decode the remaining Control Field, optional Protocol Identifier
     * field, and Info payload that may follow.
     */
    if ((decoded = decode_info(frame, data, offset, size)) < 0) {
        errno = EIO;

        goto error_decode;
    } else {
        offset += decoded;
    }

    return offset;

error_decode:
    return -1;
}

ssize_t patty_ax25_frame_info(patty_ax25_frame *frame,
                              void **info) {
    if (frame == NULL || frame->info == NULL || info == NULL) {
        errno = EINVAL;

        goto error_invalid_args;
    }

    *info = frame->info;

    return frame->len;

error_invalid_args:
    return -1;
}