Add option to change the listen(2) parameter.

[fwd] / exec.c

/* -*-c-*-
 *
 * $Id: exec.c,v 1.6 2002/02/22 23:43:32 mdw Exp $
 *
 * Source and target for executable programs
 *
 * (c) 1999 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------* 
 *
 * This file is part of the `fw' port forwarder.
 *
 * `fw' is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 * 
 * `fw' is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with `fw'; if not, write to the Free Software Foundation,
 * Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 */

/*----- Revision history --------------------------------------------------* 
 *
 * $Log: exec.c,v $
 * Revision 1.6  2002/02/22 23:43:32  mdw
 * Call @xfree@ rather than @free@.
 *
 * Revision 1.5  2002/01/13 14:49:03  mdw
 * Track @lbuf@ changes in mLib.
 *
 * Revision 1.4  2001/02/03 20:30:03  mdw
 * Support re-reading config files on SIGHUP.
 *
 * Revision 1.3  2000/07/01 11:28:52  mdw
 * Use new mLib selbuf features.
 *
 * Revision 1.2  1999/10/22 22:46:17  mdw
 * When a non-file endpoint is attached to a file, keep the file endpoint
 * open until the nonfile is done.  This stops socket sources from
 * resetting their connection limits too early.
 *
 * Revision 1.1  1999/07/26 23:33:32  mdw
 * New sources and targets.
 *
 */

/*----- Header files ------------------------------------------------------*/

#include "config.h"

#define _GNU_SOURCE

#include <ctype.h>
#include <errno.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>

#ifdef HAVE_SETRLIMIT
#  include <sys/resource.h>
#endif

#ifndef DECL_ENVIRON
  extern char **environ;
#endif

#include <pwd.h>
#include <grp.h>

#include <syslog.h>

#include <mLib/alloc.h>
#include <mLib/dstr.h>
#include <mLib/env.h>
#include <mLib/fdflags.h>
#include <mLib/report.h>
#include <mLib/sel.h>
#include <mLib/selbuf.h>
#include <mLib/sig.h>
#include <mLib/sub.h>
#include <mLib/sym.h>

#include "conf.h"
#include "endpt.h"
#include "exec.h"
#include "fattr.h"
#include "fw.h"
#include "reffd.h"
#include "scan.h"
#include "source.h"
#include "target.h"

/*----- Data structures ---------------------------------------------------*/

/* --- Resource usage --- */

#ifdef HAVE_SETRLIMIT

typedef struct xlimit {
#define R(r, n) struct rlimit n;
#include "rlimits.h"
} xlimit;

#endif

/* --- Environment variable modifications --- */

typedef struct xenv {
  struct xenv *next;
  unsigned act;
  char *name;
  char *value;
} xenv;

#define XEA_SET 0u
#define XEA_CLEAR 1u

/* --- Program options --- */

typedef struct xopts {
  unsigned ref;
  unsigned f;
  const char *dir;
  const char *root;
  uid_t uid;
  gid_t gid;
  xenv *env;
  xenv **etail;
#ifdef HAVE_SETRLIMIT
  xlimit xl;
#endif
} xopts;

#define XF_NOLOG 1u

/* --- Executable program arguments --- */

typedef struct xargs {
  unsigned ref;
  char *file;
  char *argv[1];
} xargs;

#define XARGS_SZ(n) (sizeof(xargs) + sizeof(char *) * (n))

/* --- Executable endpoints --- */

typedef struct xept {
  endpt e;
  struct xept *next, *prev;
  pid_t kid;
  endpt *f;
  char *desc;
  int st;
  xargs *xa;
  xopts *xo;
  selbuf err;
} xept;

#define XEF_CLOSE 16u
#define XEF_EXIT 32u

/* --- Executable program data --- */

typedef struct xdata {
  xargs *xa;
  xopts *xo;
} xdata;

/* --- Executable source block --- */

typedef struct xsource {
  source s;
  xdata x;
} xsource;

