putty-source/ssh/sesschan.c

/*
 * Implement the "session" channel type for the SSH server.
 */

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

#include "putty.h"
#include "ssh.h"
#include "channel.h"
#include "server.h"
#include "sftp.h"

struct agentfwd {
    ConnectionLayer *cl;
    Socket *socket;
    Plug plug;
};

typedef struct sesschan {
    SshChannel *c;

    LogContext *parent_logctx, *child_logctx;
    Conf *conf;
    const SftpServerVtable *sftpserver_vt;

    LogPolicy logpolicy;
    Seat seat;

    bool want_pty;
    struct ssh_ttymodes ttymodes;
    int wc, hc, wp, hp;
    strbuf *termtype;

    bool ignoring_input;
    bool seen_eof, seen_exit;

    Plug xfwd_plug;
    int n_x11_sockets;
    Socket *x11_sockets[MAX_X11_SOCKETS];

    agentfwd *agent;

    Backend *backend;

    bufchain subsys_input;
    SftpServer *sftpsrv;
    ScpServer *scpsrv;
    const SshServerConfig *ssc;

    Channel chan;
} sesschan;

static void sesschan_free(Channel *chan);
static size_t sesschan_send(
    Channel *chan, bool is_stderr, const void *, size_t);
static void sesschan_send_eof(Channel *chan);
static char *sesschan_log_close_msg(Channel *chan);
static bool sesschan_want_close(Channel *, bool, bool);
static void sesschan_set_input_wanted(Channel *chan, bool wanted);
static bool sesschan_run_shell(Channel *chan);
static bool sesschan_run_command(Channel *chan, ptrlen command);
static bool sesschan_run_subsystem(Channel *chan, ptrlen subsys);
static bool sesschan_enable_x11_forwarding(
    Channel *chan, bool oneshot, ptrlen authproto, ptrlen authdata,
    unsigned screen_number);
static bool sesschan_enable_agent_forwarding(Channel *chan);
static bool sesschan_allocate_pty(
    Channel *chan, ptrlen termtype, unsigned width, unsigned height,
    unsigned pixwidth, unsigned pixheight, struct ssh_ttymodes modes);
static bool sesschan_set_env(Channel *chan, ptrlen var, ptrlen value);
static bool sesschan_send_break(Channel *chan, unsigned length);
static bool sesschan_send_signal(Channel *chan, ptrlen signame);
static bool sesschan_change_window_size(
    Channel *chan, unsigned width, unsigned height,
    unsigned pixwidth, unsigned pixheight);

static const ChannelVtable sesschan_channelvt = {
    .free = sesschan_free,
    .open_confirmation = chan_remotely_opened_confirmation,
    .open_failed = chan_remotely_opened_failure,
    .send = sesschan_send,
    .send_eof = sesschan_send_eof,
    .set_input_wanted = sesschan_set_input_wanted,
    .log_close_msg = sesschan_log_close_msg,
    .want_close = sesschan_want_close,
    .rcvd_exit_status = chan_no_exit_status,
    .rcvd_exit_signal = chan_no_exit_signal,
    .rcvd_exit_signal_numeric = chan_no_exit_signal_numeric,
    .run_shell = sesschan_run_shell,
    .run_command = sesschan_run_command,
    .run_subsystem = sesschan_run_subsystem,
    .enable_x11_forwarding = sesschan_enable_x11_forwarding,
    .enable_agent_forwarding = sesschan_enable_agent_forwarding,
    .allocate_pty = sesschan_allocate_pty,
    .set_env = sesschan_set_env,
    .send_break = sesschan_send_break,
    .send_signal = sesschan_send_signal,
    .change_window_size = sesschan_change_window_size,
    .request_response = chan_no_request_response,
};

static size_t sftp_chan_send(
    Channel *chan, bool is_stderr, const void *, size_t);
static void sftp_chan_send_eof(Channel *chan);
static char *sftp_log_close_msg(Channel *chan);

