dotlock.c

/**
 * Copyright (C) 1996-2000 Michael R. Elkins <me@mutt.org>
 * Copyright (C) 1998-2001,2007 Thomas Roessler <roessler@does-not-exist.org>
 *
 * 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, see <http://www.gnu.org/licenses/>.
 */

/*
 * 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 "config.h"
#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/file.h>
#include <sys/stat.h>
#include <sys/utsname.h>
#include <time.h>
#include <unistd.h>
#include "dotlock.h"
#ifndef _POSIX_PATH_MAX
#include <limits.h>
#endif
#ifdef DL_STANDALONE
#include "git_ver.h"
#endif

#define MAXLINKS 1024 /* maximum link depth */

#ifdef DL_STANDALONE

#define LONG_STRING 1024
#define MAXLOCKATTEMPT 5

#define strfcpy(DST, SRC, LEN)                                                 \
  do                                                                           \
  {                                                                            \
    if ((LEN) > 0)                                                             \
    {                                                                          \
      *(DST + (LEN) -1) = 0;                                                   \
      strncpy(DST, SRC, (LEN) -1);                                             \
    }                                                                          \
  } while (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

#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 int DotlockFlags;
static int Retry = MAXLOCKATTEMPT;

#ifdef DL_STANDALONE
struct utsname utsname;
#endif

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

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

#ifdef DL_STANDALONE
/*
 * 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;
}
#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 (DotlockFlags & DL_FL_USEPRIV)
  {
    if (SETEGID(MailGid) != 0)
    {
      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 (DotlockFlags & DL_FL_USEPRIV)
  {
    if (SETEGID(UserGid) != 0)
    {
      exit(DL_EX_ERROR);
    }
  }
#endif
}

#ifdef DL_STANDALONE
/*
 * Usage information.
 *
 * This function doesn't return.
 *
 */
static void usage(const char *av0)
{
  fprintf(stderr, "dotlock [NeoMutt %s%s (%s)]\n", PACKAGE_VERSION, GitVer, MUTT_VERSION);
  fprintf(stderr, "usage: %s [-t|-f|-u|-d] [-p] [-r <retries>] file\n", av0);

  fputs("\noptions:"
        "\n  -t\t\ttry"
        "\n  -f\t\tforce"
        "\n  -u\t\tunlock"
        "\n  -d\t\tunlink"
        "\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 - Everything'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_check_stats(struct stat *fsb, struct stat *lsb)
{
  /* S_ISLNK (fsb->st_mode) should actually be impossible,
   * but we may have mixed up the parameters somewhere.
   * play safe.
   */

  if (S_ISLNK(lsb->st_mode) || S_ISLNK(fsb->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);
}

/*
 * 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;
}

/*
 * 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)
    {
      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)) == -1)
      {
        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;
}

static int dotlock_prepare(char *bn, size_t l, const char *f, int _fd)
{
  struct stat fsb, lsb;
  char realpath[_POSIX_PATH_MAX];
  char *basename = NULL, *dirname = NULL;
  char *p = NULL;
  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 != -1)
    fd = _fd;
  else if ((fd = open(basename, O_RDONLY)) == -1)
    return -1;

  r = fstat(fd, &fsb);

  if (_fd == -1)
    close(fd);

  if (r == -1)
    return -1;

  if (lstat(basename, &lsb) == -1)
    return -1;

  if (dotlock_check_stats(&fsb, &lsb) == -1)
    return -1;

  return 0;
}

#define HARDMAXATTEMPTS 10

/*
 * 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;
  int hard_count = 0;
  struct stat sb;
  time_t t;

  snprintf(nfslockfile, sizeof(nfslockfile), "%s.%s.%d", realpath,
#ifdef DL_STANDALONE
           utsname.nodename,
#else
           Hostname,
#endif
           (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)
    {
      return DL_EX_ERROR;
    }

    BEGIN_PRIVILEGED();
  }

  END_PRIVILEGED();

  close(fd);

  while (hard_count++ < HARDMAXATTEMPTS)
  {
    BEGIN_PRIVILEGED();
    link(nfslockfile, lockfile);
    END_PRIVILEGED();

    if (stat(nfslockfile, &sb) != 0)
    {
      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 (DotlockFlags & DL_FL_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;
}

/* remove an empty file */

static int dotlock_unlink(const char *realpath)
{
  struct stat lsb;
  int i = -1;

  if (dotlock_lock(realpath) != DL_EX_OK)
    return DL_EX_ERROR;

  if ((i = lstat(realpath, &lsb)) == 0 && lsb.st_size == 0)
    unlink(realpath);

  dotlock_unlock(realpath);

  return (i == 0) ? DL_EX_OK : DL_EX_ERROR;
}

static int dotlock_dispatch(const char *f, int fd)
{
  char realpath[_POSIX_PATH_MAX];

  /* 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, fd) != 0)
    return DL_EX_ERROR;

  /* Actually perform the locking operation. */

  if (DotlockFlags & DL_FL_TRY)
    return dotlock_try();
  else if (DotlockFlags & DL_FL_UNLOCK)
    return dotlock_unlock(realpath);
  else if (DotlockFlags & DL_FL_UNLINK)
    return dotlock_unlink(realpath);
  else /* lock */
    return dotlock_lock(realpath);
}

#ifndef 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 fd, int flags, int retry)
{
  int currdir;
  int r;

  DotlockFlags = flags;

  if ((currdir = open(".", O_RDONLY)) == -1)
    return DL_EX_ERROR;

  if (!(DotlockFlags & DL_FL_RETRY) || retry)
    Retry = MAXLOCKATTEMPT;
  else
    Retry = 0;

  r = dotlock_dispatch(path, fd);

  fchdir(currdir);
  close(currdir);

  return r;
}
#endif /* !DL_STANDALONE */


#ifdef DL_STANDALONE

#define check_flags(a)                                                         \
  if (a & DL_FL_ACTIONS)                                                       \
  usage(argv[0])

int main(int argc, char **argv)
{
  int i;
  char *p = NULL;

  /* first, drop privileges */

  if (dotlock_init_privs() == -1)
    return DL_EX_ERROR;

  /* determine the system's host name */

  uname(&utsname);
  if ((p = strchr(utsname.nodename, '.')))
    *p = '\0';

  /* parse the command line options. */
  DotlockFlags = 0;

  while ((i = getopt(argc, argv, "dtfupr:")) != EOF)
  {
    switch (i)
    {
      /* actions, mutually exclusive */
      case 't':
        check_flags(DotlockFlags);
        DotlockFlags |= DL_FL_TRY;
        break;
      case 'd':
        check_flags(DotlockFlags);
        DotlockFlags |= DL_FL_UNLINK;
        break;
      case 'u':
        check_flags(DotlockFlags);
        DotlockFlags |= DL_FL_UNLOCK;
        break;

      /* other flags */
      case 'f':
        DotlockFlags |= DL_FL_FORCE;
        break;
      case 'p':
        DotlockFlags |= DL_FL_USEPRIV;
        break;
      case 'r':
        DotlockFlags |= DL_FL_RETRY;
        Retry = atoi(optarg);
        break;

      default:
        usage(argv[0]);
    }
  }

  if (optind == argc || Retry < 0)
    usage(argv[0]);

  return dotlock_dispatch(argv[optind], -1);
}
#endif