/* --- Executable target block --- */

typedef struct xtarget {
  target t;
  xdata x;
} xtarget;

/*----- Static variables --------------------------------------------------*/

static xopts exec_opts = { 1, 0, 0, 0, -1, -1, 0, &exec_opts.env };
static xept *xept_list;
static sig xept_sig;
static sym_table env;

/*----- Fiddling about with resource limits -------------------------------*/

#ifdef HAVE_SETRLIMIT

/* --- Table mapping user-level names to OS interface bits --- */

typedef struct rlimit_ent {
  const char *name;
  const char *rname;
  int r;
  size_t off;
} rlimit_ent;

static rlimit_ent rlimits[] = {
#define R(r, n) { #n, #r, r, offsetof(xlimit, n) },
#include "rlimits.h"
  { 0, 0, 0, 0 }
};

#define RLIMIT(xl, o) ((struct rlimit *)((char *)(xl) + (o)))

/* --- @rlimit_get@ --- *
 *
 * Arguments:	@xlimit *xl@ = pointer to limit structure
 *
 * Returns:	---
 *
 * Use:		Initializes a limit structure from the current limits.
 */

static void rlimit_get(xlimit *xl)
{
  rlimit_ent *r;

  for (r = rlimits; r->name; r++) {
    if (getrlimit(r->r, RLIMIT(xl, r->off))) {
      moan("couldn't read %s: %s", r->rname, strerror(errno));
      _exit(1);
    }
  }
}

/* --- @rlimit_set@ --- *
 *
 * Arguments:	@xlimit *xl@ = pointer to limit structure
 *
 * Returns:	---
 *
 * Use:		Sets resource limits from the supplied limits structure.
 */

static void rlimit_set(xlimit *xl)
{
  rlimit_ent *r;

  for (r = rlimits; r->name; r++) {
    if (setrlimit(r->r, RLIMIT(xl, r->off))) {
      moan("couldn't set %s: %s", r->rname, strerror(errno));
      _exit(1);
    }
  }
}

/* --- @rlimit_option@ --- */

static int rlimit_option(xlimit *xl, scanner *sc)
{
  CONF_BEGIN(sc, "rlimit", "resource limit")
  enum { w_soft, w_hard, w_both } which = w_both;
  rlimit_ent *chosen;
  struct rlimit *rl;
  long v;

  /* --- Find out which resource is being fiddled --- */

  {
    rlimit_ent *r;

    chosen = 0;
    for (r = rlimits; r->name; r++) {
      if (strncmp(sc->d.buf, r->name, sc->d.len) == 0) {
	if (r->name[sc->d.len] == 0) {
	  chosen = r;
	  break;
	} else if (chosen)
	  error(sc, "ambiguous resource limit name `%s'", sc->d.buf);
	else if (CONF_QUAL)
	  chosen = r;
      }
    }
    if (!chosen)
      CONF_REJECT;
    token(sc);
    rl = RLIMIT(xl, chosen->off);
  }

  /* --- Look for hard or soft restrictions --- */

  {
    int i;
    if (sc->t == '.')
      token(sc);
    if (sc->t == CTOK_WORD) {
      if ((i = conf_enum(sc, "soft,hard",
			 ENUM_ABBREV | ENUM_NONE, "limit type")) != -1)
	which = i;
    }
  }

  /* --- Now read the new value --- */

  if (sc->t == '=')
    token(sc);
  if (sc->t != CTOK_WORD)
    error(sc, "parse error, expected limit value");

  if (conf_enum(sc, "unlimited,infinity",
		ENUM_ABBREV | ENUM_NONE, "limit value") > -1)
    v = RLIM_INFINITY;
  else {
    char *p;

    v = strtol(sc->d.buf, &p, 0);
    if (p == sc->d.buf)
      error(sc, "parse error, invalid limit value `%s'", sc->d.buf);
    switch (tolower((unsigned char)*p)) {
      case 0: break;
      case 'b': v *= 512; break;
      case 'g': v *= 1024;
      case 'm': v *= 1024;
      case 'k': v *= 1024; break;
      default: error(sc, "parse error, invalid limit scale `%c'", *p);
    }
    token(sc);
  }

