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, 2009-2020 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 <string.h>
  30 #include <unistd.h>
  31
  32 #ifndef SSIZE_MAX
  33 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
  34 #endif
  35
  36 /* SYMLINK_MAX is used only for an initial memory-allocation sanity
  37    check, so it's OK to guess too small on hosts where there is no
  38    arbitrary limit to symbolic link length.  */
  39 #ifndef SYMLINK_MAX
  40 # define SYMLINK_MAX 1024
  41 #endif
  42
  43 #define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
  44
  45 /* Call readlink to get the symbolic link value of FILE.
  46    SIZE is a hint as to how long the link is expected to be;
  47    typically it is taken from st_size.  It need not be correct.
  48    Return a pointer to that NUL-terminated string in malloc'd storage.
  49    If readlink fails, malloc fails, or if the link value is longer
  50    than SSIZE_MAX, return NULL (caller may use errno to diagnose).  */
  51
  52 char *
  53 areadlink_with_size (char const *file, size_t size)
  54 {
  55   /* Some buggy file systems report garbage in st_size.  Defend
  56      against them by ignoring outlandish st_size values in the initial
  57      memory allocation.  */
  58   size_t symlink_max = SYMLINK_MAX;
  59   size_t INITIAL_LIMIT_BOUND = 8 * 1024;
  60   size_t initial_limit = (symlink_max < INITIAL_LIMIT_BOUND
  61                           ? symlink_max + 1
  62                           : INITIAL_LIMIT_BOUND);
  63
  64   enum { stackbuf_size = 128 };
  65
  66   /* The initial buffer size for the link value.  */
  67   size_t buf_size = (size == 0 ? stackbuf_size
  68                      : size < initial_limit ? size + 1 : initial_limit);
  69
  70   while (1)
  71     {
  72       ssize_t r;
  73       size_t link_length;
  74       char stackbuf[stackbuf_size];
  75       char *buf = stackbuf;
  76       char *buffer = NULL;
  77
  78       if (! (size == 0 && buf_size == stackbuf_size))
  79         {
  80           buf = buffer = malloc (buf_size);
  81           if (!buffer)
  82             {
  83               errno = ENOMEM;
  84               return NULL;
  85             }
  86         }
  87
  88       r = readlink (file, buf, buf_size);
  89       link_length = r;
  90
  91       /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
  92          with errno == ERANGE if the buffer is too small.  */
  93       if (r < 0 && errno != ERANGE)
  94         {
  95           int saved_errno = errno;
  96           free (buffer);
  97           errno = saved_errno;
  98           return NULL;
  99         }
 100
 101       if (link_length < buf_size)
 102         {
 103           buf[link_length] = 0;
 104           if (!buffer)
 105             {
 106               buffer = malloc (link_length + 1);
 107               if (buffer)
 108                 return memcpy (buffer, buf, link_length + 1);
 109             }
 110           else if (link_length + 1 < buf_size)
 111             {
 112               /* Shrink BUFFER before returning it.  */
 113               char *shrinked_buffer = realloc (buffer, link_length + 1);
 114               if (shrinked_buffer != NULL)
 115                 buffer = shrinked_buffer;
 116             }
 117           return buffer;
 118         }
 119
 120       free (buffer);
 121       if (buf_size <= MAXSIZE / 2)
 122         buf_size *= 2;
 123       else if (buf_size < MAXSIZE)
 124         buf_size = MAXSIZE;
 125       else
 126         {
 127           errno = ENOMEM;
 128           return NULL;
 129         }
 130     }
 131 }