lib/readlinkat.c

   1 /* Read a symlink relative to an open directory.
   2    Copyright (C) 2009-2024 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 fstatat
  32 # undef readlinkat
  33
  34 ssize_t
  35 rpl_readlinkat (int fd, char const *file, char *buf, size_t bufsize)
  36 {
  37 # if READLINK_TRAILING_SLASH_BUG
  38   size_t file_len = strlen (file);
  39   if (file_len && file[file_len - 1] == '/')
  40     {
  41       /* Even if FILE without the slash is a symlink to a directory,
  42          both lstat() and stat() must resolve the trailing slash to
  43          the directory rather than the symlink.  We can therefore
  44          safely use fstatat(..., 0) to distinguish between EINVAL and
  45          ENOTDIR/ENOENT, avoiding extra overhead of rpl_fstatat().  */
  46       struct stat st;
  47       if (fstatat (fd, file, &st, 0) == 0 || errno == EOVERFLOW)
  48         errno = EINVAL;
  49       return -1;
  50     }
  51 # endif /* READLINK_TRAILING_SLASH_BUG */
  52
  53   ssize_t r = readlinkat (fd, file, buf, bufsize);
  54
  55 # if READLINK_TRUNCATE_BUG
  56   if (r < 0 && errno == ERANGE)
  57     {
  58       /* Try again with a bigger buffer.  This is just for test cases;
  59          real code invariably discards short reads.  */
  60       char stackbuf[4032];
  61       r = readlinkat (fd, file, stackbuf, sizeof stackbuf);
  62       if (r < 0)
  63         {
  64           if (errno == ERANGE)
  65             {
  66               /* Clear the buffer, which is good enough for real code.
  67                  Thankfully, no test cases try short reads of enormous
  68                  symlinks and what would be the point anyway?  */
  69               r = bufsize;
  70               memset (buf, 0, r);
  71             }
  72         }
  73       else
  74         {
  75           if (bufsize < r)
  76             r = bufsize;
  77           memcpy (buf, stackbuf, r);
  78         }
  79     }
  80 # endif
  81
  82 # if defined __CYGWIN__
  83   /* On Cygwin 3.3.6, readlinkat(AT_FDCWD,"/dev/null") returns "\\Device\\Null",
  84      which is unusable.  Better fail with EINVAL.  */
  85   if (r > 0 && strncmp (file, "/dev/", 5) == 0 && buf[0] == '\\')
  86     {
  87       errno = EINVAL;
  88       return -1;
  89     }
  90 # endif
  91
  92   return r;
  93 }
  94
  95 #else
  96
  97 /* Gnulib provides a readlink stub for mingw; use it for distinction
  98    between EINVAL and ENOENT, rather than always failing with ENOSYS.  */
  99
 100 /* POSIX 2008 says that unlike readlink, readlinkat returns 0 for
 101    success instead of the buffer length.  But this would render
 102    readlinkat worthless since readlink does not guarantee a
 103    NUL-terminated buffer.  Assume this was a bug in POSIX.  */
 104
 105 /* Read the contents of symlink FILE into buffer BUF of size BUFSIZE, in the
 106    directory open on descriptor FD.  If possible, do it without changing
 107    the working directory.  Otherwise, resort to using save_cwd/fchdir,
 108    then readlink/restore_cwd.  If either the save_cwd or the restore_cwd
 109    fails, then give a diagnostic and exit nonzero.  */
 110
 111 # define AT_FUNC_NAME readlinkat
 112 # define AT_FUNC_F1 readlink
 113 # define AT_FUNC_POST_FILE_PARAM_DECLS , char *buf, size_t bufsize
 114 # define AT_FUNC_POST_FILE_ARGS        , buf, bufsize
 115 # define AT_FUNC_RESULT ssize_t
 116 # include "at-func.c"
 117 # undef AT_FUNC_NAME
 118 # undef AT_FUNC_F1
 119 # undef AT_FUNC_POST_FILE_PARAM_DECLS
 120 # undef AT_FUNC_POST_FILE_ARGS
 121 # undef AT_FUNC_RESULT
 122
 123 #endif