  /* --- Store the limit value away --- */

  switch (which) {
    case w_both:
      rl->rlim_cur = v;
      rl->rlim_max = v;
      break;
    case w_soft:
      if (v > rl->rlim_max)
	error(sc, "soft limit %l exceeds hard limit %l for %s",
	      v, rl->rlim_max, chosen->rname);
      rl->rlim_cur = v;
      break;
    case w_hard:
      rl->rlim_max = v;
      if (rl->rlim_cur > v)
	rl->rlim_cur = v;
      break;
  }

  CONF_ACCEPT;
  CONF_END;
}

#endif

/*----- Environment fiddling ----------------------------------------------*/

/* --- @xenv_option@ --- *
 *
 * Arguments:	@xopts *xo@ = pointer to options block
 *		@scanner *sc@ = pointer to scanner
 *
 * Returns:	Nonzero if claimed
 *
 * Use:		Parses environment variable assignments.
 */

static int xenv_option(xopts *xo, scanner *sc)
{
  CONF_BEGIN(sc, "env", "environment")
  xenv *xe;

  /* --- Unset a variable --- */

  if (strcmp(sc->d.buf, "unset") == 0) {
    token(sc);
    if (sc->t != CTOK_WORD)
      error(sc, "parse error, expected environment variable name");
    xe = CREATE(xenv);
    xe->name = xstrdup(sc->d.buf);
    xe->value = 0;
    xe->act = XEA_SET;
    token(sc);
    goto link;
  }

  /* --- Clear the entire environment --- */

  if (strcmp(sc->d.buf, "clear") == 0) {
    token(sc);
    xe = CREATE(xenv);
    xe->act = XEA_CLEAR;
    goto link;
  }

  /* --- Allow `set' to be omitted if there's a prefix --- */

  if (strcmp(sc->d.buf, "set") == 0)
    token(sc);
  else if (!CONF_QUAL)
    CONF_REJECT;

  /* --- Set a variable --- */

  if (sc->t != CTOK_WORD)
    error(sc, "parse error, expected environment variable name");
  xe = CREATE(xenv);
  xe->name = xstrdup(sc->d.buf);
  token(sc);
  if (sc->t == '=')
    token(sc);
  if (sc->t != CTOK_WORD)
    error(sc, "parse error, expected environment variable value");
  xe->value = xstrdup(sc->d.buf);
  xe->act = XEA_SET;
  token(sc);
  goto link;

link:
  xe->next = 0;
  *xo->etail = xe;
  xo->etail = &xe->next;
  CONF_ACCEPT;

  /* --- Nothing else to try --- */

  CONF_END;
}

/* --- @xenv_apply@ --- *
 *
 * Arguments:	@xenv *xe@ = pointer to a variable change list
 *
 * Returns:	---
 *
 * Use:		Modifies the environment (in @env@) according to the list.
 */

static void xenv_apply(xenv *xe)
{
  while (xe) {
    switch (xe->act) {
      case XEA_SET:
	env_put(&env, xe->name, xe->value);
	break;
      case XEA_CLEAR:
	env_destroy(&env);
	sym_create(&env);
	break;
    }
    xe = xe->next;
  }
}

/* --- @xenv_destroy@ --- *
 *
 * Arguments:	@xenv *xe@ = pointer to a variable change list
 *
 * Returns:	---
 *
 * Use:		Frees the memory used by an environment variable change list.
 */

static void xenv_destroy(xenv *xe)
{
  while (xe) {
    xenv *xxe = xe;
    xe = xe->next;
    xfree(xxe->name);
    if (xxe->value)
      xfree(xxe->value);
    DESTROY(xxe);
  }
}

