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chroot_canon.c

/* Return the canonical absolute name of a given file inside chroot.
   Copyright (C) 1996,1997,1998,1999,2000,2001,2004,2005
      Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License version 2 as
   published by the Free Software Foundation.

   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., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */

#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <limits.h>
#include <sys/param.h>
#include <sys/stat.h>
#include <errno.h>
#include <stddef.h>
#include <stdint.h>

#include <ldconfig.h>

#ifndef PATH_MAX
#define PATH_MAX 1024
#endif

/* Return the canonical absolute name of file NAME as if chroot(CHROOT) was
   done first.  A canonical name does not contain any `.', `..' components
   nor any repeated path separators ('/') or symlinks.  All path components
   must exist and NAME must be absolute filename.  The result is malloc'd.
   The returned name includes the CHROOT prefix.  */

char *
chroot_canon (const char *chroot, const char *name)
{
  char *rpath;
  char *dest;
  char *extra_buf = NULL;
  char *rpath_root;
  const char *start;
  const char *end;
  const char *rpath_limit;
  int num_links = 0;
  size_t chroot_len = strlen (chroot);

  if (chroot_len < 1)
    {
      __set_errno (EINVAL);
      return NULL;
    }

  rpath = malloc (chroot_len + PATH_MAX);
  if (rpath == NULL)
    return NULL;

  rpath_limit = rpath + chroot_len + PATH_MAX;

  rpath_root = (char *) mempcpy (rpath, chroot, chroot_len) - 1;
  if (*rpath_root != '/')
    *++rpath_root = '/';
  dest = rpath_root + 1;

  for (start = end = name; *start; start = end)
    {
      struct stat64 st;
      int n;

      /* Skip sequence of multiple path-separators.  */
      while (*start == '/')
      ++start;

      /* Find end of path component.  */
      for (end = start; *end && *end != '/'; ++end)
      /* Nothing.  */;

      if (end - start == 0)
      break;
      else if (end - start == 1 && start[0] == '.')
      /* nothing */;
      else if (end - start == 2 && start[0] == '.' && start[1] == '.')
      {
        /* Back up to previous component, ignore if at root already.  */
        if (dest > rpath_root + 1)
          while ((--dest)[-1] != '/');
      }
      else
      {
        size_t new_size;

        if (dest[-1] != '/')
          *dest++ = '/';

        if (dest + (end - start) >= rpath_limit)
          {
            ptrdiff_t dest_offset = dest - rpath;
            char *new_rpath;

            new_size = rpath_limit - rpath;
            if (end - start + 1 > PATH_MAX)
            new_size += end - start + 1;
            else
            new_size += PATH_MAX;
            new_rpath = (char *) realloc (rpath, new_size);
            if (new_rpath == NULL)
            goto error;
            rpath = new_rpath;
            rpath_limit = rpath + new_size;

            dest = rpath + dest_offset;
          }

        dest = mempcpy (dest, start, end - start);
        *dest = '\0';

        if (lstat64 (rpath, &st) < 0)
          {
            if (*end == '\0')
            goto done;
            goto error;
          }

        if (S_ISLNK (st.st_mode))
          {
            char *buf = alloca (PATH_MAX);
            size_t len;

            if (++num_links > MAXSYMLINKS)
            {
              __set_errno (ELOOP);
              goto error;
            }

            n = readlink (rpath, buf, PATH_MAX);
            if (n < 0)
            {
              if (*end == '\0')
                goto done;
              goto error;
            }
            buf[n] = '\0';

            if (!extra_buf)
            extra_buf = alloca (PATH_MAX);

            len = strlen (end);
            if ((long int) (n + len) >= PATH_MAX)
            {
              __set_errno (ENAMETOOLONG);
              goto error;
            }

            /* Careful here, end may be a pointer into extra_buf... */
            memmove (&extra_buf[n], end, len + 1);
            name = end = memcpy (extra_buf, buf, n);

            if (buf[0] == '/')
            dest = rpath_root + 1;  /* It's an absolute symlink */
            else
            /* Back up to previous component, ignore if at root already: */
            if (dest > rpath_root + 1)
              while ((--dest)[-1] != '/');
          }
      }
    }
 done:
  if (dest > rpath_root + 1 && dest[-1] == '/')
    --dest;
  *dest = '\0';

  return rpath;

 error:
  free (rpath);
  return NULL;
}

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