path: root/lib/getcwd.c

/* Copyright (C) 1991-2023 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   This file 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 3 of the License,
   or (at your option) any later version.

   This file 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 <https://www.gnu.org/licenses/>.  */

#if !_LIBC
# include <config.h>
# include <stdio.h>
# include <unistd.h>
# include "pathmax.h"
#else
# define HAVE_OPENAT 1
# define D_INO_IN_DIRENT 1
# define HAVE_MSVC_INVALID_PARAMETER_HANDLER 0
# define HAVE_MINIMALLY_WORKING_GETCWD 0
#endif

#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stddef.h>

#include <fcntl.h> /* For AT_FDCWD on Solaris 9.  */

/* If this host provides the openat function or if we're using the
   gnulib replacement function with a native fdopendir, then enable
   code below to make getcwd more efficient and robust.  */
#if defined HAVE_OPENAT || (defined GNULIB_OPENAT && defined HAVE_FDOPENDIR)
# define HAVE_OPENAT_SUPPORT 1
#else
# define HAVE_OPENAT_SUPPORT 0
#endif

#ifndef __set_errno
# define __set_errno(val) (errno = (val))
#endif

#include <dirent.h>
#ifndef _D_EXACT_NAMLEN
# define _D_EXACT_NAMLEN(d) strlen ((d)->d_name)
#endif
#ifndef _D_ALLOC_NAMLEN
# define _D_ALLOC_NAMLEN(d) (_D_EXACT_NAMLEN (d) + 1)
#endif

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

#if _LIBC
# ifndef mempcpy
#  define mempcpy __mempcpy
# endif
#endif

#ifndef MAX
# define MAX(a, b) ((a) < (b) ? (b) : (a))
#endif
#ifndef MIN
# define MIN(a, b) ((a) < (b) ? (a) : (b))
#endif

/* In this file, PATH_MAX only serves as a threshold for choosing among two
   algorithms.  */
#ifndef PATH_MAX
# define PATH_MAX 8192
#endif

#if D_INO_IN_DIRENT
# define MATCHING_INO(dp, ino) ((dp)->d_ino == (ino))
#else
# define MATCHING_INO(dp, ino) true
#endif

#if HAVE_MSVC_INVALID_PARAMETER_HANDLER
# include "msvc-inval.h"
#endif

#if !_LIBC
# define GETCWD_RETURN_TYPE char *
# define __close_nocancel_nostatus close
# define __getcwd_generic rpl_getcwd
# undef stat64
# define stat64    stat
# define __fstat64 fstat
# define __fstatat64 fstatat
# define __lstat64 lstat
# define __closedir closedir
# define __opendir opendir
# define __readdir64 readdir
# define __fdopendir fdopendir
# define __openat openat
# define __rewinddir rewinddir
# define __openat64 openat
# define dirent64 dirent
#else
# include <not-cancel.h>
#endif

/* The results of opendir() in this file are not used with dirfd and fchdir,
   and we do not leak fds to any single-threaded code that could use stdio,
   therefore save some unnecessary recursion in fchdir.c.
   FIXME - if the kernel ever adds support for multi-thread safety for
   avoiding standard fds, then we should use opendir_safer and
   openat_safer.  */
#ifdef GNULIB_defined_DIR
# undef DIR
# undef opendir
# undef closedir
# undef readdir
# undef rewinddir
#else
# ifdef GNULIB_defined_opendir
#  undef opendir
# endif
# ifdef GNULIB_defined_closedir
#  undef closedir
# endif
#endif

#if defined _WIN32 && !defined __CYGWIN__
# if HAVE_MSVC_INVALID_PARAMETER_HANDLER
static char *
getcwd_nothrow (char *buf, size_t size)
{
  char *result;

  TRY_MSVC_INVAL
    {
      result = _getcwd (buf, size);
    }
  CATCH_MSVC_INVAL
    {
      result = NULL;
      errno = ERANGE;
    }
  DONE_MSVC_INVAL;

  return result;
}
# else
#  define getcwd_nothrow _getcwd
# endif
# define getcwd_system getcwd_nothrow
#else
# define getcwd_system getcwd
#endif

