/*
 * Copyright (C) 1996-8 Michael R. Elkins <me@cs.hmc.edu>
 * Copyright (C) 1998 Thomas Roessler <roessler@guug.de>
 * 
 *     This program 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.
 * 
 *     This program 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 this program; if not, write to the Free Software
 *     Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */ 

/*
 * This module either be compiled into Mutt, or it can be
 * built as a separate program. For building it
 * separately, define the DL_STANDALONE preprocessor
 * macro.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <unistd.h>
#include <dirent.h>
#include <sys/file.h>
#include <sys/stat.h>
#include <sys/utsname.h>
#include <errno.h>
#include <time.h>
#include <fcntl.h>
#include <limits.h>

#ifndef _POSIX_PATH_MAX
#include <posix1_lim.h>
#endif

#include "dotlock.h"
#include "config.h"

#ifdef HAVE_GETOPT_H
#include <getopt.h>
#endif

#ifdef DL_STANDALONE
#include "reldate.h"
#endif

# define MAXLINKS 1024 /* maximum link depth */

# ifdef DL_STANDALONE

# define LONG_STRING 1024
# define MAXLOCKATTEMPT 5

# define strfcpy(A,B,C) strncpy(A,B,C), *(A+(C)-1)=0

# ifdef USE_SETGID

#  ifdef HAVE_SETEGID
#   define SETEGID setegid
#  else
#   define SETEGID setgid
#  endif
#  ifndef S_ISLNK
#   define S_ISLNK(x) (((x) & S_IFMT) == S_IFLNK ? 1 : 0)
#  endif

# endif

# ifndef HAVE_SNPRINTF
extern int snprintf(char *, size_t, const char *, ...);
# endif

#else  /* DL_STANDALONE */

# ifdef USE_SETGID
#  error Do not try to compile dotlock as a mutt module when requiring egid switching!
# endif

# include "mutt.h"
# include "mx.h"

#endif /* DL_STANDALONE */

static short f_try = 0;
static short f_force = 0;
static short f_unlock = 0;
static short f_priv = 0;
static int   Retry = MAXLOCKATTEMPT;

#ifdef DL_STANDALONE
static char *Hostname;
#endif

#ifdef USE_SETGID
static gid_t UserGid;
static gid_t MailGid;
#endif

static int dotlock_deference_symlink(char *, size_t, const char *);
static int dotlock_prepare(char *, size_t, const char *);

#ifdef DL_STANDALONE
static int dotlock_init_privs(void);
static void usage(const char *);
#endif

static void dotlock_expand_link(char *, const char *, const char *);
static void BEGIN_PRIVILEGED(void);
static void END_PRIVILEGED(void);

/* These functions work on the current directory.
 * Invoke dotlock_prepare() before and check their
 * return value.
 */

static int dotlock_try(void);
static int dotlock_unlock(const char *);
static int dotlock_lock(const char *);


#ifdef DL_STANDALONE

int main(int argc, char **argv)
{
  int i;
  char *p;
  const char *f;
  struct utsname utsname;
  char realpath[_POSIX_PATH_MAX];


  /* first, drop privileges */
  
  if(dotlock_init_privs() == -1)
    return DL_EX_ERROR;


  /* determine the system's host name */
  
  uname(&utsname);
  if(!(Hostname = strdup(utsname.nodename)))
    return DL_EX_ERROR;
  if((p = strchr(Hostname, '.')))
    *p = '\0';


  /* parse the command line options. */
  
  while((i = getopt(argc, argv, "tfupr:")) != EOF)
  {
    switch(i)
    {
      case 't': f_try = 1; break;
      case 'f': f_force = 1; break;
      case 'u': f_unlock = 1; break;
      case 'p': f_priv = 1; break;
      case 'r': Retry = atoi(optarg); break;
      default: usage(argv[0]);
    }
  }

  if(optind == argc || f_try + f_force + f_unlock > 1 || Retry < 0)
    usage(argv[0]);
  
  f = argv[optind];


