telnet_server.c

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// SPDX-License-Identifier: GPL-2.0-or-later
 
/***************************************************************************
 *   Copyright (C) 2005 by Dominic Rath                                    *
 *   Dominic.Rath@gmx.de                                                   *
 *                                                                         *
 *   Copyright (C) 2007-2010 Øyvind Harboe                                 *
 *   oyvind.harboe@zylin.com                                               *
 *                                                                         *
 *   Copyright (C) 2008 by Spencer Oliver                                  *
 *   spen@spen-soft.co.uk                                                  *
 ***************************************************************************/
 
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif
 
#include "telnet_server.h"
#include <target/target_request.h>
#include <helper/configuration.h>
#include <helper/list.h>
 
static char *telnet_port;
 
static char *negotiate =
    "\xFF\xFB\x03"          /* IAC WILL Suppress Go Ahead */
    "\xFF\xFB\x01"          /* IAC WILL Echo */
    "\xFF\xFD\x03"          /* IAC DO Suppress Go Ahead */
    "\xFF\xFE\x01";         /* IAC DON'T Echo */
 
#define CTRL(c) (c - '@')
#define TELNET_HISTORY  ".openocd_history"
 
/* The only way we can detect that the socket is closed is the first time
 * we write to it, we will fail. Subsequent write operations will
 * succeed. Shudder!
 */
static int telnet_write(struct connection *connection, const void *data,
    int len)
{
    struct telnet_connection *t_con = connection->priv;
    if (t_con->closed)
        return ERROR_SERVER_REMOTE_CLOSED;
 
    if (connection_write(connection, data, len) == len)
        return ERROR_OK;
    t_con->closed = true;
    return ERROR_SERVER_REMOTE_CLOSED;
}
 
/* output an audible bell */
static int telnet_bell(struct connection *connection)
{
    /* ("\a" does not work, at least on windows) */
    return telnet_write(connection, "\x07", 1);
}
 
static int telnet_prompt(struct connection *connection)
{
    struct telnet_connection *t_con = connection->priv;
 
    return telnet_write(connection, t_con->prompt, strlen(t_con->prompt));
}
 
static int telnet_outputline(struct connection *connection, const char *line)
{
    int len;
 
    /* process lines in buffer */
    while (*line) {
        char *line_end = strchr(line, '\n');
 
        if (line_end)
            len = line_end-line;
        else
            len = strlen(line);
 
        telnet_write(connection, line, len);
        if (line_end) {
            telnet_write(connection, "\r\n", 2);
            line += len + 1;
        } else
            line += len;
    }
 
    return ERROR_OK;
}
 
static int telnet_output(struct command_context *cmd_ctx, const char *line)
{
    struct connection *connection = cmd_ctx->output_handler_priv;
 
    return telnet_outputline(connection, line);
}
 
static void telnet_log_callback(void *priv, const char *file, unsigned int line,
    const char *function, const char *string)
{
    struct connection *connection = priv;
    struct telnet_connection *t_con = connection->priv;
    size_t i;
    size_t tmp;
 
    /* If the prompt is not visible, simply output the message. */
    if (!t_con->prompt_visible) {
        telnet_outputline(connection, string);
        return;
    }
 
    /* Clear the command line. */
    tmp = strlen(t_con->prompt) + t_con->line_size;
 
    for (i = 0; i < tmp; i += 16)
        telnet_write(connection, "\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b",
            MIN(tmp - i, 16));
 
    for (i = 0; i < tmp; i += 16)
        telnet_write(connection, "                ", MIN(tmp - i, 16));
 
    for (i = 0; i < tmp; i += 16)
        telnet_write(connection, "\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b",
            MIN(tmp - i, 16));
 
    telnet_outputline(connection, string);
 
