Put parameter names in h/utils.h memory function prototypes.

[nmh] / sbr / netsec.c

/*
 * netsec.c -- Network security routines for handling protocols that
 *             require SASL and/or TLS.
 *
 * This code is Copyright (c) 2016, by the authors of nmh.  See the
 * COPYRIGHT file in the root directory of the nmh distribution for
 * complete copyright information.
 */

#include <h/mh.h>
#include <h/utils.h>
#include <h/netsec.h>
#include <h/oauth.h>
#include <stdarg.h>
#include <sys/select.h>

#ifdef CYRUS_SASL
#include <sasl/sasl.h>
#include <sasl/saslutil.h>
# if SASL_VERSION_FULL < 0x020125
  /* Cyrus SASL 2.1.25 introduced the sasl_callback_ft prototype,
     which has an explicit void parameter list, according to best
     practice.  So we need to cast to avoid compile warnings.
     Provide this prototype for earlier versions. */
  typedef int (*sasl_callback_ft)();
# endif /* SASL_VERSION_FULL < 0x020125 */

static int netsec_get_user(void *context, int id, const char **result,
                           unsigned int *len);
static int netsec_get_password(sasl_conn_t *conn, void *context, int id,
                               sasl_secret_t **psecret);

static int sasl_initialized = 0;

#define SASL_MAXRECVBUF 65536
#endif /* CYRUS_SASL */

#ifdef TLS_SUPPORT
#include <openssl/ssl.h>
#include <openssl/err.h>

static int tls_initialized = 0;
static SSL_CTX *sslctx = NULL;          /* SSL Context */

#endif /* TLS_SUPPORT */

/* I'm going to hardcode this for now; maybe make it adjustable later? */
#define NETSEC_BUFSIZE 65536

/*
 * Our context structure, which holds all of the relevant information
 * about a connection.
 */

struct _netsec_context {
    int ns_readfd;              /* Read descriptor for network connection */
    int ns_writefd;             /* Write descriptor for network connection */
    int ns_snoop;               /* If true, display network data */
    int ns_snoop_noend;         /* If true, didn't get a CR/LF on last line */
    netsec_snoop_callback *ns_snoop_cb; /* Snoop output callback */
    void *ns_snoop_context;     /* Context data for snoop function */
    int ns_timeout;             /* Network read timeout, in seconds */
    char *ns_userid;            /* Userid for authentication */
    unsigned char *ns_inbuffer; /* Our read input buffer */
    unsigned char *ns_inptr;    /* Our read buffer input pointer */
    unsigned int ns_inbuflen;   /* Length of data in input buffer */
    unsigned int ns_inbufsize;  /* Size of input buffer */
    unsigned char *ns_outbuffer;/* Output buffer */
    unsigned char *ns_outptr;   /* Output buffer pointer */
    unsigned int ns_outbuflen;  /* Output buffer data length */
    unsigned int ns_outbufsize; /* Output buffer size */
    char *sasl_mech;            /* User-requested mechanism */
    char *sasl_chosen_mech;     /* Mechanism chosen by SASL */
    netsec_sasl_callback sasl_proto_cb; /* SASL callback we use */
#ifdef OAUTH_SUPPORT
    char *oauth_service;        /* OAuth2 service name */
#endif /* OAUTH_SUPPORT */
#ifdef CYRUS_SASL
    char *sasl_hostname;        /* Hostname we've connected to */
    sasl_conn_t *sasl_conn;     /* SASL connection context */
    sasl_ssf_t sasl_ssf;        /* SASL Security Strength Factor */
    sasl_callback_t *sasl_cbs;  /* Callbacks used by SASL */
    nmh_creds_t sasl_creds;     /* Credentials (username/password) */
    sasl_secret_t *sasl_secret; /* SASL password structure */
    int sasl_seclayer;          /* If true, SASL security layer is enabled */
    char *sasl_tmpbuf;          /* Temporary read buffer for decodes */
    size_t sasl_maxbufsize;     /* Maximum negotiated SASL buffer size */
#endif /* CYRUS_SASL */
#ifdef TLS_SUPPORT
    BIO *ssl_io;                /* BIO used for connection I/O */
    int tls_active;             /* If true, TLS is running */
#endif /* TLS_SUPPORT */
};

/*
 * Function to read data from the actual network socket
 */

static int netsec_fillread(netsec_context *ns_context, char **errstr);

/*
 * Code to check the ASCII content of a byte array.
 */

static int checkascii(const unsigned char *byte, size_t len);

/*
 * How this code works, in general.
 *
 * _If_ we are using no encryption then we buffer the network data
 * through ns_inbuffer and ns_outbuffer.  That should be relatively
 * self-explanatory.
 *
 * If we use encryption, then ns_inbuffer and ns_outbuffer contain the
 * cleartext data.  When it comes time to send the encrypted data on the
 * (either from a flush or the buffer is full) we either use BIO_write()
 * for TLS or sasl_encode() (followed by a write() for Cyrus-SASL.  For
 * reads we either use BIO_read() (TLS) or do a network read into a
 * temporary buffer and use sasl_decode() (Cyrus-SASL).  Note that if
 * negotiate TLS then we disable SASL encryption.
 *
 * We used to use a buffering BIO for the reads/writes for TLS, but it
 * ended up being complicated to special-case the buffering for everything
 * except TLS, so the buffering is now unified, no matter which encryption
 * method is being used (even none).
 *
 * For SASL authentication, we make use of (for now) the Cyrus-SASL
 * library.  For some mechanisms, we implement those mechanisms directly
 * since the Cyrus SASL library doesn't support them (like OAuth).
 */