  /* If dotlock_prepare() succeeds [return value == 0],
   * realpath contains the basename of f, and we have
   * successfully changed our working directory to
   * `dirname $f`.  Additionally, f has been opened for
   * reading to verify that the user has at least read
   * permissions on that file.
   * 
   * For a more detailed explanation of all this, see the
   * lengthy comment below.
   */
  
  if(dotlock_prepare(realpath, sizeof(realpath), f) != 0)
    return DL_EX_ERROR;

  /* Finally, actually perform the locking operation. */
  
  if(f_try)
    return dotlock_try();
  else if(f_unlock)
    return dotlock_unlock(realpath);
  else /* lock */
    return dotlock_lock(realpath);

}

/* 
 * Determine our effective group ID, and drop 
 * privileges.
 * 
 * Return value:
 * 
 *  0 - everything went fine
 * -1 - we couldn't drop privileges.
 * 
 */


static int
dotlock_init_privs(void)
{

# ifdef USE_SETGID
  
  UserGid = getgid();
  MailGid = getegid();

  if(SETEGID(UserGid) != 0)
    return -1;

# endif

  return 0;
}
  

#else  /* DL_STANDALONE */

/* 
 * This function is intended to be invoked from within
 * mutt instead of mx.c's invoke_dotlock().
 * 
 */

int dotlock_invoke(const char *path, int flags, int retry)
{
  int currdir;
  int r;
  char realpath[_POSIX_PATH_MAX];
  
  if((currdir = open(".", O_RDONLY)) == -1)
    return DL_EX_ERROR;
  
  if(!(flags & DL_FL_RETRY) || retry)
    Retry = MAXLOCKATTEMPT;
  else
    Retry = 0;
  
  f_priv = f_try = f_unlock = f_force = 0;
  
  if(flags & DL_FL_FORCE)
    f_force = 1;

  r = DL_EX_ERROR;
  if(dotlock_prepare(realpath, sizeof(realpath), path) == -1)
    goto bail;
  
  if(flags & DL_FL_TRY)
    r = dotlock_try();
  else if(flags & DL_FL_UNLOCK)
    r = dotlock_unlock(realpath);
  else /* lock */
    r = dotlock_lock(realpath);
  
 bail:
  
  fchdir(currdir);
  close(currdir);
  
  return r;
}
    
#endif  /* DL_STANDALONE */
  
  
/*
 * Get privileges 
 * 
 * This function re-acquires the privileges we may have
 * if the user told us to do so by giving the "-p"
 * command line option.
 * 
 * BEGIN_PRIVILEGES() won't return if an error occurs.
 * 
 */

static void
BEGIN_PRIVILEGED(void)
{
#ifdef USE_SETGID
  if(f_priv)
  {
    if(SETEGID(MailGid) != 0)
    {
      /* perror("setegid"); */
      exit(DL_EX_ERROR);
    }
  }
#endif
}

/*
 * Drop privileges
 * 
 * This function drops the group privileges we may have.
 * 
 * END_PRIVILEGED() won't return if an error occurs.
 *
 */

static void
END_PRIVILEGED(void)
{
#ifdef USE_SETGID
  if(f_priv)
  {
    if(SETEGID(UserGid) != 0)
    {
      /* perror("setegid"); */
      exit(DL_EX_ERROR);
    }
  }
#endif
}

#ifdef DL_STANDALONE

/*
 * Usage information.
 * 
 * This function doesn't return.
 * 
 */

static void 
usage(const char *av0)
{
  fprintf(stderr, "dotlock [Mutt %s (%s)]\n", VERSION, ReleaseDate);
  fprintf(stderr, "usage: %s [-t|-f|-u] [-p] [-r <retries>] file\n",
	  av0);

  fputs("\noptions:"
	"\n  -t\t\ttry"
	"\n  -f\t\tforce"
	"\n  -u\t\tunlock"
	"\n  -p\t\tprivileged"
#ifndef USE_SETGID
	" (ignored)"
#endif
	"\n  -r <retries>\tRetry locking"
	"\n", stderr);
  