    /* Put the command line to its previous state. */
    telnet_prompt(connection);
    telnet_write(connection, t_con->line, t_con->line_size);
 
    for (i = t_con->line_cursor; i < t_con->line_size; i++)
        telnet_write(connection, "\b", 1);
}
 
static void telnet_load_history(struct telnet_connection *t_con)
{
    FILE *histfp;
    char buffer[TELNET_BUFFER_SIZE];
    int i = 0;
 
    char *history = get_home_dir(TELNET_HISTORY);
 
    if (!history) {
        LOG_INFO("unable to get user home directory, telnet history will be disabled");
        return;
    }
 
    histfp = fopen(history, "rb");
 
    if (histfp) {
 
        while (fgets(buffer, sizeof(buffer), histfp)) {
 
            char *p = strchr(buffer, '\n');
            if (p)
                *p = '\0';
            if (buffer[0] && i < TELNET_LINE_HISTORY_SIZE)
                t_con->history[i++] = strdup(buffer);
        }
 
        t_con->next_history = i;
        t_con->next_history %= TELNET_LINE_HISTORY_SIZE;
        /* try to set to last entry - 1, that way we skip over any exit/shutdown cmds */
        t_con->current_history = t_con->next_history > 0 ? i - 1 : 0;
        fclose(histfp);
    }
 
    free(history);
}
 
static void telnet_save_history(struct telnet_connection *t_con)
{
    FILE *histfp;
    int i;
    int num;
 
    char *history = get_home_dir(TELNET_HISTORY);
 
    if (!history) {
        LOG_INFO("unable to get user home directory, telnet history will be disabled");
        return;
    }
 
    histfp = fopen(history, "wb");
 
    if (histfp) {
 
        num = TELNET_LINE_HISTORY_SIZE;
        i = t_con->current_history + 1;
        i %= TELNET_LINE_HISTORY_SIZE;
 
        while (!t_con->history[i] && num > 0) {
            i++;
            i %= TELNET_LINE_HISTORY_SIZE;
            num--;
        }
 
        if (num > 0) {
            for (; num > 0; num--) {
                fprintf(histfp, "%s\n", t_con->history[i]);
                i++;
                i %= TELNET_LINE_HISTORY_SIZE;
            }
        }
        fclose(histfp);
    }
 
    free(history);
}
 
static int telnet_new_connection(struct connection *connection)
{
    struct telnet_connection *telnet_connection;
    struct telnet_service *telnet_service = connection->service->priv;
 
    telnet_connection = calloc(1, sizeof(struct telnet_connection));
 
    if (!telnet_connection) {
        LOG_ERROR("Failed to allocate telnet connection.");
        return ERROR_FAIL;
    }
 
    connection->priv = telnet_connection;
 
    /* initialize telnet connection information */
    telnet_connection->prompt = strdup("> ");
    telnet_connection->prompt_visible = true;
    telnet_connection->state = TELNET_STATE_DATA;
 
    /* output goes through telnet connection */
    command_set_output_handler(connection->cmd_ctx, telnet_output, connection);
 
    /* negotiate telnet options */
    telnet_write(connection, negotiate, strlen(negotiate));
 
    /* print connection banner */
    if (telnet_service->banner) {
        telnet_write(connection, telnet_service->banner, strlen(telnet_service->banner));
        telnet_write(connection, "\r\n", 2);
    }
 
    /* the prompt is always placed at the line beginning */
    telnet_write(connection, "\r", 1);
    telnet_prompt(connection);
 
    telnet_load_history(telnet_connection);
 
    log_add_callback(telnet_log_callback, connection);
 
    return ERROR_OK;
}
 
static void telnet_clear_line(struct connection *connection,
    struct telnet_connection *t_con)
{
    /* move to end of line */
    if (t_con->line_cursor < t_con->line_size)
        telnet_write(connection,
            t_con->line + t_con->line_cursor,
            t_con->line_size - t_con->line_cursor);
 
    /* backspace, overwrite with space, backspace */
    while (t_con->line_size > 0) {
        telnet_write(connection, "\b \b", 3);
        t_con->line_size--;
    }
    t_con->line_cursor = 0;
}
 
static void telnet_history_go(struct connection *connection, int idx)
{
    struct telnet_connection *t_con = connection->priv;
 
    if (t_con->history[idx]) {
        telnet_clear_line(connection, t_con);
        t_con->line_size = strlen(t_con->history[idx]);
        t_con->line_cursor = t_con->line_size;
        memcpy(t_con->line, t_con->history[idx], t_con->line_size);
        telnet_write(connection, t_con->line, t_con->line_size);
        t_con->current_history = idx;
    }
    t_con->state = TELNET_STATE_DATA;
}
 
