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, 2009-2024 Free Software Foundation, Inc.
   5
   6    This file is free software: you can redistribute it and/or modify
   7    it under the terms of the GNU Lesser General Public License as
   8    published by the Free Software Foundation; either version 2.1 of the
   9    License, or (at your option) any later version.
  10
  11    This file 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 Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser 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 /* SYMLINK_MAX is used only for an initial memory-allocation sanity
  33    check, so it's OK to guess too small on hosts where there is no
  34    arbitrary limit to symbolic link length.  */
  35 #ifndef SYMLINK_MAX
  36 # define SYMLINK_MAX 1024
  37 #endif
  38
  39 #define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
  40
  41 /* Call readlink to get the symbolic link value of FILE.
  42    SIZE is a hint as to how long the link is expected to be;
  43    typically it is taken from st_size.  It need not be correct.
  44    Return a pointer to that NUL-terminated string in malloc'd storage.
  45    If readlink fails, malloc fails, or if the link value is longer
  46    than SSIZE_MAX, return NULL (caller may use errno to diagnose).  */
  47
  48 char *
  49 areadlink_with_size (char const *file, size_t size)
  50 {
  51   /* Some buggy file systems report garbage in st_size.  Defend
  52      against them by ignoring outlandish st_size values in the initial
  53      memory allocation.  */
  54   size_t symlink_max = SYMLINK_MAX;
  55   size_t INITIAL_LIMIT_BOUND = 8 * 1024;
  56   size_t initial_limit = (symlink_max < INITIAL_LIMIT_BOUND
  57                           ? symlink_max + 1
  58                           : INITIAL_LIMIT_BOUND);
  59
  60   enum { stackbuf_size = 128 };
  61
  62   /* The initial buffer size for the link value.  */
  63   size_t buf_size = (size == 0 ? stackbuf_size
  64                      : size < initial_limit ? size + 1 : initial_limit);
  65
  66   while (1)
  67     {
  68       ssize_t r;
  69       size_t link_length;
  70       char stackbuf[stackbuf_size];
  71       char *buf = stackbuf;
  72       char *buffer = NULL;
  73
  74       if (! (size == 0 && buf_size == stackbuf_size))
  75         {
  76           buf = buffer = malloc (buf_size);
  77           if (!buffer)
  78             {
  79               errno = ENOMEM;
  80               return NULL;
  81             }
  82         }
  83
  84       r = readlink (file, buf, buf_size);
  85       link_length = r;
  86
  87       if (r < 0)
  88         {
  89           free (buffer);
  90           return NULL;
  91         }
  92
  93       if (link_length < buf_size)
  94         {
  95           buf[link_length] = 0;
  96           if (!buffer)
  97             {
  98               buffer = malloc (link_length + 1);
  99               if (buffer)
 100                 return memcpy (buffer, buf, link_length + 1);
 101             }
 102           else if (link_length + 1 < buf_size)
 103             {
 104               /* Shrink BUFFER before returning it.  */
 105               char *shrinked_buffer = realloc (buffer, link_length + 1);
 106               if (shrinked_buffer != NULL)
 107                 buffer = shrinked_buffer;
 108             }
 109           return buffer;
 110         }
 111
 112       free (buffer);
 113       if (buf_size <= MAXSIZE / 2)
 114         buf_size *= 2;
 115       else if (buf_size < MAXSIZE)
 116         buf_size = MAXSIZE;
 117       else
 118         {
 119           errno = ENOMEM;
 120           return NULL;
 121         }
 122     }
 123 }