  exit(DL_EX_ERROR);
}

#endif

/*
 * Access checking: Let's avoid to lock other users' mail
 * spool files if we aren't permitted to read them.
 * 
 * Some simple-minded access(2) checking isn't sufficient
 * here: The problem is that the user may give us a
 * deeply nested path to a file which has the same name
 * as the file he wants to lock, but different
 * permissions, say, e.g.
 * /tmp/lots/of/subdirs/var/spool/mail/root.
 * 
 * He may then try to replace /tmp/lots/of/subdirs by a
 * symbolic link to / after we have invoked access() to
 * check the file's permissions.  The lockfile we'd
 * create or remove would then actually be
 * /var/spool/mail/root.
 * 
 * To avoid this attack, we proceed as follows:
 * 
 * - First, follow symbolic links a la
 *   dotlock_deference_symlink().
 * 
 * - get the result's dirname.
 * 
 * - chdir to this directory.  If you can't, bail out.
 * 
 * - try to open the file in question, only using its
 *   basename.  If you can't, bail out.
 * 
 * - fstat that file and compare the result to a
 *   subsequent lstat (only using the basename).  If
 *   the comparison fails, bail out.
 * 
 * dotlock_prepare() is invoked from main() directly
 * after the command line parsing has been done.
 *
 * Return values:
 * 
 * 0 - Evereything's fine.  The program's new current
 *     directory is the contains the file to be locked.
 *     The string pointed to by bn contains the name of
 *     the file to be locked.
 * 
 * -1 - Something failed. Don't continue.
 * 
 * tlr, Jul 15 1998
 */

static int
dotlock_prepare(char *bn, size_t l, const char *f)
{
  struct stat fsb, lsb;
  char realpath[_POSIX_PATH_MAX];
  char *basename, *dirname;
  char *p;
  int fd;
  int r;
  
  if(dotlock_deference_symlink(realpath, sizeof(realpath), f) == -1)
    return -1;
  
  if((p = strrchr(realpath, '/')))
  {
    *p = '\0';
    basename = p + 1;
    dirname = realpath;
  }
  else
  {
    basename = realpath;
    dirname = ".";
  }

  if(strlen(basename) + 1 > l)
    return -1;
  
  strfcpy(bn, basename, l);
  
  if(chdir(dirname) == -1)
    return -1;
  
  if((fd = open(basename, O_RDONLY)) == -1)
    return -1;
  
  r = fstat(fd, &fsb);
  close(fd);
  
  if(r == -1)
    return -1;
  
  if(lstat(basename, &lsb) == -1)
    return -1;
  
  if(S_ISLNK(lsb.st_mode))
    return -1;
  
  if((lsb.st_dev != fsb.st_dev) ||
     (lsb.st_ino != fsb.st_ino) ||
     (lsb.st_mode != fsb.st_mode) ||
     (lsb.st_nlink != fsb.st_nlink) ||
     (lsb.st_uid != fsb.st_uid) ||
     (lsb.st_gid != fsb.st_gid) ||
     (lsb.st_rdev != fsb.st_rdev) ||
     (lsb.st_size != fsb.st_size))
  {
    /* something's fishy */
    return -1;
  }
  
  return 0;
}

/*
 * Expand a symbolic link.
 * 
 * This function expects newpath to have space for
 * at least _POSIX_PATH_MAX characters.
 *
 */

static void 
dotlock_expand_link (char *newpath, const char *path, const char *link)
{
  const char *lb = NULL;
  size_t len;

  /* link is full path */
  if (*link == '/')
  {
    strfcpy (newpath, link, _POSIX_PATH_MAX);
    return;
  }

  if ((lb = strrchr (path, '/')) == NULL)
  {
    /* no path in link */
    strfcpy (newpath, link, _POSIX_PATH_MAX);
    return;
  }

  len = lb - path + 1;
  memcpy (newpath, path, len);
  strfcpy (newpath + len, link, _POSIX_PATH_MAX - len);
}