/*
 * Allocate and initialize our security context
 */

netsec_context *
netsec_init(void)
{
    netsec_context *nsc = mh_xmalloc(sizeof(*nsc));

    nsc->ns_readfd = -1;
    nsc->ns_writefd = -1;
    nsc->ns_snoop = 0;
    nsc->ns_snoop_noend = 0;
    nsc->ns_snoop_cb = NULL;
    nsc->ns_snoop_context = NULL;
    nsc->ns_userid = NULL;
    nsc->ns_timeout = 60;       /* Our default */
    nsc->ns_inbufsize = NETSEC_BUFSIZE;
    nsc->ns_inbuffer = mh_xmalloc(nsc->ns_inbufsize);
    nsc->ns_inptr = nsc->ns_inbuffer;
    nsc->ns_inbuflen = 0;
    nsc->ns_outbufsize = NETSEC_BUFSIZE;
    nsc->ns_outbuffer = mh_xmalloc(nsc->ns_outbufsize);
    nsc->ns_outptr = nsc->ns_outbuffer;
    nsc->ns_outbuflen = 0;
    nsc->sasl_mech = NULL;
    nsc->sasl_chosen_mech = NULL;
    nsc->sasl_proto_cb = NULL;
#ifdef OAUTH_SUPPORT
    nsc->oauth_service = NULL;
#endif /* OAUTH_SUPPORT */
#ifdef CYRUS_SASL
    nsc->sasl_conn = NULL;
    nsc->sasl_hostname = NULL;
    nsc->sasl_cbs = NULL;
    nsc->sasl_creds = NULL;
    nsc->sasl_secret = NULL;
    nsc->sasl_ssf = 0;
    nsc->sasl_seclayer = 0;
    nsc->sasl_tmpbuf = NULL;
    nsc->sasl_maxbufsize = 0;
#endif /* CYRUS_SASL */
#ifdef TLS_SUPPORT
    nsc->ssl_io = NULL;
    nsc->tls_active = 0;
#endif /* TLS_SUPPORT */
    return nsc;
}

/*
 * Shutdown the connection completely and free all resources.
 * The connection is only closed if the flag is given.
 */

void
netsec_shutdown(netsec_context *nsc, int closeflag)
{
    if (nsc->ns_userid)
        free(nsc->ns_userid);
    if (nsc->ns_inbuffer)
        free(nsc->ns_inbuffer);
    if (nsc->ns_outbuffer)
        free(nsc->ns_outbuffer);
    if (nsc->sasl_mech)
        free(nsc->sasl_mech);
    if (nsc->sasl_chosen_mech)
        free(nsc->sasl_chosen_mech);
#ifdef OAUTH_SERVICE
    if (nsc->oauth_service)
        free(nsc->oauth_service);
#endif /* OAUTH_SERVICE */
#ifdef CYRUS_SASL
    if (nsc->sasl_conn)
        sasl_dispose(&nsc->sasl_conn);
    if (nsc->sasl_hostname)
        free(nsc->sasl_hostname);
    if (nsc->sasl_cbs)
        free(nsc->sasl_cbs);
    if (nsc->sasl_creds) {
        if (nsc->sasl_creds->password)
            memset(nsc->sasl_creds->password, 0,
                   strlen(nsc->sasl_creds->password));
        free(nsc->sasl_creds);
    }
    if (nsc->sasl_secret) {
        if (nsc->sasl_secret->len > 0) {
            memset(nsc->sasl_secret->data, 0, nsc->sasl_secret->len);
        }
        free(nsc->sasl_secret);
    }
    if (nsc->sasl_tmpbuf)
        free(nsc->sasl_tmpbuf);
#endif /* CYRUS_SASL */
#ifdef TLS_SUPPORT
    if (nsc->ssl_io)
        /*
         * I checked; BIO_free_all() will cause SSL_shutdown to be called
         * on the SSL object in the chain.
         */
        BIO_free_all(nsc->ssl_io);
#endif /* TLS_SUPPORT */

    if (closeflag) {
        if (nsc->ns_readfd != -1)
            close(nsc->ns_readfd);
        if (nsc->ns_writefd != -1 && nsc->ns_writefd != nsc->ns_readfd)
            close(nsc->ns_writefd);
    }

    free(nsc);
}

/*
 * Set the file descriptor for our context
 */

void
netsec_set_fd(netsec_context *nsc, int readfd, int writefd)
{
    nsc->ns_readfd = readfd;
    nsc->ns_writefd = writefd;
}

/*
 * Set the userid used for authentication for this context
 */

void
netsec_set_userid(netsec_context *nsc, const char *userid)
{
    nsc->ns_userid = getcpy(userid);
}

/*
 * Get the snoop flag for this connection
 */

int
netsec_get_snoop(netsec_context *nsc)
{
    return nsc->ns_snoop;
}

/*
 * Set the snoop flag for this connection
 */

void
netsec_set_snoop(netsec_context *nsc, int snoop)
{
    nsc->ns_snoop = snoop;
}

/*
 * Set the snoop callback for this connection.
 */

void netsec_set_snoop_callback(netsec_context *nsc,
                               netsec_snoop_callback callback, void *context)
{
    nsc->ns_snoop_cb = callback;
    nsc->ns_snoop_context = context;
}

/*
 * A base64-decoding snoop callback
 */

void
netsec_b64_snoop_decoder(netsec_context *nsc, const char *string, size_t len,
                         void *context)
{
    unsigned char *decoded;
    size_t decodedlen;
    int offset;
    NMH_UNUSED(nsc);

    offset = context ? *((int *) context) : 0;

    if (offset > 0) {
        /*
         * Output non-base64 data first.
         */
        fprintf(stderr, "%.*s", offset, string);
        string += offset;
        len -= offset;
    }

    if (decodeBase64(string, &decoded, &decodedlen, 1, NULL) == OK) {
        /*
         * Some mechanisms produce large binary tokens, which aren't really
         * readable.  So let's do a simple heuristic.  If the token is greater
         * than 100 characters _and_ the first 100 bytes are more than 50%
         * non-ASCII, then don't print the decoded buffer, just the
         * base64 text.
         */
        if (decodedlen > 100 && !checkascii(decoded, 100)) {
            fprintf(stderr, "%.*s\n", (int) len, string);
        } else {
            char *hexified;
            hexify(decoded, decodedlen, &hexified);
            fprintf(stderr, "b64<%s>\n", hexified);
            free(hexified);
        }
        free(decoded);
    } else {
        fprintf(stderr, "%.*s\n", (int) len, string);
    }
}

/*
 * If the ASCII content is > 50%, return 1
 */

static int
checkascii(const unsigned char *bytes, size_t len)
{
    size_t count = 0, half = len / 2;

    while (len-- > 0) {
        if (isascii(*bytes) && isprint(*bytes) && ++count > half)
            return 1;
        bytes++;
        /* No chance by this point */
        if (count + len < half)
            return 0;
    }

    return 0;
}

/*
 * Set the read timeout for this connection
 */

void
netsec_set_timeout(netsec_context *nsc, int timeout)
{
    nsc->ns_timeout = timeout;
}

/*
 * Read data from the network.  Basically, return anything in our buffer,
 * otherwise fill from the network.
 */

ssize_t
netsec_read(netsec_context *nsc, void *buffer, size_t size, char **errstr)
{
    int retlen;