static const ChannelVtable sftp_channelvt = {
    .free = sesschan_free,
    .open_confirmation = chan_remotely_opened_confirmation,
    .open_failed = chan_remotely_opened_failure,
    .send = sftp_chan_send,
    .send_eof = sftp_chan_send_eof,
    .set_input_wanted = sesschan_set_input_wanted,
    .log_close_msg = sftp_log_close_msg,
    .want_close = chan_default_want_close,
    .rcvd_exit_status = chan_no_exit_status,
    .rcvd_exit_signal = chan_no_exit_signal,
    .rcvd_exit_signal_numeric = chan_no_exit_signal_numeric,
    .run_shell = chan_no_run_shell,
    .run_command = chan_no_run_command,
    .run_subsystem = chan_no_run_subsystem,
    .enable_x11_forwarding = chan_no_enable_x11_forwarding,
    .enable_agent_forwarding = chan_no_enable_agent_forwarding,
    .allocate_pty = chan_no_allocate_pty,
    .set_env = chan_no_set_env,
    .send_break = chan_no_send_break,
    .send_signal = chan_no_send_signal,
    .change_window_size = chan_no_change_window_size,
    .request_response = chan_no_request_response,
};

static size_t scp_chan_send(
    Channel *chan, bool is_stderr, const void *, size_t);
static void scp_chan_send_eof(Channel *chan);
static void scp_set_input_wanted(Channel *chan, bool wanted);
static char *scp_log_close_msg(Channel *chan);

static const ChannelVtable scp_channelvt = {
    .free = sesschan_free,
    .open_confirmation = chan_remotely_opened_confirmation,
    .open_failed = chan_remotely_opened_failure,
    .send = scp_chan_send,
    .send_eof = scp_chan_send_eof,
    .set_input_wanted = scp_set_input_wanted,
    .log_close_msg = scp_log_close_msg,
    .want_close = chan_default_want_close,
    .rcvd_exit_status = chan_no_exit_status,
    .rcvd_exit_signal = chan_no_exit_signal,
    .rcvd_exit_signal_numeric = chan_no_exit_signal_numeric,
    .run_shell = chan_no_run_shell,
    .run_command = chan_no_run_command,
    .run_subsystem = chan_no_run_subsystem,
    .enable_x11_forwarding = chan_no_enable_x11_forwarding,
    .enable_agent_forwarding = chan_no_enable_agent_forwarding,
    .allocate_pty = chan_no_allocate_pty,
    .set_env = chan_no_set_env,
    .send_break = chan_no_send_break,
    .send_signal = chan_no_send_signal,
    .change_window_size = chan_no_change_window_size,
    .request_response = chan_no_request_response,
};

static void sesschan_eventlog(LogPolicy *lp, const char *event) {}
static void sesschan_logging_error(LogPolicy *lp, const char *event) {}
static int sesschan_askappend(
    LogPolicy *lp, Filename *filename,
    void (*callback)(void *ctx, int result), void *ctx) { return 2; }

static const LogPolicyVtable sesschan_logpolicy_vt = {
    .eventlog = sesschan_eventlog,
    .askappend = sesschan_askappend,
    .logging_error = sesschan_logging_error,
    .verbose = null_lp_verbose_no,
};

static size_t sesschan_seat_output(
    Seat *, SeatOutputType type, const void *, size_t);
static bool sesschan_seat_eof(Seat *);
static void sesschan_notify_remote_exit(Seat *seat);
static void sesschan_connection_fatal(Seat *seat, const char *message);
static bool sesschan_get_window_pixel_size(Seat *seat, int *w, int *h);