static void telnet_history_up(struct connection *connection)
{
    struct telnet_connection *t_con = connection->priv;
 
    size_t last_history = (t_con->current_history > 0) ?
                t_con->current_history - 1 :
                TELNET_LINE_HISTORY_SIZE-1;
    telnet_history_go(connection, last_history);
}
 
static void telnet_history_down(struct connection *connection)
{
    struct telnet_connection *t_con = connection->priv;
    size_t next_history;
 
    next_history = (t_con->current_history + 1) % TELNET_LINE_HISTORY_SIZE;
    telnet_history_go(connection, next_history);
}
 
static void telnet_history_add(struct connection *connection)
{
    struct telnet_connection *t_con = connection->priv;
 
    /* save only non-blank not repeating lines in the history */
    char *prev_line = t_con->history[(t_con->current_history > 0) ?
            t_con->current_history - 1 : TELNET_LINE_HISTORY_SIZE-1];
 
    if (*t_con->line && (!prev_line || strcmp(t_con->line, prev_line))) {
        /* if the history slot is already taken, free it */
        free(t_con->history[t_con->next_history]);
 
        /* add line to history */
        t_con->history[t_con->next_history] = strdup(t_con->line);
 
        /* wrap history at TELNET_LINE_HISTORY_SIZE */
        t_con->next_history = (t_con->next_history + 1) % TELNET_LINE_HISTORY_SIZE;
 
        /* current history line starts at the new entry */
        t_con->current_history = t_con->next_history;
 
        free(t_con->history[t_con->current_history]);
        t_con->history[t_con->current_history] = strdup("");
    }
}
 
static int telnet_history_print(struct connection *connection)
{
    struct telnet_connection *tc;
 
    tc = connection->priv;
 
    for (size_t i = 1; i < TELNET_LINE_HISTORY_SIZE; i++) {
        char *line;
 
        /*
         * The tc->next_history line contains empty string (unless NULL), thus
         * it is not printed.
         */
        line = tc->history[(tc->next_history + i) % TELNET_LINE_HISTORY_SIZE];
 
        if (line) {
            telnet_write(connection, line, strlen(line));
            telnet_write(connection, "\r\n\x00", 3);
        }
    }
 
    tc->line_size = 0;
    tc->line_cursor = 0;
 
    /* The prompt is always placed at the line beginning. */
    telnet_write(connection, "\r", 1);
 
    return telnet_prompt(connection);
}
 
static void telnet_move_cursor(struct connection *connection, size_t pos)
{
    struct telnet_connection *tc = connection->priv;
    size_t tmp;
 
    if (pos == tc->line_cursor) /* nothing to do */
        return;
 
    if (pos > tc->line_size) /* out of bounds */
        return;
 
    if (pos < tc->line_cursor) {
        tmp = tc->line_cursor - pos;
 
        for (size_t i = 0; i < tmp; i += 16)
            telnet_write(connection, "\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b",
                MIN(tmp - i, 16));
    } else {
        tmp = pos - tc->line_cursor;
 
        for (size_t i = 0; i < tmp; i += 16)
            telnet_write(connection, tc->line + tc->line_cursor + i,
                MIN(tmp - i, 16));
    }
 
    tc->line_cursor = pos;
}
 
/* check buffer size leaving one spare character for string null termination */
static inline bool telnet_can_insert(struct connection *connection, size_t len)
{
    struct telnet_connection *t_con = connection->priv;
 
    return t_con->line_size + len < TELNET_LINE_MAX_SIZE;
}
 
/* write to telnet console, and update the telnet_connection members
 * this function is capable of inserting in the middle of a line
 * please ensure that data does not contain special characters (\n, \r, \t, \b ...)
 *
 * returns false when it fails to insert the requested data
 */
static bool telnet_insert(struct connection *connection, const void *data, size_t len)
{
    struct telnet_connection *t_con = connection->priv;
 
    if (!telnet_can_insert(connection, len)) {
        telnet_bell(connection);
        return false;
    }
 
