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Subject: libssh2_channel_wait_closed(...) issue

libssh2_channel_wait_closed(...) issue

From: Chris Harris <chris.harris_at_kitware.com>
Date: Wed, 19 Sep 2012 16:51:44 -0400

I am running into an issue when connecting to a sshd instance that has
MaxSessions set to 1. It appears that this only allow one channel to
created at anyone time. So I am creating a channel, running a command and
then wait for the channel to be closed before create another channel... The
problem seem to be that libssh2_channel_wait_closed return 0 before the
sshd has finish cleaning up the channel, so an attempt to create another
channel at this point results in an error (-21). The reason I think
the libssh2_channel_wait_closed is returning too soon is if I add a sleep
between my execs the channels are created with out a problem. The code
below illustrates the problem when connected to a sshd instance with
MaxSessions=1. Any help resolving this issue would be gratefully received.

Regards,

Chris

/*
 * Sample showing how to use libssh2 to execute a command remotely.
 *
 * The sample code has fixed values for host name, user name, password
 * and command to run.
 *
 * Run it like this:
 *
 * $ ./ssh2_exec 127.0.0.1 user password "uptime"
 *
 */

#include "libssh2_config.h"
#include <libssh2.h>

#ifdef HAVE_WINSOCK2_H
# include <winsock2.h>
#endif
#ifdef HAVE_SYS_SOCKET_H
# include <sys/socket.h>
#endif
#ifdef HAVE_NETINET_IN_H
# include <netinet/in.h>
#endif
#ifdef HAVE_SYS_SELECT_H
# include <sys/select.h>
#endif
# ifdef HAVE_UNISTD_H
#include <unistd.h>
#endif
#ifdef HAVE_ARPA_INET_H
# include <arpa/inet.h>
#endif

#include <sys/time.h>
#include <sys/types.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
#include <ctype.h>

static int waitsocket(int socket_fd, LIBSSH2_SESSION *session)
{
    struct timeval timeout;
    int rc;
    fd_set fd;
    fd_set *writefd = NULL;
    fd_set *readfd = NULL;
    int dir;

    timeout.tv_sec = 10;
    timeout.tv_usec = 0;

    FD_ZERO(&fd);

    FD_SET(socket_fd, &fd);

    /* now make sure we wait in the correct direction */
    dir = libssh2_session_block_directions(session);

    if(dir & LIBSSH2_SESSION_BLOCK_INBOUND)
        readfd = &fd;

    if(dir & LIBSSH2_SESSION_BLOCK_OUTBOUND)
        writefd = &fd;

    rc = select(socket_fd + 1, readfd, writefd, NULL, &timeout);

    return rc;
}

void exec(int sock, LIBSSH2_SESSION *session, const char *commandline)
{
 LIBSSH2_CHANNEL *channel;
 int rc;
 int exitcode;
 char *exitsignal=(char *)"none";
 int bytecount = 0;

/* Exec non-blocking on the remove host */
 while( (channel = libssh2_channel_open_session(session)) == NULL &&
        libssh2_session_last_error(session,NULL,NULL,0) ==
        LIBSSH2_ERROR_EAGAIN )
 {
     waitsocket(sock, session);
 }
 if( channel == NULL )
 {
     rc = libssh2_session_last_error(session,NULL,NULL,0);
     fprintf(stderr,"Error: %d\n", rc);
     exit( 1 );
 }
 while( (rc = libssh2_channel_exec(channel, commandline)) ==
        LIBSSH2_ERROR_EAGAIN )
 {
     waitsocket(sock, session);
 }
 if( rc != 0 )
 {
     fprintf(stderr,"Error\n");
     exit( 1 );
 }
 for( ;; )
 {
     /* loop until we block */
     int rc;
     do
     {
         char buffer[0x4000];
         rc = libssh2_channel_read( channel, buffer, sizeof(buffer) );
         if( rc > 0 )
         {
             int i;
             bytecount += rc;
             fprintf(stderr, "We read:\n");
             for( i=0; i < rc; ++i )
                 fputc( buffer[i], stderr);
             fprintf(stderr, "\n");
         }
         else {
             if( rc != LIBSSH2_ERROR_EAGAIN )
                 /* no need to output this for the EAGAIN case */
                 fprintf(stderr, "libssh2_channel_read returned %d\n", rc);
         }
     }
     while( rc > 0 );

     /* this is due to blocking that would occur otherwise so we loop on
        this condition */
     if( rc == LIBSSH2_ERROR_EAGAIN )
     {
         waitsocket(sock, session);
     }
     else
         break;
 }
 exitcode = 127;
 while( (rc = libssh2_channel_close(channel)) == LIBSSH2_ERROR_EAGAIN )
     waitsocket(sock, session);

 while( (rc = libssh2_channel_wait_closed(channel)) == LIBSSH2_ERROR_EAGAIN
)
     waitsocket(sock, session);

 if( rc == 0 )
 {
     exitcode = libssh2_channel_get_exit_status( channel );
     libssh2_channel_get_exit_signal(channel, &exitsignal,
                                     NULL, NULL, NULL, NULL, NULL);
 }

 if (exitsignal)
     printf("\nGot signal: %s\n", exitsignal);
 else
     printf("\nEXIT: %d bytecount: %d\n", exitcode, bytecount);

 libssh2_channel_free(channel);
 channel = NULL;
}

int main(int argc, char *argv[])
{
    const char *hostname = "127.0.0.1";
    const char *commandline = "uptime";
    const char *username = "user";
    const char *password = "password";
    unsigned long hostaddr;
    int sock;
    struct sockaddr_in sin;
    const char *fingerprint;
    LIBSSH2_SESSION *session;
    int rc;
    size_t len;
    LIBSSH2_KNOWNHOSTS *nh;
    int type;

