emacs/lib/readlinkat.c

/* Read a symlink relative to an open directory.
   Copyright (C) 2009-2021 Free Software Foundation, Inc.

   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 3 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 <https://www.gnu.org/licenses/>.  */

/* written by Eric Blake */

#include <config.h>

/* Specification.  */
#include <unistd.h>

#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>

#if HAVE_READLINKAT

# undef fstatat
# undef readlinkat

ssize_t
rpl_readlinkat (int fd, char const *file, char *buf, size_t bufsize)
{
# if READLINK_TRAILING_SLASH_BUG
  size_t file_len = strlen (file);
  if (file_len && file[file_len - 1] == '/')
    {
      /* Even if FILE without the slash is a symlink to a directory,
         both lstat() and stat() must resolve the trailing slash to
         the directory rather than the symlink.  We can therefore
         safely use fstatat(..., 0) to distinguish between EINVAL and
         ENOTDIR/ENOENT, avoiding extra overhead of rpl_fstatat().  */
      struct stat st;
      if (fstatat (fd, file, &st, 0) == 0 || errno == EOVERFLOW)
        errno = EINVAL;
      return -1;
    }
# endif /* READLINK_TRAILING_SLASH_BUG */

  ssize_t r = readlinkat (fd, file, buf, bufsize);

# if READLINK_TRUNCATE_BUG
  if (r < 0 && errno == ERANGE)
    {
      /* Try again with a bigger buffer.  This is just for test cases;
         real code invariably discards short reads.  */
      char stackbuf[4032];
      r = readlinkat (fd, file, stackbuf, sizeof stackbuf);
      if (r < 0)
        {
          if (errno == ERANGE)
            {
              /* Clear the buffer, which is good enough for real code.
                 Thankfully, no test cases try short reads of enormous
                 symlinks and what would be the point anyway?  */
              r = bufsize;
              memset (buf, 0, r);
            }
        }
      else
        {
          if (bufsize < r)
            r = bufsize;
          memcpy (buf, stackbuf, r);
        }
    }
# endif

  return r;
}

#else

/* Gnulib provides a readlink stub for mingw; use it for distinction
   between EINVAL and ENOENT, rather than always failing with ENOSYS.  */

/* POSIX 2008 says that unlike readlink, readlinkat returns 0 for
   success instead of the buffer length.  But this would render
   readlinkat worthless since readlink does not guarantee a
   NUL-terminated buffer.  Assume this was a bug in POSIX.  */

/* Read the contents of symlink FILE into buffer BUF of size BUFSIZE, in the
   directory open on descriptor FD.  If possible, do it without changing
   the working directory.  Otherwise, resort to using save_cwd/fchdir,
   then readlink/restore_cwd.  If either the save_cwd or the restore_cwd
   fails, then give a diagnostic and exit nonzero.  */

# define AT_FUNC_NAME readlinkat
# define AT_FUNC_F1 readlink
# define AT_FUNC_POST_FILE_PARAM_DECLS , char *buf, size_t bufsize
# define AT_FUNC_POST_FILE_ARGS        , buf, bufsize
# define AT_FUNC_RESULT ssize_t
# include "at-func.c"
# undef AT_FUNC_NAME
# undef AT_FUNC_F1
# undef AT_FUNC_POST_FILE_PARAM_DECLS
# undef AT_FUNC_POST_FILE_ARGS
# undef AT_FUNC_RESULT

#endif