/* Get the name of the current working directory, and put it in SIZE
   bytes of BUF.  Returns NULL with errno set if the directory couldn't be
   determined or SIZE was too small.  If successful, returns BUF.  In GNU,
   if BUF is NULL, an array is allocated with 'malloc'; the array is SIZE
   bytes long, unless SIZE == 0, in which case it is as big as necessary.  */

GETCWD_RETURN_TYPE
__getcwd_generic (char *buf, size_t size)
{
  /* Lengths of big file name components and entire file names, and a
     deep level of file name nesting.  These numbers are not upper
     bounds; they are merely large values suitable for initial
     allocations, designed to be large enough for most real-world
     uses.  */
  enum
    {
      BIG_FILE_NAME_COMPONENT_LENGTH = 255,
      BIG_FILE_NAME_LENGTH = MIN (4095, PATH_MAX - 1),
      DEEP_NESTING = 100
    };

#if HAVE_OPENAT_SUPPORT
  int fd = AT_FDCWD;
  bool fd_needs_closing = false;
# if defined __linux__
  bool proc_fs_not_mounted = false;
# endif
#else
  char dots[DEEP_NESTING * sizeof ".." + BIG_FILE_NAME_COMPONENT_LENGTH + 1];
  char *dotlist = dots;
  size_t dotsize = sizeof dots;
  size_t dotlen = 0;
#endif
  DIR *dirstream = NULL;
  dev_t rootdev, thisdev;
  ino_t rootino, thisino;
  char *dir;
  register char *dirp;
  struct stat64 st;
  size_t allocated = size;
  size_t used;

#if HAVE_MINIMALLY_WORKING_GETCWD
  /* If AT_FDCWD is not defined, the algorithm below is O(N**2) and
     this is much slower than the system getcwd (at least on
     GNU/Linux).  So trust the system getcwd's results unless they
     look suspicious.

     Use the system getcwd even if we have openat support, since the
     system getcwd works even when a parent is unreadable, while the
     openat-based approach does not.

     But on AIX 5.1..7.1, the system getcwd is not even minimally
     working: If the current directory name is slightly longer than
     PATH_MAX, it omits the first directory component and returns
     this wrong result with errno = 0.  */

# undef getcwd
  dir = getcwd_system (buf, size);
  if (dir || (size && errno == ERANGE))
    return dir;

  /* Solaris getcwd (NULL, 0) fails with errno == EINVAL, but it has
     internal magic that lets it work even if an ancestor directory is
     inaccessible, which is better in many cases.  So in this case try
     again with a buffer that's almost always big enough.  */
  if (errno == EINVAL && buf == NULL && size == 0)
    {
      char big_buffer[BIG_FILE_NAME_LENGTH + 1];
      dir = getcwd_system (big_buffer, sizeof big_buffer);
      if (dir)
        return strdup (dir);
    }

# if HAVE_PARTLY_WORKING_GETCWD
  /* The system getcwd works, except it sometimes fails when it
     shouldn't, setting errno to ERANGE, ENAMETOOLONG, or ENOENT.    */
  if (errno != ERANGE && errno != ENAMETOOLONG && errno != ENOENT)
    return NULL;
# endif
#endif
  if (size == 0)
    {
      if (buf != NULL)
        {
          __set_errno (EINVAL);
          return NULL;
        }

      allocated = BIG_FILE_NAME_LENGTH + 1;
    }

  if (buf == NULL)
    {
      dir = malloc (allocated);
      if (dir == NULL)
        return NULL;
    }
  else
    dir = buf;

  dirp = dir + allocated;
  *--dirp = '\0';

  if (__lstat64 (".", &st) < 0)
    goto lose;
  thisdev = st.st_dev;
  thisino = st.st_ino;

  if (__lstat64 ("/", &st) < 0)
    goto lose;
  rootdev = st.st_dev;
  rootino = st.st_ino;

  while (!(thisdev == rootdev && thisino == rootino))
    {
      struct dirent64 *d;
      dev_t dotdev;
      ino_t dotino;
      bool mount_point;
      int parent_status;
      size_t dirroom;
      size_t namlen;
      bool use_d_ino = true;