    if (t_con->line_cursor < t_con->line_size) {
        /* we have some content after the cursor */
        memmove(t_con->line + t_con->line_cursor + len,
                t_con->line + t_con->line_cursor,
                t_con->line_size - t_con->line_cursor);
    }
 
    strncpy(t_con->line + t_con->line_cursor, data, len);
 
    telnet_write(connection,
            t_con->line + t_con->line_cursor,
            t_con->line_size + len - t_con->line_cursor);
 
    t_con->line_size += len;
    t_con->line_cursor += len;
 
    for (size_t i = t_con->line_cursor; i < t_con->line_size; i++)
        telnet_write(connection, "\b", 1);
 
    return true;
}
 
static void telnet_delete_character(struct connection *connection)
{
    struct telnet_connection *t_con = connection->priv;
 
    if (t_con->line_cursor == 0)
        return;
 
    if (t_con->line_cursor != t_con->line_size) {
        size_t i;
        telnet_write(connection, "\b", 1);
        t_con->line_cursor--;
        t_con->line_size--;
        memmove(t_con->line + t_con->line_cursor,
                t_con->line + t_con->line_cursor + 1,
                t_con->line_size -
                t_con->line_cursor);
 
        telnet_write(connection,
                t_con->line + t_con->line_cursor,
                t_con->line_size -
                t_con->line_cursor);
        telnet_write(connection, " \b", 2);
        for (i = t_con->line_cursor; i < t_con->line_size; i++)
            telnet_write(connection, "\b", 1);
    } else {
        t_con->line_size--;
        t_con->line_cursor--;
        /* back space: move the 'printer' head one char
         * back, overwrite with space, move back again */
        telnet_write(connection, "\b \b", 3);
    }
}
 
static void telnet_remove_character(struct connection *connection)
{
    struct telnet_connection *t_con = connection->priv;
 
    if (t_con->line_cursor < t_con->line_size) {
        size_t i;
        t_con->line_size--;
        /* remove char from line buffer */
        memmove(t_con->line + t_con->line_cursor,
                t_con->line + t_con->line_cursor + 1,
                t_con->line_size - t_con->line_cursor);
 
        /* print remainder of buffer */
        telnet_write(connection, t_con->line + t_con->line_cursor,
                t_con->line_size - t_con->line_cursor);
        /* overwrite last char with whitespace */
        telnet_write(connection, " \b", 2);
 
        /* move back to cursor position*/
        for (i = t_con->line_cursor; i < t_con->line_size; i++)
            telnet_write(connection, "\b", 1);
    }
}
 
static int telnet_exec_line(struct connection *connection)
{
    struct telnet_connection *t_con = connection->priv;
    struct command_context *command_context = connection->cmd_ctx;
    int retval;
 
    telnet_write(connection, "\r\n\x00", 3);
 
    if (strcmp(t_con->line, "history") == 0) {
        retval = telnet_history_print(connection);
 
        if (retval != ERROR_OK)
            return retval;
 
        return ERROR_OK;
    }
 
    telnet_history_add(connection);
 
    t_con->line_size = 0;
 
    /* to suppress prompt in log callback during command execution */
    t_con->prompt_visible = false;
 
    if (strcmp(t_con->line, "shutdown") == 0)
        telnet_save_history(t_con);
 
    retval = command_run_line(command_context, t_con->line);
 
    t_con->line_cursor = 0;
    t_con->prompt_visible = true;
 
    if (retval == ERROR_COMMAND_CLOSE_CONNECTION)
        /* "shutdown" or "exit" executed. */
        return ERROR_SERVER_REMOTE_CLOSED;
 
    /* the prompt is always placed at the line beginning */
    telnet_write(connection, "\r", 1);
 
    retval = telnet_prompt(connection);
    if (retval == ERROR_SERVER_REMOTE_CLOSED)
        return ERROR_SERVER_REMOTE_CLOSED;
 
    return ERROR_OK;
}
 
static void telnet_cut_line_to_end(struct connection *connection)
{
    struct telnet_connection *t_con = connection->priv;
 