#ifdef WIN32
    WSADATA wsadata;
    WSAStartup(MAKEWORD(2,0), &wsadata);
#endif
    if (argc > 1)
        /* must be ip address only */
        hostname = argv[1];

    if (argc > 2) {
        username = argv[2];
    }
    if (argc > 3) {
        password = argv[3];
    }
    if (argc > 4) {
        commandline = argv[4];
    }

    rc = libssh2_init (0);
    if (rc != 0) {
        fprintf (stderr, "libssh2 initialization failed (%d)\n", rc);
        return 1;
    }

    hostaddr = inet_addr(hostname);

    /* Ultra basic "connect to port 22 on localhost"
     * Your code is responsible for creating the socket establishing the
     * connection
     */
    sock = socket(AF_INET, SOCK_STREAM, 0);

    sin.sin_family = AF_INET;
    sin.sin_port = htons(22);
    sin.sin_addr.s_addr = hostaddr;
    if (connect(sock, (struct sockaddr*)(&sin),
                sizeof(struct sockaddr_in)) != 0) {
        fprintf(stderr, "failed to connect!\n");
        return -1;
    }

    /* Create a session instance */
    session = libssh2_session_init();
    if (!session)
        return -1;

    /* tell libssh2 we want it all done non-blocking */
    libssh2_session_set_blocking(session, 0);

    /* ... start it up. This will trade welcome banners, exchange keys,
     * and setup crypto, compression, and MAC layers
     */
    while ((rc = libssh2_session_handshake(session, sock)) ==
           LIBSSH2_ERROR_EAGAIN);
    if (rc) {
        fprintf(stderr, "Failure establishing SSH session: %d\n", rc);
        return -1;
    }

    nh = libssh2_knownhost_init(session);
    if(!nh) {
        /* eeek, do cleanup here */
        return 2;
    }

    /* read all hosts from here */
    libssh2_knownhost_readfile(nh, "known_hosts",
                               LIBSSH2_KNOWNHOST_FILE_OPENSSH);

    /* store all known hosts to here */
    libssh2_knownhost_writefile(nh, "dumpfile",
                                LIBSSH2_KNOWNHOST_FILE_OPENSSH);

    fingerprint = libssh2_session_hostkey(session, &len, &type);
    if(fingerprint) {
        struct libssh2_knownhost *host;
#if LIBSSH2_VERSION_NUM >= 0x010206
        /* introduced in 1.2.6 */
        int check = libssh2_knownhost_checkp(nh, hostname, 22,
                                             fingerprint, len,
                                             LIBSSH2_KNOWNHOST_TYPE_PLAIN|
                                             LIBSSH2_KNOWNHOST_KEYENC_RAW,
                                             &host);
#else
        /* 1.2.5 or older */
        int check = libssh2_knownhost_check(nh, hostname,
                                            fingerprint, len,
                                            LIBSSH2_KNOWNHOST_TYPE_PLAIN|
                                            LIBSSH2_KNOWNHOST_KEYENC_RAW,
                                            &host);
#endif
        fprintf(stderr, "Host check: %d, key: %s\n", check,
                (check <= LIBSSH2_KNOWNHOST_CHECK_MISMATCH)?
                host->key:"<none>");

        /*****
         * At this point, we could verify that 'check' tells us the key is
         * fine or bail out.
         *****/
    }
    else {
        /* eeek, do cleanup here */
        return 3;
    }
    libssh2_knownhost_free(nh);

    if ( strlen(password) != 0 ) {
        /* We could authenticate via password */
        while ((rc = libssh2_userauth_password(session, username,
password)) ==
               LIBSSH2_ERROR_EAGAIN);
        if (rc) {
            fprintf(stderr, "Authentication by password failed.\n");
            goto shutdown;
        }
    }
    else {
        /* Or by public key */
        while ((rc = libssh2_userauth_publickey_fromfile(session, username,
                                                         "/home/user/"
                                                         ".ssh/id_rsa.pub",
                                                         "/home/user/"
                                                         ".ssh/id_rsa",
                                                         password)) ==
               LIBSSH2_ERROR_EAGAIN);
        if (rc) {
            fprintf(stderr, "\tAuthentication by public key failed\n");
            goto shutdown;
        }
    }

#if 0
    libssh2_trace(session, ~0 );
#endif

    exec(sock, session, commandline);
    // Adding this solve the issue sleep(1);
    exec(sock, session, commandline);

shutdown:

    libssh2_session_disconnect(session,
                               "Normal Shutdown, Thank you for playing");
    libssh2_session_free(session);

#ifdef WIN32
    closesocket(sock);
#else
    close(sock);
#endif
    fprintf(stderr, "all done\n");

    libssh2_exit();

    return 0;
}

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Received on 2012-09-19

the libssh2 team