lib/areadlinkat-with-size.c

   1 /* readlinkat wrapper to return the link name in malloc'd storage.
   2    Unlike xreadlinkat, only call exit on failure to change directory.
   3
   4    Copyright (C) 2001, 2003-2007, 2009-2017 Free Software Foundation, Inc.
   5
   6    This program is free software: you can redistribute it and/or modify
   7    it under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 3 of the License, or
   9    (at your option) any later version.
  10
  11    This program is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU General Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License
  17    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  18
  19 /* Written by Jim Meyering <jim@meyering.net>
  20    and Eric Blake <ebb9@byu.net>.  */
  21
  22 #include <config.h>
  23
  24 #include "areadlink.h"
  25
  26 #include <errno.h>
  27 #include <limits.h>
  28 #include <stdint.h>
  29 #include <stdlib.h>
  30 #include <unistd.h>
  31
  32 #if HAVE_READLINKAT
  33
  34 # ifndef SSIZE_MAX
  35 #  define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
  36 # endif
  37
  38 /* SYMLINK_MAX is used only for an initial memory-allocation sanity
  39    check, so it's OK to guess too small on hosts where there is no
  40    arbitrary limit to symbolic link length.  */
  41 # ifndef SYMLINK_MAX
  42 #  define SYMLINK_MAX 1024
  43 # endif
  44
  45 # define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
  46
  47 /* Call readlinkat to get the symbolic link value of FILE, relative to FD.
  48    SIZE is a hint as to how long the link is expected to be;
  49    typically it is taken from st_size.  It need not be correct.
  50    Return a pointer to that NUL-terminated string in malloc'd storage.
  51    If readlinkat fails, malloc fails, or if the link value is longer
  52    than SSIZE_MAX, return NULL (caller may use errno to diagnose).
  53    However, failure to change directory during readlinkat will issue
  54    a diagnostic and exit.  */
  55
  56 char *
  57 areadlinkat_with_size (int fd, char const *file, size_t size)
  58 {
  59   /* Some buggy file systems report garbage in st_size.  Defend
  60      against them by ignoring outlandish st_size values in the initial
  61      memory allocation.  */
  62   size_t symlink_max = SYMLINK_MAX;
  63   size_t INITIAL_LIMIT_BOUND = 8 * 1024;
  64   size_t initial_limit = (symlink_max < INITIAL_LIMIT_BOUND
  65                           ? symlink_max + 1
  66                           : INITIAL_LIMIT_BOUND);
  67
  68   /* The initial buffer size for the link value.  */
  69   size_t buf_size = size < initial_limit ? size + 1 : initial_limit;
  70
  71   while (1)
  72     {
  73       ssize_t r;
  74       size_t link_length;
  75       char *buffer = malloc (buf_size);
  76
  77       if (buffer == NULL)
  78         return NULL;
  79       r = readlinkat (fd, file, buffer, buf_size);
  80       link_length = r;
  81
  82       /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
  83          with errno == ERANGE if the buffer is too small.  */
  84       if (r < 0 && errno != ERANGE)
  85         {
  86           int saved_errno = errno;
  87           free (buffer);
  88           errno = saved_errno;
  89           return NULL;
  90         }
  91
  92       if (link_length < buf_size)
  93         {
  94           buffer[link_length] = 0;
  95           return buffer;
  96         }
  97
  98       free (buffer);
  99       if (buf_size <= MAXSIZE / 2)
 100         buf_size *= 2;
 101       else if (buf_size < MAXSIZE)
 102         buf_size = MAXSIZE;
 103       else
 104         {
 105           errno = ENOMEM;
 106           return NULL;
 107         }
 108     }
 109 }
 110
 111 #else /* !HAVE_READLINKAT */
 112
 113
 114 /* It is more efficient to change directories only once and call
 115    areadlink_with_size, rather than repeatedly call the replacement
 116    readlinkat.  */
 117
 118 # define AT_FUNC_NAME areadlinkat_with_size
 119 # define AT_FUNC_F1 areadlink_with_size
 120 # define AT_FUNC_POST_FILE_PARAM_DECLS , size_t size
 121 # define AT_FUNC_POST_FILE_ARGS        , size
 122 # define AT_FUNC_RESULT char *
 123 # define AT_FUNC_FAIL NULL
 124 # include "at-func.c"
 125 # undef AT_FUNC_NAME
 126 # undef AT_FUNC_F1
 127 # undef AT_FUNC_POST_FILE_PARAM_DECLS
 128 # undef AT_FUNC_POST_FILE_ARGS
 129 # undef AT_FUNC_RESULT
 130 # undef AT_FUNC_FAIL
 131
 132 #endif /* !HAVE_READLINKAT */