lib/readlinkat.c

   1 /* Read a symlink relative to an open directory.
   2    Copyright (C) 2009-2020 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation; either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 /* written by Eric Blake */
  18
  19 #include <config.h>
  20
  21 /* Specification.  */
  22 #include <unistd.h>
  23
  24 #include <errno.h>
  25 #include <stdlib.h>
  26 #include <string.h>
  27 #include <sys/stat.h>
  28
  29 #if HAVE_READLINKAT
  30
  31 # undef readlinkat
  32
  33 ssize_t
  34 rpl_readlinkat (int fd, char const *file, char *buf, size_t len)
  35 {
  36 # if READLINK_TRAILING_SLASH_BUG
  37   size_t file_len = strlen (file);
  38   if (file_len && file[file_len - 1] == '/')
  39     {
  40       /* Even if FILE without the slash is a symlink to a directory,
  41          both lstat() and stat() must resolve the trailing slash to
  42          the directory rather than the symlink.  We can therefore
  43          safely use stat() to distinguish between EINVAL and
  44          ENOTDIR/ENOENT, avoiding extra overhead of rpl_lstat().  */
  45       struct stat st;
  46       if (stat (file, &st) == 0)
  47         errno = EINVAL;
  48       return -1;
  49     }
  50 # endif /* READLINK_TRAILING_SLASH_BUG */
  51   return readlinkat (fd, file, buf, len);
  52 }
  53
  54 #else
  55
  56 /* Gnulib provides a readlink stub for mingw; use it for distinction
  57    between EINVAL and ENOENT, rather than always failing with ENOSYS.  */
  58
  59 /* POSIX 2008 says that unlike readlink, readlinkat returns 0 for
  60    success instead of the buffer length.  But this would render
  61    readlinkat worthless since readlink does not guarantee a
  62    NUL-terminated buffer.  Assume this was a bug in POSIX.  */
  63
  64 /* Read the contents of symlink FILE into buffer BUF of size LEN, in the
  65    directory open on descriptor FD.  If possible, do it without changing
  66    the working directory.  Otherwise, resort to using save_cwd/fchdir,
  67    then readlink/restore_cwd.  If either the save_cwd or the restore_cwd
  68    fails, then give a diagnostic and exit nonzero.  */
  69
  70 # define AT_FUNC_NAME readlinkat
  71 # define AT_FUNC_F1 readlink
  72 # define AT_FUNC_POST_FILE_PARAM_DECLS , char *buf, size_t len
  73 # define AT_FUNC_POST_FILE_ARGS        , buf, len
  74 # define AT_FUNC_RESULT ssize_t
  75 # include "at-func.c"
  76 # undef AT_FUNC_NAME
  77 # undef AT_FUNC_F1
  78 # undef AT_FUNC_POST_FILE_PARAM_DECLS
  79 # undef AT_FUNC_POST_FILE_ARGS
  80 # undef AT_FUNC_RESULT
  81
  82 #endif