/*----- Miscellaneous good things -----------------------------------------*/

/* --- @x_tidy@ --- *
 *
 * Arguments:	@xargs *xa@ = pointer to an arguments block
 *		@xopts *xo@ = pointer to an options block
 *
 * Returns:	---
 *
 * Use:		Releases a reference to argument and options blocks.
 */

static void x_tidy(xargs *xa, xopts *xo)
{
  xa->ref--;
  if (!xa->ref)
    xfree(xa);

  xo->ref--;
  if (!xo->ref) {
    xenv_destroy(xo->env);
    DESTROY(xo);
  }
}

/*----- Executable endpoints ----------------------------------------------*/

/* --- @attach@ --- */

static void xept_error(char */*p*/, size_t /*len*/, void */*v*/);

static void xept_attach(endpt *e, reffd *in, reffd *out)
{
  xept *xe = (xept *)e;
  pid_t kid;
  int fd[2];

  /* --- Make a pipe for standard error --- */

  if (pipe(fd)) {
    fw_log(-1, "[%s] couldn't create pipe: %s", xe->desc, strerror(errno));
    return;
  }
  fdflags(fd[0], O_NONBLOCK, O_NONBLOCK, FD_CLOEXEC, FD_CLOEXEC);

  /* --- Fork a child, and handle an error if there was one --- */

  if ((kid = fork()) == -1) {
    fw_log(-1, "[%s] couldn't fork: %s", xe->desc, strerror(errno));
    close(fd[0]);
    close(fd[1]);
    return;
  }

  /* --- Do the child thing --- */

  if (kid == 0) {
    xopts *xo = xe->xo;

    /* --- Fiddle with the file descriptors --- *
     *
     * Attach the other endpoint's descriptors to standard input and output.
     * Attach my pipe to standard error.  Mark everything as blocking and
     * not-to-be-closed-on-exec at this end.
     */

    close(fd[0]);
    if (dup2(in->fd, STDIN_FILENO) < 0 ||
	dup2(out->fd, STDOUT_FILENO) < 0 ||
	dup2(fd[1], STDERR_FILENO) < 0) {
      moan("couldn't manipulate file descriptors: %s", strerror(errno));
      _exit(1);
    }

    if (in->fd > 2)
      close(in->fd);
    if (out->fd > 2)
      close(out->fd);

    fdflags(STDIN_FILENO, O_NONBLOCK, 0, FD_CLOEXEC, 0);
    fdflags(STDOUT_FILENO, O_NONBLOCK, 0, FD_CLOEXEC, 0);
    fdflags(STDERR_FILENO, O_NONBLOCK, 0, FD_CLOEXEC, 0);

    /* --- First of all set the @chroot@ prison --- */

    if (xo->root && chroot(xo->root)) {
      moan("couldn't set `%s' as filesystem root: %s",
	   xo->root, strerror(errno));
      _exit(1);
    }

    /* --- Now set the current directory --- */

    if (xo->dir ? chdir(xo->dir) : xo->root ? chdir("/") : 0) {
      moan("couldn't set `%s' as current directory: %s",
	   xo->dir ? xo->dir : "/", strerror(errno));
      _exit(1);
    }

    /* --- Set the resource limits --- */

#ifdef HAVE_SETRLIMIT
    rlimit_set(&xo->xl);
#endif

    /* --- Set group id --- */

    if (xo->gid != -1) {
      if (setgid(xo->gid)) {
	moan("couldn't set gid %i: %s", xo->gid, strerror(errno));
	_exit(1);
      }
#ifdef HAVE_SETGROUPS
      if (setgroups(1, &xo->gid))
	moan("warning: couldn't set group list to %i: %s", xo->gid,
	     strerror(errno));
#endif
    }

    /* --- Set uid --- */

    if (xo->uid != -1) {
      if (setuid(xo->uid)) {
	moan("couldn't set uid %i: %s", xo->uid, strerror(errno));
	_exit(1);
      }
    }