static const SeatVtable sesschan_seat_vt = {
    .output = sesschan_seat_output,
    .eof = sesschan_seat_eof,
    .sent = nullseat_sent,
    .banner = nullseat_banner,
    .get_userpass_input = nullseat_get_userpass_input,
    .notify_session_started = nullseat_notify_session_started,
    .notify_remote_exit = sesschan_notify_remote_exit,
    .notify_remote_disconnect = nullseat_notify_remote_disconnect,
    .connection_fatal = sesschan_connection_fatal,
    .update_specials_menu = nullseat_update_specials_menu,
    .get_ttymode = nullseat_get_ttymode,
    .set_busy_status = nullseat_set_busy_status,
    .confirm_ssh_host_key = nullseat_confirm_ssh_host_key,
    .confirm_weak_crypto_primitive = nullseat_confirm_weak_crypto_primitive,
    .confirm_weak_cached_hostkey = nullseat_confirm_weak_cached_hostkey,
    .prompt_descriptions = nullseat_prompt_descriptions,
    .is_utf8 = nullseat_is_never_utf8,
    .echoedit_update = nullseat_echoedit_update,
    .get_x_display = nullseat_get_x_display,
    .get_windowid = nullseat_get_windowid,
    .get_window_pixel_size = sesschan_get_window_pixel_size,
    .stripctrl_new = nullseat_stripctrl_new,
    .set_trust_status = nullseat_set_trust_status,
    .can_set_trust_status = nullseat_can_set_trust_status_no,
    .has_mixed_input_stream = nullseat_has_mixed_input_stream_no,
    .verbose = nullseat_verbose_no,
    .interactive = nullseat_interactive_no,
    .get_cursor_position = nullseat_get_cursor_position,
};

Channel *sesschan_new(SshChannel *c, LogContext *logctx,
                      const SftpServerVtable *sftpserver_vt,
                      const SshServerConfig *ssc)
{
    sesschan *sess = snew(sesschan);
    memset(sess, 0, sizeof(sesschan));

    sess->c = c;
    sess->chan.vt = &sesschan_channelvt;
    sess->chan.initial_fixed_window_size = 0;
    sess->parent_logctx = logctx;
    sess->ssc = ssc;

    /* Start with a completely default Conf */
    sess->conf = conf_new();
    load_open_settings(NULL, sess->conf);

    /* Set close-on-exit = true to suppress pty.c's "[pterm: process
     * terminated with status x]" message */
    conf_set_int(sess->conf, CONF_close_on_exit, FORCE_ON);

    sess->seat.vt = &sesschan_seat_vt;
    sess->logpolicy.vt = &sesschan_logpolicy_vt;
    sess->child_logctx = log_init(&sess->logpolicy, sess->conf);

    sess->sftpserver_vt = sftpserver_vt;

    bufchain_init(&sess->subsys_input);

    return &sess->chan;
}

static void sesschan_free(Channel *chan)
{
    sesschan *sess = container_of(chan, sesschan, chan);
    int i;

    delete_callbacks_for_context(sess);
    conf_free(sess->conf);
    if (sess->backend)
        backend_free(sess->backend);
    bufchain_clear(&sess->subsys_input);
    if (sess->sftpsrv)
        sftpsrv_free(sess->sftpsrv);
    for (i = 0; i < sess->n_x11_sockets; i++)
        sk_close(sess->x11_sockets[i]);
    if (sess->agent)
        agentfwd_free(sess->agent);

    sfree(sess);
}

static size_t sesschan_send(Channel *chan, bool is_stderr,
                            const void *data, size_t length)
{
    sesschan *sess = container_of(chan, sesschan, chan);

    if (!sess->backend || sess->ignoring_input)
        return 0;

    backend_send(sess->backend, data, length);
    return backend_sendbuffer(sess->backend);
}

static void sesschan_send_eof(Channel *chan)
{
    sesschan *sess = container_of(chan, sesschan, chan);
    if (sess->backend)
        backend_special(sess->backend, SS_EOF, 0);
}

static char *sesschan_log_close_msg(Channel *chan)
{
    return dupstr("Session channel closed");
}