    /*
     * If our buffer is empty, then we should fill it now
     */

    if (nsc->ns_inbuflen == 0) {
        if (netsec_fillread(nsc, errstr) != OK)
            return NOTOK;
    }

    /*
     * netsec_fillread only returns if the buffer is full, so we can
     * assume here that this has something in it.
     */

    retlen = size > nsc->ns_inbuflen ? nsc->ns_inbuflen : size;

    memcpy(buffer, nsc->ns_inptr, retlen);

    if (retlen == (int) nsc->ns_inbuflen) {
        /*
         * We've emptied our buffer, so reset everything.
         */
        nsc->ns_inptr = nsc->ns_inbuffer;
        nsc->ns_inbuflen = 0;
    } else {
        nsc->ns_inptr += size;
        nsc->ns_inbuflen -= size;
    }

    return OK;
}

/*
 * Get a "line" (CR/LF) terminated from the network.
 *
 * Okay, we play some games here, so pay attention:
 *
 * - Unlike every other function, we return a pointer to the
 *   existing buffer.  This pointer is valid until you call another
 *   read function again.
 * - We NUL-terminate the buffer right at the end, before the CR-LF terminator.
 * - Technically we look for a LF; if we find a CR right before it, then
 *   we back up one.
 * - If your data may contain embedded NULs, this won't work.  You should
 *   be using netsec_read() in that case.
 */

char *
netsec_readline(netsec_context *nsc, size_t *len, char **errstr)
{
    unsigned char *ptr = nsc->ns_inptr;
    size_t count = 0, offset;

retry:
    /*
     * Search through our existing buffer for a LF
     */

    while (count < nsc->ns_inbuflen) {
        count++;
        if (*ptr++ == '\n') {
            char *sptr = (char *) nsc->ns_inptr;
            if (count > 1 && *(ptr - 2) == '\r')
                ptr--;
            *--ptr = '\0';
            if (len)
                *len = ptr - nsc->ns_inptr;
            nsc->ns_inptr += count;
            nsc->ns_inbuflen -= count;
            if (nsc->ns_snoop) {
#ifdef CYRUS_SASL
                if (nsc->sasl_seclayer)
                    fprintf(stderr, "(sasl-decrypted) ");
#endif /* CYRUS_SASL */
#ifdef TLS_SUPPORT
                if (nsc->tls_active)
                    fprintf(stderr, "(tls-decrypted) ");
#endif /* TLS_SUPPORT */
                fprintf(stderr, "<= ");
                if (nsc->ns_snoop_cb)
                    nsc->ns_snoop_cb(nsc, sptr, strlen(sptr),
                                     nsc->ns_snoop_context);
                else
                    fprintf(stderr, "%s\n", sptr);
            }
            return sptr;
        }
    }

    /*
     * Hm, we didn't find a \n.  If we've already searched half of the input
     * buffer, return an error.
     */

    if (count >= nsc->ns_inbufsize / 2) {
        netsec_err(errstr, "Unable to find a line terminator after %d bytes",
                   count);
        return NULL;
    }

    /*
     * Okay, get some more network data.  This may move inptr, so regenerate
     * our ptr value;
     */

    offset = ptr - nsc->ns_inptr;

    if (netsec_fillread(nsc, errstr) != OK)
        return NULL;

    ptr = nsc->ns_inptr + offset;

    goto retry;

    return NULL;        /* Should never reach this */
}

/*
 * Fill our read buffer with some data from the network.
 */

static int
netsec_fillread(netsec_context *nsc, char **errstr)
{
    unsigned char *end;
    char *readbuf;
    size_t readbufsize, remaining, startoffset;
    int rc;

    /*
     * If inbuflen is zero, that means the buffer has been emptied
     * completely.  In that case move inptr back to the start.
     */

    if (nsc->ns_inbuflen == 0) {
        nsc->ns_inptr = nsc->ns_inbuffer;
    }

#if defined(CYRUS_SASL) || defined(TLS_SUPPORT)
retry:
#endif /* CYRUS_SASL || TLS_SUPPORT */
    /*
     * If we are using TLS and there's anything pending, then skip the
     * select call
     */
#ifdef TLS_SUPPORT
    if (!nsc->tls_active || BIO_pending(nsc->ssl_io) == 0)
#endif /* TLS_SUPPORT */
    {
        struct timeval tv;
        fd_set rfds;

        FD_ZERO(&rfds);
        FD_SET(nsc->ns_readfd, &rfds);

        tv.tv_sec = nsc->ns_timeout;
        tv.tv_usec = 0;

        rc = select(nsc->ns_readfd + 1, &rfds, NULL, NULL, &tv);

        if (rc == -1) {
            netsec_err(errstr, "select() while reading failed: %s",
                       strerror(errno));
            return NOTOK;
        }

        if (rc == 0) {
            netsec_err(errstr, "read() timed out after %d seconds",
                       nsc->ns_timeout);
            return NOTOK;
        }

        /*
         * At this point, we know that rc is 1, so there's not even any
         * point to check to see if our descriptor is set in rfds.
         */
    }

    /*
     * Some explanation:
     *
     * startoffset is the offset from the beginning of the input
     * buffer to data that is in our input buffer, but has not yet
     * been consumed.  This can be non-zero if functions like
     * netsec_readline() leave leftover data.
     *
     * remaining is the remaining amount of unconsumed data in the input
     * buffer.
     *
     * end is a pointer to the end of the valid data + 1; it's where
     * the next read should go.
     */

    startoffset = nsc->ns_inptr - nsc->ns_inbuffer;
    remaining = nsc->ns_inbufsize - (startoffset + nsc->ns_inbuflen);
    end = nsc->ns_inptr + nsc->ns_inbuflen;

    /*
     * If we're past the halfway point in our read buffers, shuffle everything
     * back to the beginning.
     */

    if (startoffset > nsc->ns_inbufsize / 2) {
        memmove(nsc->ns_inbuffer, nsc->ns_inptr, nsc->ns_inbuflen);
        nsc->ns_inptr = nsc->ns_inbuffer;
        startoffset = 0;
        remaining = nsc->ns_inbufsize - nsc->ns_inbuflen;
        end = nsc->ns_inptr + nsc->ns_inbuflen;
    }

    /*
     * If we are using TLS, then just read via the BIO.  But we still
     * use our local buffer.
     */
#ifdef TLS_SUPPORT
    if (nsc->tls_active) {
        rc = BIO_read(nsc->ssl_io, end, remaining);
        if (rc == 0) {
            SSL *ssl;
            int errcode;