    /* --- Play with signal dispositions --- */

    signal(SIGPIPE, SIG_DFL);

    /* --- Fiddle with the environment --- */

    xenv_apply(exec_opts.env);
    xenv_apply(xe->xo->env);
    environ = env_export(&env);

    /* --- Run the program --- */

    execvp(xe->xa->file, xe->xa->argv);
    moan("couldn't execute `%s': %s", xe->xa->file, strerror(errno));
    _exit(127);
  }

  /* --- The child's done; see to the parent --- */

  xe->kid = kid;
  selbuf_init(&xe->err, sel, fd[0], xept_error, xe);
  close(fd[1]);
  xe->next = xept_list;
  xe->prev = 0;
  if (xept_list)
    xept_list->prev = xe;
  xept_list = xe;
  if (!(xe->xo->f & XF_NOLOG))
    fw_log(-1, "[%s] started with pid %i", xe->desc, kid);
  fw_inc();
  return;
}

/* --- @xept_file@ --- */

static void xept_file(endpt *e, endpt *f)
{
  xept *xe = (xept *)e;
  xe->f = f;
}

/* --- @xept_close@ --- */

static void xept_close(endpt *e)
{
  xept *xe = (xept *)e;
  if (xe->kid == -1) {
    if (xe->f)
      xe->f->ops->close(xe->f);
    x_tidy(xe->xa, xe->xo);
    DESTROY(xe);
    fw_dec();
  }
}

/* --- @xept_destroy@ --- */

static void xept_destroy(xept *xe)
{
  /* --- First emit the news about the process --- */

  if (xe->xo->f & XF_NOLOG)
    /* Nothin' doin' */;
  else if (WIFEXITED(xe->st)) {
    if (WEXITSTATUS(xe->st) == 0)
      fw_log(-1, "[%s] pid %i exited successfully", xe->desc, xe->kid);
    else {
      fw_log(-1, "[%s] pid %i failed: status %i",
	     xe->desc, xe->kid, WEXITSTATUS(xe->st));
    }
  } else if (WIFSIGNALED(xe->st)) {
    const char *s;
#ifdef HAVE_STRSIGNAL
    s = strsignal(WTERMSIG(xe->st));
#elif HAVE__SYS_SIGLIST
    s = _sys_siglist[WTERMSIG(xe->st)];
#else
    char buf[32];
    sprintf(buf, "signal %i", WTERMSIG(xe->st));
    s = buf;
#endif
    fw_log(-1, "[%s] pid %i failed: %s", xe->desc, xe->kid, s);
  } else
    fw_log(-1, "[%s] pid %i failed: unrecognized status", xe->desc, xe->kid);

  /* --- Free up the parent-side resources --- */

  if (xe->next)
    xe->next->prev = xe->prev;
  if (xe->prev)
    xe->prev->next = xe->next;
  else
    xept_list = xe->next;

  xfree(xe->desc);
  if (xe->f)
    xe->f->ops->close(xe->f);
  x_tidy(xe->xa, xe->xo);
  fw_dec();
  DESTROY(xe);
}

/* --- @xept_chld@ --- *
 *
 * Arguments:	@int n@ = signal number
 *		@void *p@ = uninteresting pointer
 *
 * Returns:	---
 *
 * Use:		Deals with child death situations.
 */

static void xept_chld(int n, void *p)
{
  pid_t kid;
  int st;

  while ((kid = waitpid(-1, &st, WNOHANG)) > 0) {
    xept *xe = xept_list;
    while (xe) {
      xept *xxe = xe;
      xe = xe->next;
      if (kid == xxe->kid) {
	xxe->st = st;
	xxe->e.f |= XEF_EXIT;
	if (xxe->e.f & XEF_CLOSE)
	  xept_destroy(xxe);
	break;
      }
    }
  }
}