static void sesschan_set_input_wanted(Channel *chan, bool wanted)
{
    sesschan *sess = container_of(chan, sesschan, chan);
    /* Request the back end to resume sending input, if it had become
     * throttled by the channel window shortening */
    if (wanted && sess->backend)
        backend_unthrottle(sess->backend, 0);
}

static void sesschan_start_backend(sesschan *sess, const char *cmd)
{
    /*
     * List of environment variables that we should not pass through
     * from the login session Uppity was run in (which, it being a
     * test server, there will usually be one of). These variables
     * will be set as part of X or agent forwarding, and shouldn't be
     * confusingly set in the absence of that.
     *
     * (DISPLAY must also be cleared, but pty.c will do that anyway
     * when our get_x_display method returns NULL.)
     */
    static const char *const env_to_unset[] = {
        "XAUTHORITY", "SSH_AUTH_SOCK", "SSH_AGENT_PID",
        NULL /* terminator */
    };

    sess->backend = pty_backend_create(
        &sess->seat, sess->child_logctx, sess->conf, NULL, cmd,
        sess->ttymodes, !sess->want_pty, sess->ssc->session_starting_dir,
        env_to_unset);
    backend_size(sess->backend, sess->wc, sess->hc);
}

bool sesschan_run_shell(Channel *chan)
{
    sesschan *sess = container_of(chan, sesschan, chan);

    if (sess->backend)
        return false;

    sesschan_start_backend(sess, NULL);
    return true;
}

bool sesschan_run_command(Channel *chan, ptrlen command)
{
    sesschan *sess = container_of(chan, sesschan, chan);

    if (sess->backend)
        return false;

    /* FIXME: make this possible to configure off */
    if ((sess->scpsrv = scp_recognise_exec(sess->c, sess->sftpserver_vt,
                                           command)) != NULL) {
        sess->chan.vt = &scp_channelvt;
        logevent(sess->parent_logctx, "Starting built-in SCP server");
        return true;
    }

    char *command_str = mkstr(command);
    sesschan_start_backend(sess, command_str);
    sfree(command_str);

    return true;
}

bool sesschan_run_subsystem(Channel *chan, ptrlen subsys)
{
    sesschan *sess = container_of(chan, sesschan, chan);

    if (ptrlen_eq_string(subsys, "sftp") && sess->sftpserver_vt) {
        sess->sftpsrv = sftpsrv_new(sess->sftpserver_vt);
        sess->chan.vt = &sftp_channelvt;
        logevent(sess->parent_logctx, "Starting built-in SFTP subsystem");
        return true;
    }

    return false;
}

static void fwd_log(Plug *plug, PlugLogType type, SockAddr *addr, int port,
                    const char *error_msg, int error_code)
{ /* don't expect any weirdnesses from a listening socket */ }
static void fwd_closing(Plug *plug, PlugCloseType type, const char *error_msg)
{ /* not here, either */ }

static int xfwd_accepting(Plug *p, accept_fn_t constructor, accept_ctx_t ctx)
{
    sesschan *sess = container_of(p, sesschan, xfwd_plug);
    Plug *plug;
    Channel *chan;
    Socket *s;
    SocketPeerInfo *pi;
    const char *err;

    chan = portfwd_raw_new(sess->c->cl, &plug, false);
    s = constructor(ctx, plug);
    if ((err = sk_socket_error(s)) != NULL) {
        portfwd_raw_free(chan);
        return 1;
    }
    pi = sk_peer_info(s);
    portfwd_raw_setup(chan, s, ssh_serverside_x11_open(sess->c->cl, chan, pi));
    sk_free_peer_info(pi);

    return 0;
}

static const PlugVtable xfwd_plugvt = {
    .log = fwd_log,
    .closing = fwd_closing,
    .accepting = xfwd_accepting,
};

bool sesschan_enable_x11_forwarding(
    Channel *chan, bool oneshot, ptrlen authproto, ptrlen authdata_hex,
    unsigned screen_number)
{
    sesschan *sess = container_of(chan, sesschan, chan);
    strbuf *authdata_bin;
    size_t i;