            /*
             * Check to see if we're supposed to retry; if so,
             * then go back and read again.
             */

            if (BIO_should_retry(nsc->ssl_io))
                goto retry;

            /*
             * Okay, fine.  Get the real error out of the SSL context.
             */

            if (BIO_get_ssl(nsc->ssl_io, &ssl) < 1) {
                netsec_err(errstr, "SSL_read() returned 0, but cannot "
                           "retrieve SSL context");
                return NOTOK;
            }

            errcode = SSL_get_error(ssl, rc);
            if (errcode == SSL_ERROR_ZERO_RETURN) {
                netsec_err(errstr, "TLS peer closed remote connection");
            } else {
                netsec_err(errstr, "TLS network read failed: %s",
                           ERR_error_string(ERR_peek_last_error(), NULL));
            }
            if (nsc->ns_snoop)
                ERR_print_errors_fp(stderr);
            return NOTOK;
        } else if (rc < 0) {
            /* Definitely an error */
            netsec_err(errstr, "Read on TLS connection failed: %s",
                       ERR_error_string(ERR_get_error(), NULL));
            return NOTOK;
        }

        nsc->ns_inbuflen += rc;

        return OK;
    }
#endif /* TLS_SUPPORT */

    /*
     * Okay, time to read some data.  Either we're just doing it straight
     * or we're passing it through sasl_decode() first.
     */

#ifdef CYRUS_SASL
    if (nsc->sasl_seclayer) {
        readbuf = nsc->sasl_tmpbuf;
        readbufsize = nsc->sasl_maxbufsize;
    } else
#endif /* CYRUS_SASL */
    {
        readbuf = (char *) end;
        readbufsize = remaining;
    }

    /*
     * At this point, we should have active data on the connection (see
     * select() above) so this read SHOULDN'T block.  Hopefully.
     */

    rc = read(nsc->ns_readfd, readbuf, readbufsize);

    if (rc == 0) {
        netsec_err(errstr, "Received EOF on network read");
        return NOTOK;
    }

    if (rc < 0) {
        netsec_err(errstr, "Network read failed: %s", strerror(errno));
        return NOTOK;
    }

    /*
     * Okay, so we've had a successful read.  If we are doing SASL security
     * layers, pass this through sasl_decode().  sasl_decode() can return
     * 0 bytes decoded; if that happens, jump back to the beginning.  Otherwise
     * we can just update our length pointer.
     */

#ifdef CYRUS_SASL
    if (nsc->sasl_seclayer) {
        const char *tmpout;
        unsigned int tmpoutlen;

        rc = sasl_decode(nsc->sasl_conn, nsc->sasl_tmpbuf, rc,
                         &tmpout, &tmpoutlen);

        if (rc != SASL_OK) {
            netsec_err(errstr, "Unable to decode SASL network data: %s",
                       sasl_errdetail(nsc->sasl_conn));
            return NOTOK;
        }

        if (tmpoutlen == 0)
            goto retry;

        /*
         * Just in case ...
         */

        if (tmpoutlen > remaining) {
            netsec_err(errstr, "Internal error: SASL decode buffer overflow!");
            return NOTOK;
        }

        memcpy(end, tmpout, tmpoutlen);

        nsc->ns_inbuflen += tmpoutlen;
    } else
#endif /* CYRUS_SASL */
        nsc->ns_inbuflen += rc;

    return OK;
}

/*
 * Write data to our network connection.  Really, fill up the buffer as
 * much as we can, and flush it out if necessary.  netsec_flush() does
 * the real work.
 */

int
netsec_write(netsec_context *nsc, const void *buffer, size_t size,
             char **errstr)
{
    const unsigned char *bufptr = buffer;
    int rc, remaining;

    /* Just in case */

    if (size == 0)
        return OK;

    /*
     * Run a loop copying in data to our local buffer; when we're done with
     * any buffer overflows then just copy any remaining data in.
     */

    while ((int) size >= (remaining = nsc->ns_outbufsize - nsc->ns_outbuflen)) {
        memcpy(nsc->ns_outptr, bufptr, remaining);

        /*
         * In theory I should increment outptr, but netsec_flush just resets
         * it anyway.
         */
        nsc->ns_outbuflen = nsc->ns_outbufsize;

        rc = netsec_flush(nsc, errstr);

        if (rc != OK)
            return NOTOK;

        bufptr += remaining;
        size -= remaining;
    }

    /*
     * Copy any leftover data into the buffer.
     */

    if (size > 0) {
        memcpy(nsc->ns_outptr, bufptr, size);
        nsc->ns_outptr += size;
        nsc->ns_outbuflen += size;
    }

    return OK;
}

/*
 * Our network printf() routine, which really just calls netsec_vprintf().
 */

int
netsec_printf(netsec_context *nsc, char **errstr, const char *format, ...)
{
    va_list ap;
    int rc;

    va_start(ap, format);
    rc = netsec_vprintf(nsc, errstr, format, ap);
    va_end(ap);

    return rc;
}

/*
 * Write bytes to the network using printf()-style formatting.
 *
 * Again, for the most part copy stuff into our buffer to be flushed
 * out later.
 */

int
netsec_vprintf(netsec_context *nsc, char **errstr, const char *format,
               va_list ap)
{
    int rc;

    /*
     * Cheat a little.  If we can fit the data into our outgoing buffer,
     * great!  If not, generate a flush and retry once.
     */

retry:
    rc = vsnprintf((char *) nsc->ns_outptr,
                   nsc->ns_outbufsize - nsc->ns_outbuflen, format, ap);

    if (rc >= (int) (nsc->ns_outbufsize - nsc->ns_outbuflen)) {
        /*
         * This means we have an overflow.  Note that we don't actually
         * make use of the terminating NUL, but according to the spec
         * vsnprintf() won't write to the last byte in the string; that's
         * why we have to use >= in the comparison above.
         */
        if (nsc->ns_outbuffer == nsc->ns_outptr) {
            /*
             * Whoops, if the buffer pointer was the same as the start of the
             * buffer, that means we overflowed the internal buffer.
             * At that point, just give up.
             */
            netsec_err(errstr, "Internal error: wanted to printf() a total of "
                       "%d bytes, but our buffer size was only %d bytes",
                       rc, nsc->ns_outbufsize);
            return NOTOK;
        } else {
            /*
             * Generate a flush (which may be inefficient, but hopefully
             * it isn't) and then try again.
             */
            if (netsec_flush(nsc, errstr) != OK)
                return NOTOK;
            /*
             * After this, outbuffer should == outptr, so we shouldn't
             * hit this next time around.
             */
            goto retry;
        }
    }