/* --- @xept_error@ --- *
 *
 * Arguments:	@char *p@ = pointer to string read from stderr
 *		@size_t len@ = length of the string
 *		@void *v@ = pointer to by endpoint
 *
 * Returns:	---
 *
 * Use:		Handles error reports from a child process.
 */

static void xept_error(char *p, size_t len, void *v)
{
  xept *xe = v;
  if (p)
    fw_log(-1, "[%s] pid %i: %s", xe->desc, xe->kid, p);
  else {
    close(xe->err.reader.fd);
    selbuf_destroy(&xe->err);
    xe->e.f |= XEF_CLOSE;
    if (xe->e.f & XEF_EXIT)
      xept_destroy(xe);
  }
}

/* --- Endpoint operations --- */

static endpt_ops xept_ops = { xept_attach, xept_file, 0, xept_close };

/*----- General operations on sources and targets -------------------------*/

/* --- @exec_init@ --- *
 *
 * Arguments:	---
 *
 * Returns:	---
 *
 * Use:		Initializes the executable problem source and target.
 */

void exec_init(void)
{
  rlimit_get(&exec_opts.xl);
  sig_add(&xept_sig, SIGCHLD, xept_chld, 0);
  sym_create(&env);
  env_import(&env, environ);
}

/* --- @exec_option@ --- */

static int exec_option(xdata *x, scanner *sc)
{
  xopts *xo = x ? x->xo : &exec_opts;

  CONF_BEGIN(sc, "exec", "executable");

  /* --- Logging settings --- */

  if (strcmp(sc->d.buf, "logging") == 0 ||
      strcmp(sc->d.buf, "log") == 0) {
    token(sc);
    if (sc->t == '=')
      token(sc);
    if (conf_enum(sc, "no,yes", ENUM_ABBREV, "logging status"))
      xo->f &= ~XF_NOLOG;
    else
      xo->f |= XF_NOLOG;
    CONF_ACCEPT;
  }

  /* --- Current directory setting --- *
   *
   * Lots of possibilities to guard against possible brainoes.
   */

  if (strcmp(sc->d.buf, "dir") == 0 ||
      strcmp(sc->d.buf, "cd") == 0 ||
      strcmp(sc->d.buf, "chdir") == 0 ||
      strcmp(sc->d.buf, "cwd") == 0) {
    dstr d = DSTR_INIT;
    token(sc);
    if (sc->t == '=')
      token(sc);
    conf_name(sc, '/', &d);
    xo->dir = xstrdup(d.buf);
    dstr_destroy(&d);
    CONF_ACCEPT;
  }

  /* --- Set a chroot prison --- */

  if (strcmp(sc->d.buf, "root") == 0 ||
      strcmp(sc->d.buf, "chroot") == 0) { 
    dstr d = DSTR_INIT;
    token(sc);
    if (sc->t == '=')
      token(sc);
    conf_name(sc, '/', &d);
    xo->root = xstrdup(d.buf);
    dstr_destroy(&d);
    CONF_ACCEPT;
  }

  /* --- Set the target user id --- */

  if (strcmp(sc->d.buf, "uid") == 0 ||
      strcmp(sc->d.buf, "user") == 0) {
    token(sc);
    if (sc->t == '=')
      token(sc);
    if (sc->t != CTOK_WORD)
      error(sc, "parse error, expected user name or uid");
    if (isdigit((unsigned char)*sc->d.buf))
      xo->uid = atoi(sc->d.buf);
    else {
      struct passwd *pw = getpwnam(sc->d.buf);
      if (!pw)
	error(sc, "unknown user name `%s'", sc->d.buf);
      xo->uid = pw->pw_uid;
    }
    token(sc);
    CONF_ACCEPT;
  }

  /* --- Set the target group id --- */

  if (strcmp(sc->d.buf, "gid") == 0 ||
      strcmp(sc->d.buf, "group") == 0) {
    token(sc);
    if (sc->t == '=')
      token(sc);
    if (sc->t != CTOK_WORD)
      error(sc, "parse error, expected group name or gid");
    if (isdigit((unsigned char)*sc->d.buf))
      xo->gid = atoi(sc->d.buf);
    else {
      struct group *gr = getgrnam(sc->d.buf);
      if (!gr)
	error(sc, "unknown user name `%s'", sc->d.buf);
      xo->gid = gr->gr_gid;
    }
    token(sc);
    CONF_ACCEPT;
  }