      /* Look at the parent directory.  */
#if HAVE_OPENAT_SUPPORT
      fd = __openat64 (fd, "..", O_RDONLY);
      if (fd < 0)
        goto lose;
      fd_needs_closing = true;
      parent_status = __fstat64 (fd, &st);
#else
      dotlist[dotlen++] = '.';
      dotlist[dotlen++] = '.';
      dotlist[dotlen] = '\0';
      parent_status = __lstat64 (dotlist, &st);
#endif
      if (parent_status != 0)
        goto lose;

      if (dirstream && __closedir (dirstream) != 0)
        {
          dirstream = NULL;
          goto lose;
        }

      /* Figure out if this directory is a mount point.  */
      dotdev = st.st_dev;
      dotino = st.st_ino;
      mount_point = dotdev != thisdev;

      /* Search for the last directory.  */
#if HAVE_OPENAT_SUPPORT
      dirstream = __fdopendir (fd);
      if (dirstream == NULL)
        goto lose;
      fd_needs_closing = false;
#else
      dirstream = __opendir (dotlist);
      if (dirstream == NULL)
        goto lose;
      dotlist[dotlen++] = '/';
#endif
      for (;;)
        {
          /* Clear errno to distinguish EOF from error if readdir returns
             NULL.  */
          __set_errno (0);
          d = __readdir64 (dirstream);

          /* When we've iterated through all directory entries without finding
             one with a matching d_ino, rewind the stream and consider each
             name again, but this time, using lstat.  This is necessary in a
             chroot on at least one system (glibc-2.3.6 + linux 2.6.12), where
             .., ../.., ../../.., etc. all had the same device number, yet the
             d_ino values for entries in / did not match those obtained
             via lstat.  */
          if (d == NULL && errno == 0 && use_d_ino)
            {
              use_d_ino = false;
              __rewinddir (dirstream);
              d = __readdir64 (dirstream);
            }

          if (d == NULL)
            {
              if (errno == 0)
                /* EOF on dirstream, which can mean e.g., that the current
                   directory has been removed.  */
                __set_errno (ENOENT);
              goto lose;
            }
          if (d->d_name[0] == '.' &&
              (d->d_name[1] == '\0' ||
               (d->d_name[1] == '.' && d->d_name[2] == '\0')))
            continue;

          if (use_d_ino)
            {
              bool match = (MATCHING_INO (d, thisino) || mount_point);
              if (! match)
                continue;
            }

          {
            int entry_status;
#if HAVE_OPENAT_SUPPORT
            entry_status = __fstatat64 (fd, d->d_name, &st, AT_SYMLINK_NOFOLLOW);
#else
            /* Compute size needed for this file name, or for the file
               name ".." in the same directory, whichever is larger.
               Room for ".." might be needed the next time through
               the outer loop.  */
            size_t name_alloc = _D_ALLOC_NAMLEN (d);
            size_t filesize = dotlen + MAX (sizeof "..", name_alloc);

            if (filesize < dotlen)
              goto memory_exhausted;

            if (dotsize < filesize)
              {
                /* My, what a deep directory tree you have, Grandma.  */
                size_t newsize = MAX (filesize, dotsize * 2);
                size_t i;
                if (newsize < dotsize)
                  goto memory_exhausted;
                if (dotlist != dots)
                  free (dotlist);
                dotlist = malloc (newsize);
                if (dotlist == NULL)
                  goto lose;
                dotsize = newsize;

                i = 0;
                do
                  {
                    dotlist[i++] = '.';
                    dotlist[i++] = '.';
                    dotlist[i++] = '/';
                  }
                while (i < dotlen);
              }

            memcpy (dotlist + dotlen, d->d_name, _D_ALLOC_NAMLEN (d));
            entry_status = __lstat64 (dotlist, &st);
#endif
            /* We don't fail here if we cannot stat() a directory entry.
               This can happen when (network) file systems fail.  If this
               entry is in fact the one we are looking for we will find
               out soon as we reach the end of the directory without
               having found anything.  */
            if (entry_status == 0 && S_ISDIR (st.st_mode)
                && st.st_dev == thisdev && st.st_ino == thisino)
              break;
          }
        }

      dirroom = dirp - dir;
      namlen = _D_EXACT_NAMLEN (d);

      if (dirroom <= namlen)
        {
          if (size != 0)
            {
              __set_errno (ERANGE);
              goto lose;
            }
          else
            {
              char *tmp;
              size_t oldsize = allocated;

              allocated += MAX (allocated, namlen);
              if (allocated < oldsize
                  || ! (tmp = realloc (dir, allocated)))
                goto memory_exhausted;