    if (nsc->ns_snoop) {
        int outlen = rc;
        if (outlen > 0 && nsc->ns_outptr[outlen - 1] == '\n') {
            outlen--;
            if (outlen > 0 && nsc->ns_outptr[outlen - 1] == '\r')
                outlen--;
        } else {
            nsc->ns_snoop_noend = 1;
        }
        if (outlen > 0 || nsc->ns_snoop_noend == 0) {
#ifdef CYRUS_SASL
            if (nsc->sasl_seclayer)
                fprintf(stderr, "(sasl-encrypted) ");
#endif /* CYRUS_SASL */
#ifdef TLS_SUPPORT
            if (nsc->tls_active)
                fprintf(stderr, "(tls-encrypted) ");
#endif /* TLS_SUPPORT */
            fprintf(stderr, "=> ");
            if (nsc->ns_snoop_cb)
                nsc->ns_snoop_cb(nsc, (char *) nsc->ns_outptr, outlen,
                                 nsc->ns_snoop_context);
            else
                 fprintf(stderr, "%.*s\n", outlen, nsc->ns_outptr); 
        } else {
            nsc->ns_snoop_noend = 0;
        }
    }

    nsc->ns_outptr += rc;
    nsc->ns_outbuflen += rc;

    return OK;
}

/*
 * Flush out any buffered data in our output buffers.  This routine is
 * actually where the real network writes take place.
 */

int
netsec_flush(netsec_context *nsc, char **errstr)
{
    const char *netoutbuf = (const char *) nsc->ns_outbuffer;
    unsigned int netoutlen = nsc->ns_outbuflen;
    int rc;

    /*
     * Small optimization
     */

    if (netoutlen == 0)
        return OK;

    /*
     * If SASL security layers are in effect, run the data through
     * sasl_encode() first.
     */
#ifdef CYRUS_SASL
    if (nsc->sasl_seclayer) {
        rc = sasl_encode(nsc->sasl_conn, (const char *) nsc->ns_outbuffer,
                         nsc->ns_outbuflen, &netoutbuf, &netoutlen);

        if (rc != SASL_OK) {
            netsec_err(errstr, "SASL data encoding failed: %s",
                       sasl_errdetail(nsc->sasl_conn));
            return NOTOK;
        }

    }
#endif /* CYRUS_SASL */

    /*
     * If TLS is active, then use those functions to write out the
     * data.
     */
#ifdef TLS_SUPPORT
    if (nsc->tls_active) {
        if (BIO_write(nsc->ssl_io, netoutbuf, netoutlen) <= 0) {
            netsec_err(errstr, "Error writing to TLS connection: %s",
                       ERR_error_string(ERR_get_error(), NULL));
            return NOTOK;
        }
    } else
#endif /* TLS_SUPPORT */
    {
        rc = write(nsc->ns_writefd, netoutbuf, netoutlen);

        if (rc < 0) {
            netsec_err(errstr, "write() failed: %s", strerror(errno));
            return NOTOK;
        }
    }

    nsc->ns_outptr = nsc->ns_outbuffer;
    nsc->ns_outbuflen = 0;

    return OK;
}

/*
 * Set various SASL protocol parameters
 */

int
netsec_set_sasl_params(netsec_context *nsc, const char *hostname,
                       const char *service, const char *mechanism,
                       netsec_sasl_callback callback, char **errstr)
{
#ifdef CYRUS_SASL
    sasl_callback_t *sasl_cbs;
    int retval;

    if (! sasl_initialized) {
        retval = sasl_client_init(NULL);
        if (retval != SASL_OK) {
            netsec_err(errstr, "SASL client initialization failed: %s",
                       sasl_errstring(retval, NULL, NULL));
            return NOTOK;
        }
        sasl_initialized++;
    }

    /*
     * Allocate an array of SASL callbacks for this connection.
     * Right now we just allocate an array of four callbacks.
     */

    sasl_cbs = mh_xmalloc(sizeof(*sasl_cbs) * 4);

    sasl_cbs[0].id = SASL_CB_USER;
    sasl_cbs[0].proc = (sasl_callback_ft) netsec_get_user;
    sasl_cbs[0].context = nsc;

    sasl_cbs[1].id = SASL_CB_AUTHNAME;
    sasl_cbs[1].proc = (sasl_callback_ft) netsec_get_user;
    sasl_cbs[1].context = nsc;

    sasl_cbs[2].id = SASL_CB_PASS;
    sasl_cbs[2].proc = (sasl_callback_ft) netsec_get_password;
    sasl_cbs[2].context = nsc;

    sasl_cbs[3].id = SASL_CB_LIST_END;
    sasl_cbs[3].proc = NULL;
    sasl_cbs[3].context = NULL;

    nsc->sasl_cbs = sasl_cbs;

    retval = sasl_client_new(service, hostname, NULL, NULL, nsc->sasl_cbs, 0,
                             &nsc->sasl_conn);

    if (retval) {
        netsec_err(errstr, "SASL new client allocation failed: %s",
                   sasl_errstring(retval, NULL, NULL));
        return NOTOK;
    }

    nsc->sasl_hostname = getcpy(hostname);
#else /* CYRUS_SASL */
    NMH_UNUSED(hostname);
    NMH_UNUSED(service);
    NMH_UNUSED(errstr);
#endif /* CYRUS_SASL */