  /* --- Now try resource limit settings --- */

  if (rlimit_option(&xo->xl, sc))
    CONF_ACCEPT;

  /* --- And then environment settings --- */

  if (xenv_option(xo, sc))
    CONF_ACCEPT;

  /* --- Nothing found --- */

  CONF_END;
}

/* --- @exec_desc@ --- */

static void exec_desc(xdata *x, dstr *d)
{
  char **p;
  char sep = '[';
  dstr_puts(d, "exec ");
  if (strcmp(x->xa->file, x->xa->argv[0]) != 0) {
    dstr_puts(d, x->xa->file);
    dstr_putc(d, ' ');
  }
  for (p = x->xa->argv; *p; p++) {
    dstr_putc(d, sep);
    dstr_puts(d, *p);
    sep = ' ';
  }
  dstr_putc(d, ']');
  dstr_putz(d);
}

/* --- @exec_read@ --- */

static void exec_read(xdata *x, scanner *sc)
{
  size_t base = 0;
  dstr d = DSTR_INIT;
  xargs *xa;

  /* --- Read the first word --- *
   *
   * This is either a shell command or the actual program to run.
   */

  if (sc->t == CTOK_WORD) {
    dstr_putd(&d, &sc->d); d.len++;
    base = d.len;
    token(sc);
  }

  /* --- See if there's a list of arguments --- *
   *
   * If not, then the thing I saw was a shell command, so build the proper
   * arguments for that.
   */

  if (sc->t != '[') {
    char *p;
    if (!base)
      error(sc, "parse error, expected shell command or argument list");
    xa = xmalloc(XARGS_SZ(3) + 8 + 3 + d.len);
    p = (char *)(xa->argv + 4);
    xa->ref = 1;
    xa->file = p;
    xa->argv[0] = p; memcpy(p, "/bin/sh", 8); p += 8;
    xa->argv[1] = p; memcpy(p, "-c", 3); p += 3;
    xa->argv[2] = p; memcpy(p, d.buf, d.len); p += d.len;
    xa->argv[3] = 0;
  }

  /* --- Snarf in a list of arguments --- */

  else {
    int argc = 0;
    char *p, *q;
    char **v;

    /* --- Strip off the leading `[' --- */

    token(sc);

    /* --- Read a sequence of arguments --- */

    while (sc->t == CTOK_WORD) {
      dstr_putd(&d, &sc->d); d.len++;
      token(sc);
      argc++;
    }

    /* --- Expect the closing `]' --- */

    if (sc->t != ']')
      error(sc, "parse error, missing `]'");
    token(sc);

    /* --- If there are no arguments, whinge --- */

    if (!argc)
      error(sc, "must specify at least one argument");

    /* --- Allocate a lump of memory for the array --- */

    xa = xmalloc(XARGS_SZ(argc) + d.len);
    xa->ref = 1;
    v = xa->argv;
    p = (char *)(v + argc + 1);
    memcpy(p, d.buf, d.len);
    q = p + d.len;
    xa->file = p;
    p += base;

    /* --- Start dumping addresses into the @argv@ array --- */

    for (;;) {
      *v++ = p;
      while (*p++ && p < q)
	;
      if (p >= q)
	break;
    }
    *v++ = 0;
  }

  /* --- Do some other setting up --- */

  dstr_destroy(&d);
  x->xa = xa;
  x->xo = CREATE(xopts);
  *x->xo = exec_opts;
  x->xo->ref = 1;
  return;
}