    /* FIXME: currently this function does not save to clipboard */
 
    if (t_con->line_cursor < t_con->line_size) {
        /* overwrite with space, until end of line, move back */
        for (size_t i = t_con->line_cursor; i < t_con->line_size; i++)
            telnet_write(connection, " ", 1);
        for (size_t i = t_con->line_cursor; i < t_con->line_size; i++)
            telnet_write(connection, "\b", 1);
        t_con->line[t_con->line_cursor] = '\0';
        t_con->line_size = t_con->line_cursor;
    }
}
 
static void telnet_interrupt(struct connection *connection)
{
    struct telnet_connection *t_con = connection->priv;
 
    /* print '^C' at line end, and display a new command prompt */
    telnet_move_cursor(connection, t_con->line_size);
    telnet_write(connection, "^C\n\r", 4);
    t_con->line_cursor = 0;
    t_con->line_size = 0;
    telnet_prompt(connection);
}
 
static void telnet_auto_complete(struct connection *connection)
{
    struct telnet_connection *t_con = connection->priv;
    struct command_context *command_context = connection->cmd_ctx;
 
    struct cmd_match {
        char *cmd;
        struct list_head lh;
    };
 
    OOCD_LIST_HEAD(matches);
 
    /* - user command sequence, either at line beginning
     *   or we start over after these characters ';', '[', '{'
     * - user variable sequence, start after the character '$'
     *   and do not contain white spaces */
    bool is_variable_auto_completion = false;
    bool have_spaces = false;
    size_t seq_start = (t_con->line_cursor == 0) ? 0 : (t_con->line_cursor - 1);
    while (1) {
        char c = t_con->line[seq_start];
 
        if (c == ';' || c == '[' || c == '{') {
            seq_start++;
            break;
        } else if (c == ' ') {
            have_spaces = true;
        } else if (c == '$' && !have_spaces) {
            is_variable_auto_completion = true;
            seq_start++;
            break;
        }
 
        if (seq_start == 0)
            break;
 
        seq_start--;
    }
 
    /* user command position in the line, ignore leading spaces */
    size_t usr_cmd_pos = seq_start;
    while ((usr_cmd_pos < t_con->line_cursor) &&
            isspace((unsigned char)t_con->line[usr_cmd_pos]))
        usr_cmd_pos++;
 
    /* check user command length */
    if (t_con->line_cursor < usr_cmd_pos) {
        telnet_bell(connection);
        return;
    }
    size_t usr_cmd_len = t_con->line_cursor - usr_cmd_pos;
 
    /* optimize multiple spaces in the user command,
     * because info commands does not tolerate multiple spaces */
    size_t optimized_spaces = 0;
    char query[usr_cmd_len + 1];
 
    for (size_t i = 0; i < usr_cmd_len; i++) {
        if ((i < usr_cmd_len - 1) && isspace((unsigned char)t_con->line[usr_cmd_pos + i])
                && isspace((unsigned char)t_con->line[usr_cmd_pos + i + 1])) {
            optimized_spaces++;
            continue;
        }
 
        query[i - optimized_spaces] = t_con->line[usr_cmd_pos + i];
    }
 
    usr_cmd_len -= optimized_spaces;
    query[usr_cmd_len] = '\0';
 
    /* filter commands */
    char *query_cmd;
 
    if (is_variable_auto_completion)
        query_cmd = alloc_printf("lsort [info vars {%s*}]", query);
    else
        query_cmd = alloc_printf("_telnet_autocomplete_helper {%s*}", query);
 
    if (!query_cmd) {
        LOG_ERROR("Out of memory");
        return;
    }
 
    int retval = Jim_EvalSource(command_context->interp, __FILE__, __LINE__, query_cmd);
    free(query_cmd);
    if (retval != JIM_OK)
        return;
 
    Jim_Obj *list = Jim_GetResult(command_context->interp);
    Jim_IncrRefCount(list);
 
    /* common prefix length of the matched commands */
    size_t common_len = 0;
    char *first_match = NULL; /* used to compute the common prefix length */
 
    int len = Jim_ListLength(command_context->interp, list);
    for (int i = 0; i < len; i++) {
        Jim_Obj *elem = Jim_ListGetIndex(command_context->interp, list, i);
        Jim_IncrRefCount(elem);
 
        char *name = (char *)Jim_GetString(elem, NULL);
 