    if (oneshot)
        return false;                  /* not supported */

    /*
     * Decode the authorisation data from ASCII hex into binary.
     */
    if (authdata_hex.len % 2)
        return false;                  /* expected an even number of digits */
    authdata_bin = strbuf_new_nm();
    for (i = 0; i < authdata_hex.len; i += 2) {
        const unsigned char *hex = authdata_hex.ptr;
        char hexbuf[3];

        if (!isxdigit((unsigned char)hex[i]) ||
            !isxdigit((unsigned char)hex[i+1])) {
            strbuf_free(authdata_bin);
            return false;              /* not hex */
        }

        hexbuf[0] = hex[i];
        hexbuf[1] = hex[i+1];
        hexbuf[2] = '\0';
        put_byte(authdata_bin, strtoul(hexbuf, NULL, 16));
    }

    sess->xfwd_plug.vt = &xfwd_plugvt;

    char *screensuffix = dupprintf(".%u", screen_number);

    sess->n_x11_sockets = platform_make_x11_server(
        &sess->xfwd_plug, appname, 10, screensuffix,
        authproto, ptrlen_from_strbuf(authdata_bin),
        sess->x11_sockets, sess->conf);

    sfree(screensuffix);
    strbuf_free(authdata_bin);
    return sess->n_x11_sockets != 0;
}

static int agentfwd_accepting(
    Plug *p, accept_fn_t constructor, accept_ctx_t ctx)
{
    agentfwd *agent = container_of(p, agentfwd, plug);
    Plug *plug;
    Channel *chan;
    Socket *s;
    const char *err;

    chan = portfwd_raw_new(agent->cl, &plug, false);
    s = constructor(ctx, plug);
    if ((err = sk_socket_error(s)) != NULL) {
        portfwd_raw_free(chan);
        return 1;
    }
    portfwd_raw_setup(chan, s, ssh_serverside_agent_open(agent->cl, chan));

    return 0;
}

static const PlugVtable agentfwd_plugvt = {
    .log = fwd_log,
    .closing = fwd_closing,
    .accepting = agentfwd_accepting,
};

agentfwd *agentfwd_new(ConnectionLayer *cl, char **socketname_out)
{
    agentfwd *agent = snew(agentfwd);
    agent->cl = cl;
    agent->plug.vt = &agentfwd_plugvt;

    char *dir_prefix = dupprintf("/tmp/%s-agentfwd", appname);
    char *error = NULL, *socketname = NULL;
    agent->socket = platform_make_agent_socket(
        &agent->plug, dir_prefix, &error, &socketname);
    sfree(dir_prefix);
    sfree(error);

    if (!agent->socket) {
        sfree(agent);
        sfree(socketname);
        return NULL;
    }

    *socketname_out = socketname;
    return agent;
}

void agentfwd_free(agentfwd *agent)
{
    if (agent->socket)
        sk_close(agent->socket);
    sfree(agent);
}

bool sesschan_enable_agent_forwarding(Channel *chan)
{
    sesschan *sess = container_of(chan, sesschan, chan);
    char *socketname;

    assert(!sess->agent);

    sess->agent = agentfwd_new(sess->c->cl, &socketname);

    if (!sess->agent)
        return false;

    conf_set_str_str(sess->conf, CONF_environmt, "SSH_AUTH_SOCK", socketname);
    sfree(socketname);
    return true;
}

bool sesschan_allocate_pty(
    Channel *chan, ptrlen termtype, unsigned width, unsigned height,
    unsigned pixwidth, unsigned pixheight, struct ssh_ttymodes modes)
{
    sesschan *sess = container_of(chan, sesschan, chan);
    char *s;

    if (sess->want_pty)
        return false;

    s = mkstr(termtype);
    conf_set_str(sess->conf, CONF_termtype, s);
    sfree(s);