/*
 * Deference a chain of symbolic links
 * 
 * The final path is written to d.
 *
 */

static int
dotlock_deference_symlink(char *d, size_t l, const char *path)
{
  struct stat sb;
  char realpath[_POSIX_PATH_MAX];
  const char *pathptr = path;
  int count = 0;
  
  while(count++ < MAXLINKS)
  {
    if(lstat(pathptr, &sb) == -1)
    {
      /* perror(pathptr); */
      return -1;
    }
    
    if(S_ISLNK (sb.st_mode))
    {
      char linkfile[_POSIX_PATH_MAX];
      char linkpath[_POSIX_PATH_MAX];
      int len;

      if((len = readlink(pathptr, linkfile, sizeof(linkfile))) == -1)
      {
	/* perror(pathptr); */
	return -1;
      }
      
      linkfile[len] = '\0';
      dotlock_expand_link(linkpath, pathptr, linkfile);
      strfcpy(realpath, linkpath, sizeof(realpath));
      pathptr = realpath;
    }
    else
      break;
  }

  strfcpy(d, pathptr, l);
  return 0;
}

/*
 * Dotlock a file.
 * 
 * realpath is assumed _not_ to be an absolute path to
 * the file we are about to lock.  Invoke
 * dotlock_prepare() before using this function!
 * 
 */

static int
dotlock_lock(const char *realpath)
{
  char lockfile[_POSIX_PATH_MAX + LONG_STRING];
  char nfslockfile[_POSIX_PATH_MAX + LONG_STRING];
  size_t prev_size = 0;
  int fd;
  int count = 0;
  struct stat sb;
  time_t t;
  
  snprintf(nfslockfile, sizeof(nfslockfile), "%s.%s.%d",
	   realpath, Hostname, (int) getpid());
  snprintf(lockfile, sizeof(lockfile), "%s.lock", realpath);

  
  BEGIN_PRIVILEGED();

  unlink(nfslockfile);

  while ((fd = open (nfslockfile, O_WRONLY | O_EXCL | O_CREAT, 0)) < 0)
  {
    END_PRIVILEGED();

  
    if (errno != EAGAIN)
    {
      /* perror ("cannot open NFS lock file"); */
      return DL_EX_ERROR;
    }

    
    BEGIN_PRIVILEGED();
  }

  END_PRIVILEGED();

  
  close(fd);
  
  while(1)
  {

    BEGIN_PRIVILEGED();
    link(nfslockfile, lockfile);
    END_PRIVILEGED();

    if(stat(nfslockfile, &sb) != 0)
    {
      /* perror("stat"); */
      return DL_EX_ERROR;
    }
    
    if(sb.st_nlink == 2)
      break;
    
    if(count == 0)
      prev_size = sb.st_size;
    
    if(prev_size == sb.st_size && ++count > Retry)
    {
      if(f_force)
      {
	BEGIN_PRIVILEGED();
	unlink(lockfile);
	END_PRIVILEGED();

	count = 0;
	continue;
      }
      else
      {
	BEGIN_PRIVILEGED();
	unlink(nfslockfile);
	END_PRIVILEGED();
	return DL_EX_EXIST;
      }
    }
    
    prev_size = sb.st_size;
    
    /* don't trust sleep(3) as it may be interrupted
     * by users sending signals. 
     */
    
    t = time(NULL);
    do {
      sleep(1);
    } while (time(NULL) == t);
  }
  
  BEGIN_PRIVILEGED();
  unlink(nfslockfile);
  END_PRIVILEGED();

  return DL_EX_OK;
}


/*
 * Unlock a file. 
 * 
 * The same comment as for dotlock_lock() applies here.
 * 
 */

static int
dotlock_unlock(const char *realpath)
{
  char lockfile[_POSIX_PATH_MAX + LONG_STRING];
  int i;

  snprintf(lockfile, sizeof(lockfile), "%s.lock",
	   realpath);
  
  BEGIN_PRIVILEGED();
  i = unlink(lockfile);
  END_PRIVILEGED();
  
  if(i == -1)
    return DL_EX_ERROR;
  
  return DL_EX_OK;
}


/*
 * Check if a file can be locked at all.
 * 
 * The same comment as for dotlock_lock() applies here.
 * 
 */

static int
dotlock_try(void)
{
#ifdef USE_SETGID
  struct stat sb;
#endif

  if(access(".", W_OK) == 0)
    return DL_EX_OK;

#ifdef USE_SETGID
  if(stat(".", &sb) == 0)
  {
    if((sb.st_mode & S_IWGRP) == S_IWGRP && sb.st_gid == MailGid)
      return DL_EX_NEED_PRIVS;
  }
#endif

  return DL_EX_IMPOSSIBLE;
}