        /* validate the command */
        bool ignore_cmd = false;
        if (!is_variable_auto_completion) {
            Jim_Cmd *jim_cmd = Jim_GetCommand(command_context->interp, elem, JIM_NONE);
 
            if (!jim_cmd) {
                /* Why we are here? Let's ignore it! */
                ignore_cmd = true;
            } else if (jimcmd_is_oocd_command(jim_cmd)) {
                struct command *cmd = jimcmd_privdata(jim_cmd);
 
                if (cmd && !cmd->handler) {
                    /* Initial part of a multi-word command. Ignore it! */
                    ignore_cmd = true;
                } else if (cmd && cmd->mode == COMMAND_CONFIG) {
                    /* Not executable after config phase. Ignore it! */
                    ignore_cmd = true;
                }
            }
        }
 
        /* save the command in the prediction list */
        if (!ignore_cmd) {
            struct cmd_match *match = calloc(1, sizeof(struct cmd_match));
            if (!match) {
                LOG_ERROR("Out of memory");
                Jim_DecrRefCount(command_context->interp, elem);
                break; /* break the for loop */
            }
 
            if (list_empty(&matches)) {
                common_len = strlen(name);
                first_match = name;
            } else {
                size_t new_common_len = usr_cmd_len; /* save some loops */
 
                while (new_common_len < common_len && first_match[new_common_len] == name[new_common_len])
                    new_common_len++;
 
                common_len = new_common_len;
            }
 
            match->cmd = name;
            list_add_tail(&match->lh, &matches);
        }
 
        Jim_DecrRefCount(command_context->interp, elem);
    }
    /* end of command filtering */
 
    /* proceed with auto-completion */
    if (list_empty(&matches))
        telnet_bell(connection);
    else if (common_len == usr_cmd_len && list_is_singular(&matches) && t_con->line_cursor == t_con->line_size)
        telnet_insert(connection, " ", 1);
    else if (common_len > usr_cmd_len) {
        int completion_size = common_len - usr_cmd_len;
        if (telnet_insert(connection, first_match + usr_cmd_len, completion_size)) {
            /* in bash this extra space is only added when the cursor in at the end of line */
            if (list_is_singular(&matches) && t_con->line_cursor == t_con->line_size)
                telnet_insert(connection, " ", 1);
        }
    } else if (!list_is_singular(&matches)) {
        telnet_write(connection, "\n\r", 2);
 
        struct cmd_match *match;
        list_for_each_entry(match, &matches, lh) {
            telnet_write(connection, match->cmd, strlen(match->cmd));
            telnet_write(connection, "\n\r", 2);
        }
 
        telnet_prompt(connection);
        telnet_write(connection, t_con->line, t_con->line_size);
 
        /* restore the terminal visible cursor location */
        for (size_t i = t_con->line_cursor; i < t_con->line_size; i++)
            telnet_write(connection, "\b", 1);
    }
 
    /* destroy the command_list */
    struct cmd_match *tmp, *match;
    list_for_each_entry_safe(match, tmp, &matches, lh)
        free(match);
 
    Jim_DecrRefCount(command_context->interp, list);
}
 
static int telnet_input(struct connection *connection)
{
    int bytes_read;
    unsigned char buffer[TELNET_BUFFER_SIZE];
    unsigned char *buf_p;
    struct telnet_connection *t_con = connection->priv;
 
    bytes_read = connection_read(connection, buffer, TELNET_BUFFER_SIZE);
 
    if (bytes_read == 0)
        return ERROR_SERVER_REMOTE_CLOSED;
    else if (bytes_read == -1) {
        LOG_ERROR("error during read: %s", strerror(errno));
        return ERROR_SERVER_REMOTE_CLOSED;
    }
 
    buf_p = buffer;
    while (bytes_read) {
        switch (t_con->state) {
            case TELNET_STATE_DATA:
                if (*buf_p == 0xff) {
                    t_con->state = TELNET_STATE_IAC;
                } else {
                    if (isprint(*buf_p)) {  /* printable character */
                        telnet_insert(connection, buf_p, 1);
                    } else { /* non-printable */
                        if (*buf_p == 0x1b) {   /* escape */
                            t_con->state = TELNET_STATE_ESCAPE;
                            t_con->last_escape = '\x00';
                        } else if ((*buf_p == 0xd) || (*buf_p == 0xa)) { /* CR/LF */
                            int retval;
 