    /*
     * According to the RFC, mechanisms can only be uppercase letter, numbers,
     * and a hypen or underscore.  So make sure we uppercase any letters
     * in case the user passed in lowercase.
     */

    if (mechanism) {
        char *p;
        nsc->sasl_mech = getcpy(mechanism);

        for (p = nsc->sasl_mech; *p; p++)
            if (isascii((unsigned char) *p))    /* Just in case */
                *p = toupper((unsigned char) *p);
    }

    nsc->sasl_proto_cb = callback;

    return OK;
}

#ifdef CYRUS_SASL
/*
 * Our userid callback; return the specified username to the SASL
 * library when asked.
 */

int netsec_get_user(void *context, int id, const char **result,
                    unsigned int *len)
{
    netsec_context *nsc = (netsec_context *) context;

    if (! result || (id != SASL_CB_USER && id != SASL_CB_AUTHNAME))
        return SASL_BADPARAM;

    if (nsc->ns_userid == NULL) {
        /*
         * Pass the 1 third argument to nmh_get_credentials() so that
         * a default user if the -user switch wasn't supplied, and so
         * that a default password will be supplied.  That's used when
         * those values really don't matter, and only with legacy/.netrc,
         * i.e., with a credentials profile entry.
         */

        if (nsc->sasl_creds == NULL) {
            nsc->sasl_creds = mh_xmalloc(sizeof(*nsc->sasl_creds));
            nsc->sasl_creds->user = NULL;
            nsc->sasl_creds->password = NULL;
        }

        if (nmh_get_credentials(nsc->sasl_hostname, nsc->ns_userid, 1,
                                nsc->sasl_creds) != OK)
            return SASL_BADPARAM;

        if (nsc->ns_userid != nsc->sasl_creds->user) {
            if (nsc->ns_userid)
                free(nsc->ns_userid);
            nsc->ns_userid = getcpy(nsc->sasl_creds->user);
        }
    }

    *result = nsc->ns_userid;
    if (len)
        *len = strlen(nsc->ns_userid);

    return SASL_OK;
}

/*
 * Retrieve a password and return it to SASL
 */

static int
netsec_get_password(sasl_conn_t *conn, void *context, int id,
                    sasl_secret_t **psecret)
{
    netsec_context *nsc = (netsec_context *) context;
    int len;

    NMH_UNUSED(conn);

    if (! psecret || id != SASL_CB_PASS)
        return SASL_BADPARAM;

    if (nsc->sasl_creds == NULL) {
        nsc->sasl_creds = mh_xmalloc(sizeof(*nsc->sasl_creds));
        nsc->sasl_creds->user = NULL;
        nsc->sasl_creds->password = NULL;
    }

    if (nsc->sasl_creds->password == NULL) {
        /*
         * Pass the 0 third argument to nmh_get_credentials() so
         * that the default password isn't used.  With legacy/.netrc
         * credentials support, we'll only get here if the -user
         * switch to send(1)/post(8) wasn't used.
         */

        if (nmh_get_credentials(nsc->sasl_hostname, nsc->ns_userid, 0,
                                nsc->sasl_creds) != OK) {
            return SASL_BADPARAM;
        }
    }

    len = strlen(nsc->sasl_creds->password);

    /*
     * sasl_secret_t includes 1 bytes for "data" already, so that leaves
     * us room for a terminating NUL
     */

    *psecret = (sasl_secret_t *) malloc(sizeof(sasl_secret_t) + len);

    if (! *psecret)
        return SASL_NOMEM;

    (*psecret)->len = len;
    strcpy((char *) (*psecret)->data, nsc->sasl_creds->password);

    nsc->sasl_secret = *psecret;

    return SASL_OK;
}
#endif /* CYRUS_SASL */

/*
 * Negotiate SASL on this connection
 */

int
netsec_negotiate_sasl(netsec_context *nsc, const char *mechlist, char **errstr)
{
#ifdef CYRUS_SASL
    sasl_security_properties_t secprops;
    const char *chosen_mech;
    const unsigned char *saslbuf;
    unsigned char *outbuf;
    unsigned int saslbuflen, outbuflen;
    sasl_ssf_t *ssf;
    int *outbufmax;
#endif
#ifdef OAUTH_SUPPORT
    unsigned char *xoauth_client_res;
    size_t xoauth_client_res_len;
#endif /* OAUTH_SUPPORT */
#if defined CYRUS_SASL || defined OAUTH_SUPPORT
    int rc;
#endif /* CYRUS_SASL || OAUTH_SUPPORT */

    /*
     * If we've been passed a requested mechanism, check our mechanism
     * list from the protocol.  If it's not supported, return an error.
     */

    if (nsc->sasl_mech) {
        char **str, *mlist = getcpy(mechlist);
        int i;

        str = brkstring(mlist, " ", NULL);

        for (i = 0; str[i] != NULL; i++) {
            if (strcasecmp(nsc->sasl_mech, str[i]) == 0) {
                break;
            }
        }

        i = (str[i] == NULL);

        free(mlist);

        if (i) {
            netsec_err(errstr, "Chosen mechanism %s not supported by server",
                       nsc->sasl_mech);
            return NOTOK;
        }
    }

#ifdef OAUTH_SUPPORT
    if (nsc->sasl_mech && strcasecmp(nsc->sasl_mech, "XOAUTH2") == 0) {
        /*
         * This should be relatively straightforward, but requires some
         * help from the plugin.  Basically, if XOAUTH2 is a success,
         * the callback has to return success, but no output data.  If
         * there is output data, it will be assumed that it is the JSON
         * error message.
         */

        if (! nsc->oauth_service) {
            netsec_err(errstr, "Internal error: OAuth2 service name not given");
            return NOTOK;
        }

        nsc->sasl_chosen_mech = getcpy(nsc->sasl_mech);

        if (mh_oauth_do_xoauth(nsc->ns_userid, nsc->oauth_service,
                               &xoauth_client_res, &xoauth_client_res_len,
                               nsc->ns_snoop ? stderr : NULL) != OK) {
            netsec_err(errstr, "Internal error: Unable to get OAuth2 "
                       "bearer token");
            return NOTOK;
        }

        rc = nsc->sasl_proto_cb(NETSEC_SASL_START, xoauth_client_res,
                                xoauth_client_res_len, NULL, 0, errstr);
        free(xoauth_client_res);

        if (rc != OK)
            return NOTOK;

        /*
         * Okay, we need to do a NETSEC_SASL_FINISH now.  If we return
         * success, we indicate that with no output data.  But if we
         * fail, then send a blank message and get the resulting
         * error.
         */

        rc = nsc->sasl_proto_cb(NETSEC_SASL_FINISH, NULL, 0, NULL, 0, errstr);

        if (rc != OK) {
            /*
             * We're going to assume the error here is a JSON response;
             * we ignore it and send a blank message in response.  We should
             * then get a failure messages with a useful error.  We should
             * NOT get a success message at this point.
             */
            free(*errstr);
            nsc->sasl_proto_cb(NETSEC_SASL_WRITE, NULL, 0, NULL, 0, NULL);
            rc = nsc->sasl_proto_cb(NETSEC_SASL_FINISH, NULL, 0, NULL, 0,
                                    errstr);
            if (rc == 0) {
                netsec_err(errstr, "Unexpected success after OAuth failure!");
            }
            return NOTOK;
        }
        return OK;
    }
#endif /* OAUTH_SUPPORT */