    sess->want_pty = true;
    sess->ttymodes  = modes;
    sess->wc = width;
    sess->hc = height;
    sess->wp = pixwidth;
    sess->hp = pixheight;

    return true;
}

bool sesschan_set_env(Channel *chan, ptrlen var, ptrlen value)
{
    sesschan *sess = container_of(chan, sesschan, chan);

    char *svar = mkstr(var), *svalue = mkstr(value);
    conf_set_str_str(sess->conf, CONF_environmt, svar, svalue);
    sfree(svar);
    sfree(svalue);

    return true;
}

bool sesschan_send_break(Channel *chan, unsigned length)
{
    sesschan *sess = container_of(chan, sesschan, chan);

    if (sess->backend) {
        /* We ignore the break length. We could pass it through as the
         * 'arg' parameter, and have pty.c collect it and pass it on
         * to tcsendbreak, but since tcsendbreak in turn assigns
         * implementation-defined semantics to _its_ duration
         * parameter, this all just sounds too difficult. */
        backend_special(sess->backend, SS_BRK, 0);
        return true;
    }
    return false;
}

bool sesschan_send_signal(Channel *chan, ptrlen signame)
{
    sesschan *sess = container_of(chan, sesschan, chan);

    /* Start with a code that definitely isn't a signal (or indeed a
     * special command at all), to indicate 'nothing matched'. */
    SessionSpecialCode code = SS_EXITMENU;

    #define SIGNAL_SUB(name) \
        if (ptrlen_eq_string(signame, #name)) code = SS_SIG ## name;
    #define SIGNAL_MAIN(name, text) SIGNAL_SUB(name)
    #include "signal-list.h"
    #undef SIGNAL_MAIN
    #undef SIGNAL_SUB

    if (code == SS_EXITMENU)
        return false;

    backend_special(sess->backend, code, 0);
    return true;
}

bool sesschan_change_window_size(
    Channel *chan, unsigned width, unsigned height,
    unsigned pixwidth, unsigned pixheight)
{
    sesschan *sess = container_of(chan, sesschan, chan);

    if (!sess->want_pty)
        return false;

    sess->wc = width;
    sess->hc = height;
    sess->wp = pixwidth;
    sess->hp = pixheight;

    if (sess->backend)
        backend_size(sess->backend, sess->wc, sess->hc);

    return true;
}

static size_t sesschan_seat_output(
    Seat *seat, SeatOutputType type, const void *data, size_t len)
{
    sesschan *sess = container_of(seat, sesschan, seat);
    return sshfwd_write_ext(sess->c, type == SEAT_OUTPUT_STDERR, data, len);
}

static void sesschan_check_close_callback(void *vctx)
{
    sesschan *sess = (sesschan *)vctx;

    /*
     * Once we've seen incoming EOF from the backend (aka EIO from the
     * pty master) and also passed on the process's exit status, we
     * should proactively initiate closure of the session channel.
     */
    if (sess->seen_eof && sess->seen_exit)
        sshfwd_initiate_close(sess->c, NULL);
}

static bool sesschan_want_close(Channel *chan, bool seen_eof, bool rcvd_eof)
{
    sesschan *sess = container_of(chan, sesschan, chan);

    /*
     * Similarly to above, we don't want to initiate channel closure
     * until we've sent the process's exit status, _even_ if EOF of
     * the actual data stream has happened in both directions.
     */
    return (sess->seen_eof && sess->seen_exit);
}

static bool sesschan_seat_eof(Seat *seat)
{
    sesschan *sess = container_of(seat, sesschan, seat);

    sshfwd_write_eof(sess->c);
    sess->seen_eof = true;

    queue_toplevel_callback(sesschan_check_close_callback, sess);
    return true;
}

static void sesschan_notify_remote_exit(Seat *seat)
{
    sesschan *sess = container_of(seat, sesschan, seat);

    if (!sess->backend)
        return;