                            /* skip over combinations with CR/LF and NUL characters */
                            if ((bytes_read > 1) && ((*(buf_p + 1) == 0xa) ||
                                    (*(buf_p + 1) == 0xd))) {
                                buf_p++;
                                bytes_read--;
                            }
                            if ((bytes_read > 1) && (*(buf_p + 1) == 0)) {
                                buf_p++;
                                bytes_read--;
                            }
                            t_con->line[t_con->line_size] = 0;
 
                            retval = telnet_exec_line(connection);
                            if (retval != ERROR_OK)
                                return retval;
                        } else if ((*buf_p == 0x7f) || (*buf_p == 0x8)) {   /* delete character */
                            telnet_delete_character(connection);
                        } else if (*buf_p == 0x15) {    /* clear line */
                            telnet_clear_line(connection, t_con);
                        } else if (*buf_p == CTRL('B')) {   /* cursor left */
                            telnet_move_cursor(connection, t_con->line_cursor - 1);
                            t_con->state = TELNET_STATE_DATA;
                        } else if (*buf_p == CTRL('C')) {   /* interrupt */
                            telnet_interrupt(connection);
                        } else if (*buf_p == CTRL('F')) {   /* cursor right */
                            telnet_move_cursor(connection, t_con->line_cursor + 1);
                            t_con->state = TELNET_STATE_DATA;
                        } else if (*buf_p == CTRL('P')) {   /* cursor up */
                            telnet_history_up(connection);
                        } else if (*buf_p == CTRL('N')) {   /* cursor down */
                            telnet_history_down(connection);
                        } else if (*buf_p == CTRL('A')) {   /* move the cursor to the beginning of the line */
                            telnet_move_cursor(connection, 0);
                        } else if (*buf_p == CTRL('E')) {   /* move the cursor to the end of the line */
                            telnet_move_cursor(connection, t_con->line_size);
                        } else if (*buf_p == CTRL('K')) {   /* kill line to end */
                            telnet_cut_line_to_end(connection);
                        } else if (*buf_p == '\t') {
                            telnet_auto_complete(connection);
                        } else {
                            LOG_DEBUG("unhandled nonprintable: %2.2x", *buf_p);
                        }
                    }
                }
                break;
            case TELNET_STATE_IAC:
                switch (*buf_p) {
                case 0xfe:
                    t_con->state = TELNET_STATE_DONT;
                    break;
                case 0xfd:
                    t_con->state = TELNET_STATE_DO;
                    break;
                case 0xfc:
                    t_con->state = TELNET_STATE_WONT;
                    break;
                case 0xfb:
                    t_con->state = TELNET_STATE_WILL;
                    break;
                }
                break;
            case TELNET_STATE_SB:
                break;
            case TELNET_STATE_SE:
                break;
            case TELNET_STATE_WILL:
            case TELNET_STATE_WONT:
            case TELNET_STATE_DO:
            case TELNET_STATE_DONT:
                t_con->state = TELNET_STATE_DATA;
                break;
            case TELNET_STATE_ESCAPE:
                if (t_con->last_escape == '[') {
                    if (*buf_p == 'D') {    /* cursor left */
                        telnet_move_cursor(connection, t_con->line_cursor - 1);
                        t_con->state = TELNET_STATE_DATA;
                    } else if (*buf_p == 'C') { /* cursor right */
                        telnet_move_cursor(connection, t_con->line_cursor + 1);
                        t_con->state = TELNET_STATE_DATA;
                    } else if (*buf_p == 'A') { /* cursor up */
                        telnet_history_up(connection);
                    } else if (*buf_p == 'B') { /* cursor down */
                        telnet_history_down(connection);
                    } else if (*buf_p == 'F') { /* end key */
                        telnet_move_cursor(connection, t_con->line_size);
                        t_con->state = TELNET_STATE_DATA;
                    } else if (*buf_p == 'H') { /* home key */
                        telnet_move_cursor(connection, 0);
                        t_con->state = TELNET_STATE_DATA;
                    } else if (*buf_p == '3') {
                        t_con->last_escape = *buf_p;
                    } else {
                        t_con->state = TELNET_STATE_DATA;
                    }
                } else if (t_con->last_escape == '3') {
                    /* Remove character */
                    if (*buf_p == '~') {
                        telnet_remove_character(connection);
                        t_con->state = TELNET_STATE_DATA;
                    } else
                        t_con->state = TELNET_STATE_DATA;
                } else if (t_con->last_escape == '\x00') {
                    if (*buf_p == '[')
                        t_con->last_escape = *buf_p;
                    else
                        t_con->state = TELNET_STATE_DATA;
                } else {
                    LOG_ERROR("BUG: unexpected value in t_con->last_escape");
                    t_con->state = TELNET_STATE_DATA;
                }
 