#ifdef CYRUS_SASL
    /*
     * In netsec_set_sasl_params, we've already done all of our setup with
     * sasl_client_init() and sasl_client_new().  So time to set security
     * properties, call sasl_client_start(), and generate the protocol
     * messages.
     */

    memset(&secprops, 0, sizeof(secprops));
    secprops.maxbufsize = SASL_MAXRECVBUF;

    /*
     * If we're using TLS, do not negotiate a security layer
     */

    secprops.max_ssf = 
#ifdef TLS_SUPPORT
                nsc->tls_active ? 0 :
#endif /* TLS_SUPPORT */
                UINT_MAX;

    rc = sasl_setprop(nsc->sasl_conn, SASL_SEC_PROPS, &secprops);

    if (rc != SASL_OK) {
        netsec_err(errstr, "SASL security property initialization failed: %s",
                   sasl_errstring(rc, NULL, NULL));
        return NOTOK;
    }

    /*
     * Start the actual protocol negotiation, and go through the
     * sasl_client_step() loop (after sasl_client_start, of course).
     */

    rc = sasl_client_start(nsc->sasl_conn,
                           nsc->sasl_mech ? nsc->sasl_mech : mechlist, NULL,
                           (const char **) &saslbuf, &saslbuflen,
                           &chosen_mech);

    if (rc != SASL_OK && rc != SASL_CONTINUE) {
        netsec_err(errstr, "SASL client start failed: %s",
                   sasl_errdetail(nsc->sasl_conn));
        return NOTOK;
    }

    nsc->sasl_chosen_mech = getcpy(chosen_mech);

    if (nsc->sasl_proto_cb(NETSEC_SASL_START, saslbuf, saslbuflen, NULL, 0,
                           errstr) != OK)
        return NOTOK;

    /*
     * We've written out our first message; enter in the step loop
     */

    while (rc == SASL_CONTINUE) {
        /*
         * Call our SASL callback, which will handle the details of
         * reading data from the network.
         */

        if (nsc->sasl_proto_cb(NETSEC_SASL_READ, NULL, 0, &outbuf, &outbuflen,
                               errstr) != OK) {
            nsc->sasl_proto_cb(NETSEC_SASL_CANCEL, NULL, 0, NULL, 0, NULL);
            return NOTOK;
        }

        rc = sasl_client_step(nsc->sasl_conn, (char *) outbuf, outbuflen, NULL,
                              (const char **) &saslbuf, &saslbuflen);

        if (outbuf)
            free(outbuf);

        if (rc != SASL_OK && rc != SASL_CONTINUE) {
            netsec_err(errstr, "SASL client negotiation failed: %s",
                       sasl_errdetail(nsc->sasl_conn));
            nsc->sasl_proto_cb(NETSEC_SASL_CANCEL, NULL, 0, NULL, 0, NULL);
            return NOTOK;
        }

        if (nsc->sasl_proto_cb(NETSEC_SASL_WRITE, saslbuf, saslbuflen,
                               NULL, 0, errstr) != OK) {
            nsc->sasl_proto_cb(NETSEC_SASL_CANCEL, NULL, 0, NULL, 0, NULL);
            return NOTOK;
        }
    }

    /*
     * SASL exchanges should be complete, process the final response message
     * from the server.
     */

    if (nsc->sasl_proto_cb(NETSEC_SASL_FINISH, NULL, 0, NULL, 0,
                           errstr) != OK) {
        /*
         * At this point we can't really send an abort since the SASL dialog
         * has completed, so just bubble back up the error message.
         */

        return NOTOK;
    }

    /*
     * At this point, SASL should be complete.  Get a few properties
     * from the authentication exchange.
     */

    rc = sasl_getprop(nsc->sasl_conn, SASL_SSF, (const void **) &ssf);

    if (rc != SASL_OK) {
        netsec_err(errstr, "Cannot retrieve SASL negotiated security "
                  "strength factor: %s", sasl_errstring(rc, NULL, NULL));
        return NOTOK;
    }

    nsc->sasl_ssf = *ssf;

    if (nsc->sasl_ssf > 0) {
        rc = sasl_getprop(nsc->sasl_conn, SASL_MAXOUTBUF,
                          (const void **) &outbufmax);

        if (rc != SASL_OK) {
            netsec_err(errstr, "Cannot retrieve SASL negotiated output "
                       "buffer size: %s", sasl_errstring(rc, NULL, NULL));
            return NOTOK;
        }

        /*
         * If our output buffer isn't the same size as the input buffer,
         * reallocate it and set the new size (since we won't encode any
         * data larger than that).
         */

        nsc->sasl_maxbufsize = *outbufmax;

        if (nsc->ns_outbufsize != nsc->sasl_maxbufsize) {
            nsc->ns_outbufsize = nsc->sasl_maxbufsize;
            nsc->ns_outbuffer = mh_xrealloc(nsc->ns_outbuffer,
                                            nsc->ns_outbufsize);
            /*
             * There shouldn't be any data in the buffer, but for
             * consistency's sake discard it.
             */
            nsc->ns_outptr = nsc->ns_outbuffer;
            nsc->ns_outbuflen = 0;
        }

        /*
         * Allocate a buffer to do temporary reads into, before we
         * call sasl_decode()
         */

        nsc->sasl_tmpbuf = mh_xmalloc(nsc->sasl_maxbufsize);

        /*
         * Okay, this is a bit weird.  Make sure that the input buffer
         * is at least TWICE the size of the max buffer size.  That's
         * because if we're consuming data but want to extend the current
         * buffer, we want to be sure there's room for another full buffer's
         * worth of data.
         */

        if (nsc->ns_inbufsize < nsc->sasl_maxbufsize * 2) {
            size_t offset = nsc->ns_inptr - nsc->ns_inbuffer;
            nsc->ns_inbufsize = nsc->sasl_maxbufsize * 2;
            nsc->ns_inbuffer = mh_xrealloc(nsc->ns_inbuffer, nsc->ns_inbufsize);
            nsc->ns_inptr = nsc->ns_inbuffer + offset;
        }

        nsc->sasl_seclayer = 1;
    }

    return OK;
#else
    /*
     * If we're at this point, then either we have NEITHER OAuth2 or
     * Cyrus-SASL compiled in, or have OAuth2 but didn't give the XOAUTH2
     * mechanism on the command line.
     */

    if (! nsc->sasl_mech)
        netsec_err(errstr, "SASL library support not available; please "
                   "specify a SASL mechanism to use");
    else
        netsec_err(errstr, "No support for the %s SASL mechanism",
                   nsc->sasl_mech);

    return NOTOK;
#endif /* CYRUS_SASL */
}

/*
 * Retrieve our chosen SASL mechanism
 */

char *
netsec_get_sasl_mechanism(netsec_context *nsc)
{
    return nsc->sasl_chosen_mech;
}