    bool got_signal = false;
    if (!sess->ssc->exit_signal_numeric) {
        char *sigmsg;
        ptrlen signame = pty_backend_exit_signame(sess->backend, &sigmsg);

        if (signame.len) {
            if (!sigmsg)
                sigmsg = dupstr("");

            sshfwd_send_exit_signal(
                sess->c, signame, false, ptrlen_from_asciz(sigmsg));

            got_signal = true;
        }

        sfree(sigmsg);
    } else {
        int signum = pty_backend_exit_signum(sess->backend);

        if (signum >= 0) {
            sshfwd_send_exit_signal_numeric(sess->c, signum, false,
                                            PTRLEN_LITERAL(""));
            got_signal = true;
        }
    }

    if (!got_signal)
        sshfwd_send_exit_status(sess->c, backend_exitcode(sess->backend));

    sess->seen_exit = true;
    queue_toplevel_callback(sesschan_check_close_callback, sess);
}

static void sesschan_connection_fatal(Seat *seat, const char *message)
{
    sesschan *sess = container_of(seat, sesschan, seat);

    /* Closest translation I can think of */
    sshfwd_send_exit_signal(
        sess->c, PTRLEN_LITERAL("HUP"), false, ptrlen_from_asciz(message));

    sess->ignoring_input = true;
}

static bool sesschan_get_window_pixel_size(Seat *seat, int *width, int *height)
{
    sesschan *sess = container_of(seat, sesschan, seat);

    *width = sess->wp;
    *height = sess->hp;

    return true;
}

/* ----------------------------------------------------------------------
 * Built-in SFTP subsystem.
 */

static size_t sftp_chan_send(Channel *chan, bool is_stderr,
                             const void *data, size_t length)
{
    sesschan *sess = container_of(chan, sesschan, chan);

    bufchain_add(&sess->subsys_input, data, length);

    while (bufchain_size(&sess->subsys_input) >= 4) {
        char lenbuf[4];
        unsigned pktlen;
        struct sftp_packet *pkt, *reply;

        bufchain_fetch(&sess->subsys_input, lenbuf, 4);
        pktlen = GET_32BIT_MSB_FIRST(lenbuf);

        if (bufchain_size(&sess->subsys_input) - 4 < pktlen)
            break;                     /* wait for more data */

        bufchain_consume(&sess->subsys_input, 4);
        pkt = sftp_recv_prepare(pktlen);
        bufchain_fetch_consume(&sess->subsys_input, pkt->data, pktlen);
        sftp_recv_finish(pkt);
        reply = sftp_handle_request(sess->sftpsrv, pkt);
        sftp_pkt_free(pkt);

        sftp_send_prepare(reply);
        sshfwd_write(sess->c, reply->data, reply->length);
        sftp_pkt_free(reply);
    }

    return 0;
}

static void sftp_chan_send_eof(Channel *chan)
{
    sesschan *sess = container_of(chan, sesschan, chan);
    sshfwd_write_eof(sess->c);
}

static char *sftp_log_close_msg(Channel *chan)
{
    return dupstr("Session channel (SFTP) closed");
}

/* ----------------------------------------------------------------------
 * Built-in SCP subsystem.
 */

static size_t scp_chan_send(Channel *chan, bool is_stderr,
                            const void *data, size_t length)
{
    sesschan *sess = container_of(chan, sesschan, chan);
    return scp_send(sess->scpsrv, data, length);
}

static void scp_chan_send_eof(Channel *chan)
{
    sesschan *sess = container_of(chan, sesschan, chan);
    scp_eof(sess->scpsrv);
}

static char *scp_log_close_msg(Channel *chan)
{
    return dupstr("Session channel (SCP) closed");
}

static void scp_set_input_wanted(Channel *chan, bool wanted)
{
    sesschan *sess = container_of(chan, sesschan, chan);
    scp_throttle(sess->scpsrv, !wanted);
}