              /* Move current contents up to the end of the buffer.
                 This is guaranteed to be non-overlapping.  */
              dirp = memcpy (tmp + allocated - (oldsize - dirroom),
                             tmp + dirroom,
                             oldsize - dirroom);
              dir = tmp;
            }
        }
      dirp -= namlen;
      memcpy (dirp, d->d_name, namlen);
      *--dirp = '/';

      thisdev = dotdev;
      thisino = dotino;

#if HAVE_OPENAT_SUPPORT
      /* On some platforms, a system call returns the directory that FD points
         to.  This is useful if some of the ancestor directories of the
         directory are unreadable, because in this situation the loop that
         climbs up the ancestor hierarchy runs into an EACCES error.
         For example, in some Android app, /data/data/com.termux is readable,
         but /data/data and /data are not.  */
# if defined __linux__
      /* On Linux, in particular, if /proc is mounted,
           readlink ("/proc/self/fd/<fd>")
         returns the directory, if its length is < 4096.  (If the length is
         >= 4096, it fails with error ENAMETOOLONG, even if the buffer that we
         pass to the readlink function would be large enough.)  */
      if (!proc_fs_not_mounted)
        {
          char namebuf[14 + 10 + 1];
          sprintf (namebuf, "/proc/self/fd/%u", (unsigned int) fd);
          char linkbuf[4096];
          ssize_t linklen = readlink (namebuf, linkbuf, sizeof linkbuf);
          if (linklen < 0)
            {
              if (errno != ENAMETOOLONG)
                /* If this call was not successful, the next one will likely be
                   not successful either.  */
                proc_fs_not_mounted = true;
            }
          else
            {
              dirroom = dirp - dir;
              if (dirroom < linklen)
                {
                  if (size != 0)
                    {
                      __set_errno (ERANGE);
                      goto lose;
                    }
                  else
                    {
                      char *tmp;
                      size_t oldsize = allocated;

                      allocated += linklen - dirroom;
                      if (allocated < oldsize
                          || ! (tmp = realloc (dir, allocated)))
                        goto memory_exhausted;

                      /* Move current contents up to the end of the buffer.  */
                      dirp = memmove (tmp + dirroom + (allocated - oldsize),
                                      tmp + dirroom,
                                      oldsize - dirroom);
                      dir = tmp;
                    }
                }
              dirp -= linklen;
              memcpy (dirp, linkbuf, linklen);
              break;
            }
        }
# endif
#endif
    }

  if (dirstream && __closedir (dirstream) != 0)
    {
      dirstream = NULL;
      goto lose;
    }

  if (dirp == &dir[allocated - 1])
    *--dirp = '/';

#if ! HAVE_OPENAT_SUPPORT
  if (dotlist != dots)
    free (dotlist);
#endif

  used = dir + allocated - dirp;
  memmove (dir, dirp, used);

  if (size == 0)
    /* Ensure that the buffer is only as large as necessary.  */
    buf = (used < allocated ? realloc (dir, used) : dir);

  if (buf == NULL)
    /* Either buf was NULL all along, or 'realloc' failed but
       we still have the original string.  */
    buf = dir;

  return buf;

 memory_exhausted:
  __set_errno (ENOMEM);
 lose:
  {
    int save = errno;
    if (dirstream)
      __closedir (dirstream);
#if HAVE_OPENAT_SUPPORT
    if (fd_needs_closing)
       __close_nocancel_nostatus (fd);
#else
    if (dotlist != dots)
      free (dotlist);
#endif
    if (buf == NULL)
      free (dir);
    __set_errno (save);
  }
  return NULL;
}

#if defined _LIBC && !defined GETCWD_RETURN_TYPE
libc_hidden_def (__getcwd)
weak_alias (__getcwd, getcwd)
#endif