/*
 * Set an OAuth2 service name, if we support it.
 */

int
netsec_set_oauth_service(netsec_context *nsc, const char *service)
{
#ifdef OAUTH_SUPPORT
    nsc->oauth_service = getcpy(service);
    return OK;
#else /* OAUTH_SUPPORT */
    NMH_UNUSED(nsc);
    NMH_UNUSED(service);
    return NOTOK;
#endif /* OAUTH_SUPPORT */
}

/*
 * Initialize (and enable) TLS for this connection
 */

int
netsec_set_tls(netsec_context *nsc, int tls, char **errstr)
{
    if (tls) {
#ifdef TLS_SUPPORT
        SSL *ssl;
        BIO *rbio, *wbio, *ssl_bio;;

        if (! tls_initialized) {
            SSL_library_init();
            SSL_load_error_strings();

            /*
             * Create the SSL context; this has the properties for all
             * SSL connections (we are only doing one), though.  Make sure
             * we only support secure (known as of now) TLS protocols.
             */

            sslctx = SSL_CTX_new(SSLv23_client_method());

            if (! sslctx) {
                netsec_err(errstr, "Unable to initialize OpenSSL context: %s",
                           ERR_error_string(ERR_get_error(), NULL));
                return NOTOK;
            }

            SSL_CTX_set_options(sslctx, SSL_OP_NO_SSLv2 | SSL_OP_NO_SSLv3 |
                                SSL_OP_NO_TLSv1);

            tls_initialized++;
        }

        if (nsc->ns_readfd == -1 || nsc->ns_writefd == -1) {
            netsec_err(errstr, "Invalid file descriptor in netsec context");
            return NOTOK;
        }

        /*
         * Create the SSL structure which holds the data for a single
         * TLS connection.
         */

        ssl = SSL_new(sslctx);

        if (! ssl) {
            netsec_err(errstr, "Unable to create SSL connection: %s",
                       ERR_error_string(ERR_get_error(), NULL));
            return NOTOK;
        }

        /*
         * Never bother us, since we are using blocking sockets.
         */

        SSL_set_mode(ssl, SSL_MODE_AUTO_RETRY);

        /*
         * This is a bit weird, so pay attention.
         *
         * We create a socket BIO, and bind it to our SSL connection.
         * That means reads and writes to the SSL connection will use our
         * supplied socket.
         *
         * Then we create an SSL BIO, and assign our current SSL connection
         * to it.  This is done so our code stays simple if we want to use
         * any buffering BIOs (right now we do our own buffering).
         * So the chain looks like:
         *
         * SSL BIO -> socket BIO.
         */

        rbio = BIO_new_socket(nsc->ns_readfd, BIO_NOCLOSE);

        if (! rbio) {
            netsec_err(errstr, "Unable to create a read socket BIO: %s",
                       ERR_error_string(ERR_get_error(), NULL));
            SSL_free(ssl);
            return NOTOK;
        }

        wbio = BIO_new_socket(nsc->ns_writefd, BIO_NOCLOSE);

        if (! wbio) {
            netsec_err(errstr, "Unable to create a write socket BIO: %s",
                       ERR_error_string(ERR_get_error(), NULL));
            SSL_free(ssl);
            BIO_free(rbio);
            return NOTOK;
        }

        SSL_set_bio(ssl, rbio, wbio);
        SSL_set_connect_state(ssl);

        ssl_bio = BIO_new(BIO_f_ssl());

        if (! ssl_bio) {
            netsec_err(errstr, "Unable to create a SSL BIO: %s",
                       ERR_error_string(ERR_get_error(), NULL));
            SSL_free(ssl);
            return NOTOK;
        }

        BIO_set_ssl(ssl_bio, ssl, BIO_CLOSE);
        nsc->ssl_io = ssl_bio;

        return OK;
    } else {
        BIO_free_all(nsc->ssl_io);
        nsc->ssl_io = NULL;

        return OK;
    }
#else /* TLS_SUPPORT */
        netsec_err(errstr, "TLS is not supported");

        return NOTOK;
    }
#endif /* TLS_SUPPORT */
}

/*
 * Start TLS negotiation on this connection
 */

int
netsec_negotiate_tls(netsec_context *nsc, char **errstr)
{
#ifdef TLS_SUPPORT
    if (! nsc->ssl_io) {
        netsec_err(errstr, "TLS has not been configured for this connection");
        return NOTOK;
    }

    if (BIO_do_handshake(nsc->ssl_io) < 1) {
        netsec_err(errstr, "TLS negotiation failed: %s",
                   ERR_error_string(ERR_get_error(), NULL));
        return NOTOK;
    }

    if (nsc->ns_snoop) {
        SSL *ssl;

        if (BIO_get_ssl(nsc->ssl_io, &ssl) < 1) {
            fprintf(stderr, "WARNING: cannot determine SSL ciphers\n");
        } else {
            const SSL_CIPHER *cipher = SSL_get_current_cipher(ssl);
            fprintf(stderr, "TLS negotiation successful: %s(%d) %s\n",
                    SSL_CIPHER_get_name(cipher),
                    SSL_CIPHER_get_bits(cipher, NULL),
                    SSL_CIPHER_get_version(cipher));
        }
    }

    nsc->tls_active = 1;

    return OK;
#else /* TLS_SUPPORT */
    netsec_err(errstr, "TLS not supported");

    return NOTOK;
#endif /* TLS_SUPPORT */
}

/*
 * Generate an (allocated) error string
 */

void
netsec_err(char **errstr, const char *fmt, ...)
{
    va_list ap;
    size_t errbufsize;
    char *errbuf = NULL;
    int rc = 127;

    if (! errstr)
        return;

    do {
        errbufsize = rc + 1;
        errbuf = mh_xrealloc(errbuf, errbufsize);
        va_start(ap, fmt);
        rc = vsnprintf(errbuf, errbufsize, fmt, ap);
        va_end(ap);
    } while (rc >= (int) errbufsize);

    *errstr = errbuf;
}