                break;
            default:
                LOG_ERROR("unknown telnet state");
                return ERROR_FAIL;
        }
 
        bytes_read--;
        buf_p++;
    }
 
    return ERROR_OK;
}
 
static int telnet_connection_closed(struct connection *connection)
{
    struct telnet_connection *t_con = connection->priv;
    int i;
 
    log_remove_callback(telnet_log_callback, connection);
 
    free(t_con->prompt);
    t_con->prompt = NULL;
 
    /* save telnet history */
    telnet_save_history(t_con);
 
    for (i = 0; i < TELNET_LINE_HISTORY_SIZE; i++) {
        free(t_con->history[i]);
        t_con->history[i] = NULL;
    }
 
    /* if this connection registered a debug-message receiver delete it */
    delete_debug_msg_receiver(connection->cmd_ctx, NULL);
 
    free(connection->priv);
    connection->priv = NULL;
 
    return ERROR_OK;
}
 
static const struct service_driver telnet_service_driver = {
    .name = "telnet",
    .new_connection_during_keep_alive_handler = NULL,
    .new_connection_handler = telnet_new_connection,
    .input_handler = telnet_input,
    .connection_closed_handler = telnet_connection_closed,
    .keep_client_alive_handler = NULL,
};
 
int telnet_init(char *banner)
{
    if (strcmp(telnet_port, "disabled") == 0) {
        LOG_INFO("telnet server disabled");
        return ERROR_OK;
    }
 
    struct telnet_service *telnet_service =
        malloc(sizeof(struct telnet_service));
 
    if (!telnet_service) {
        LOG_ERROR("Failed to allocate telnet service.");
        return ERROR_FAIL;
    }
 
    telnet_service->banner = banner;
 
    int ret = add_service(&telnet_service_driver, telnet_port, CONNECTION_LIMIT_UNLIMITED,
        telnet_service);
 
    if (ret != ERROR_OK) {
        free(telnet_service);
        return ret;
    }
 
    return ERROR_OK;
}
 
bool telnet_is_from_telnet_session(struct command_context *cmd_ctx)
{
    return cmd_ctx->output_handler == telnet_output;
}
 
COMMAND_HANDLER(handle_telnet_port_command)
{
    return CALL_COMMAND_HANDLER(server_pipe_command, &telnet_port);
}
 
static const struct command_registration telnet_subcommand_handlers[] = {
    {
        .name = "port",
        .handler = handle_telnet_port_command,
        .mode = COMMAND_CONFIG,
        .help = "Specify port on which to listen "
            "for incoming telnet connections.  "
            "Read help on 'gdb port'.",
        .usage = "[port_num]",
    },
    COMMAND_REGISTRATION_DONE
};
 
static const struct command_registration telnet_command_handlers[] = {
    {
        .name = "telnet",
        .chain = telnet_subcommand_handlers,
        .mode = COMMAND_CONFIG,
        .help = "telnet commands",
        .usage = "",
    },
    COMMAND_REGISTRATION_DONE
};
 
int telnet_register_commands(struct command_context *cmd_ctx)
{
    telnet_port = strdup("4444");
    return register_commands(cmd_ctx, NULL, telnet_command_handlers);
}
 
void telnet_service_free(void)
{
    free(telnet_port);
}