lib/areadlink-with-size.c

   1 /* readlink wrapper to return the link name in malloc'd storage.
   2    Unlike xreadlink and xreadlink_with_size, don't ever call exit.
   3
   4    Copyright (C) 2001, 2003-2007, 2009-2019 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 <https://www.gnu.org/licenses/>.  */
  18
  19 /* Written by Jim Meyering <jim@meyering.net>  */
  20
  21 #include <config.h>
  22
  23 #include "areadlink.h"
  24
  25 #include <errno.h>
  26 #include <limits.h>
  27 #include <stdint.h>
  28 #include <stdlib.h>
  29 #include <unistd.h>
  30
  31 #ifndef SSIZE_MAX
  32 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
  33 #endif
  34
  35 /* SYMLINK_MAX is used only for an initial memory-allocation sanity
  36    check, so it's OK to guess too small on hosts where there is no
  37    arbitrary limit to symbolic link length.  */
  38 #ifndef SYMLINK_MAX
  39 # define SYMLINK_MAX 1024
  40 #endif
  41
  42 #define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
  43
  44 /* Call readlink to get the symbolic link value of FILE.
  45    SIZE is a hint as to how long the link is expected to be;
  46    typically it is taken from st_size.  It need not be correct.
  47    Return a pointer to that NUL-terminated string in malloc'd storage.
  48    If readlink fails, malloc fails, or if the link value is longer
  49    than SSIZE_MAX, return NULL (caller may use errno to diagnose).  */
  50
  51 char *
  52 areadlink_with_size (char const *file, size_t size)
  53 {
  54   /* Some buggy file systems report garbage in st_size.  Defend
  55      against them by ignoring outlandish st_size values in the initial
  56      memory allocation.  */
  57   size_t symlink_max = SYMLINK_MAX;
  58   size_t INITIAL_LIMIT_BOUND = 8 * 1024;
  59   size_t initial_limit = (symlink_max < INITIAL_LIMIT_BOUND
  60                           ? symlink_max + 1
  61                           : INITIAL_LIMIT_BOUND);
  62
  63   /* The initial buffer size for the link value.  */
  64   size_t buf_size = size < initial_limit ? size + 1 : initial_limit;
  65
  66   while (1)
  67     {
  68       ssize_t r;
  69       size_t link_length;
  70       char *buffer = malloc (buf_size);
  71
  72       if (buffer == NULL)
  73         return NULL;
  74       r = readlink (file, buffer, buf_size);
  75       link_length = r;
  76
  77       /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
  78          with errno == ERANGE if the buffer is too small.  */
  79       if (r < 0 && errno != ERANGE)
  80         {
  81           int saved_errno = errno;
  82           free (buffer);
  83           errno = saved_errno;
  84           return NULL;
  85         }
  86
  87       if (link_length < buf_size)
  88         {
  89           buffer[link_length] = 0;
  90           return buffer;
  91         }
  92
  93       free (buffer);
  94       if (buf_size <= MAXSIZE / 2)
  95         buf_size *= 2;
  96       else if (buf_size < MAXSIZE)
  97         buf_size = MAXSIZE;
  98       else
  99         {
 100           errno = ENOMEM;
 101           return NULL;
 102         }
 103     }
 104 }