/* --- @exec_endpt@ --- */

static endpt *exec_endpt(xdata *x, const char *desc)
{
  xept *xe = CREATE(xept);
  xe->e.ops = &xept_ops;
  xe->e.other = 0;
  xe->e.t = 0;
  xe->e.f = 0;
  xe->xa = x->xa; xe->xa->ref++;
  xe->xo = x->xo; xe->xo->ref++;
  xe->kid = -1;
  xe->f = 0;
  xe->desc = xstrdup(desc);
  return (&xe->e);
}

/* --- @exec_destroy@ --- */

static void exec_destroy(xdata *x)
{
  x_tidy(x->xa, x->xo);
}

/*----- Source definition -------------------------------------------------*/

/* --- @option@ --- */

static int xsource_option(source *s, scanner *sc)
{
  xsource *xs = (xsource *)s;
  return (exec_option(xs ? &xs->x : 0, sc));
}

/* --- @read@ --- */

static source *xsource_read(scanner *sc)
{
  xsource *xs;

  if (!conf_prefix(sc, "exec"))
    return (0);
  xs = CREATE(xsource);
  xs->s.ops = &xsource_ops;
  xs->s.desc = 0;
  exec_read(&xs->x, sc);
  return (&xs->s);
}

/* --- @attach@ --- */

static void xsource_destroy(source */*s*/);

static void xsource_attach(source *s, scanner *sc, target *t)
{
  xsource *xs = (xsource *)s;
  endpt *e, *ee;

  /* --- Set up the source description string --- */

  {
    dstr d = DSTR_INIT;
    exec_desc(&xs->x, &d);
    dstr_puts(&d, " -> ");
    dstr_puts(&d, t->desc);
    xs->s.desc = xstrdup(d.buf);
    dstr_destroy(&d);
  }

  /* --- Create the endpoints --- */

  if ((ee = t->ops->create(t, xs->s.desc)) == 0)
    goto tidy;
  if ((e = exec_endpt(&xs->x, xs->s.desc)) == 0) {
    ee->ops->close(ee);
    goto tidy;
  }
  endpt_join(e, ee);

  /* --- Dispose of source and target --- */

tidy:
  t->ops->destroy(t);
  xsource_destroy(&xs->s);
}

/* --- @destroy@ --- */

static void xsource_destroy(source *s)
{
  xsource *xs = (xsource *)s;
  xfree(xs->s.desc);
  exec_destroy(&xs->x);
  DESTROY(xs);
}

/* --- Executable source operations --- */

source_ops xsource_ops = {
  "exec",
  xsource_option, xsource_read, xsource_attach, xsource_destroy
};

/*----- Exec target description -------------------------------------------*/

/* --- @option@ --- */

static int xtarget_option(target *t, scanner *sc)
{
  xtarget *xt = (xtarget *)t;
  return (exec_option(xt ? &xt->x : 0, sc));
}

/* --- @read@ --- */

static target *xtarget_read(scanner *sc)
{
  xtarget *xt;
  dstr d = DSTR_INIT;

  if (!conf_prefix(sc, "exec"))
    return (0);
  xt = CREATE(xtarget);
  xt->t.ops = &xtarget_ops;
  exec_read(&xt->x, sc);
  exec_desc(&xt->x, &d);
  xt->t.desc = xstrdup(d.buf);
  dstr_destroy(&d);
  return (&xt->t);
}

/* --- @create@ --- */

static endpt *xtarget_create(target *t, const char *desc)
{
  xtarget *xt = (xtarget *)t;
  endpt *e = exec_endpt(&xt->x, desc);
  return (e);
}

/* --- @destroy@ --- */

static void xtarget_destroy(target *t)
{
  xtarget *xt = (xtarget *)t;
  xfree(xt->t.desc);
  exec_destroy(&xt->x);
  DESTROY(xt);
}

/* --- Exec target operations --- */

target_ops xtarget_ops = {
  "exec",
  xtarget_option, xtarget_read, xtarget_create, xtarget_destroy
};

/*----- That's all, folks -------------------------------------------------*/