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, 2009-2021 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    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 <string.h>
  31 #include <unistd.h>
  32
  33 #if HAVE_READLINKAT
  34
  35 # ifndef SSIZE_MAX
  36 #  define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
  37 # endif
  38
  39 /* SYMLINK_MAX is used only for an initial memory-allocation sanity
  40    check, so it's OK to guess too small on hosts where there is no
  41    arbitrary limit to symbolic link length.  */
  42 # ifndef SYMLINK_MAX
  43 #  define SYMLINK_MAX 1024
  44 # endif
  45
  46 # define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
  47
  48 /* Call readlinkat to get the symbolic link value of FILE, relative to FD.
  49    SIZE is a hint as to how long the link is expected to be;
  50    typically it is taken from st_size.  It need not be correct.
  51    Return a pointer to that NUL-terminated string in malloc'd storage.
  52    If readlinkat fails, malloc fails, or if the link value is longer
  53    than SSIZE_MAX, return NULL (caller may use errno to diagnose).
  54    However, failure to change directory during readlinkat will issue
  55    a diagnostic and exit.  */
  56
  57 char *
  58 areadlinkat_with_size (int fd, char const *file, size_t size)
  59 {
  60   /* Some buggy file systems report garbage in st_size.  Defend
  61      against them by ignoring outlandish st_size values in the initial
  62      memory allocation.  */
  63   size_t symlink_max = SYMLINK_MAX;
  64   size_t INITIAL_LIMIT_BOUND = 8 * 1024;
  65   size_t initial_limit = (symlink_max < INITIAL_LIMIT_BOUND
  66                           ? symlink_max + 1
  67                           : INITIAL_LIMIT_BOUND);
  68
  69   enum { stackbuf_size = 128 };
  70
  71   /* The initial buffer size for the link value.  */
  72   size_t buf_size = (size == 0 ? stackbuf_size
  73                      : size < initial_limit ? size + 1 : initial_limit);
  74
  75   while (1)
  76     {
  77       ssize_t r;
  78       size_t link_length;
  79       char stackbuf[stackbuf_size];
  80       char *buf = stackbuf;
  81       char *buffer = NULL;
  82
  83       if (! (size == 0 && buf_size == stackbuf_size))
  84         {
  85           buf = buffer = malloc (buf_size);
  86           if (!buffer)
  87             /* We can assume errno == ENOMEM here, since all platforms that have
  88                readlinkat() have a POSIX compliant malloc().  */
  89             return NULL;
  90         }
  91
  92       r = readlinkat (fd, file, buf, buf_size);
  93       link_length = r;
  94
  95       if (r < 0)
  96         {
  97           int saved_errno = errno;
  98           free (buffer);
  99           errno = saved_errno;
 100           return NULL;
 101         }
 102
 103       if (link_length < buf_size)
 104         {
 105           buf[link_length] = 0;
 106           if (!buffer)
 107             {
 108               buffer = malloc (link_length + 1);
 109               if (buffer)
 110                 return memcpy (buffer, buf, link_length + 1);
 111             }
 112           else if (link_length + 1 < buf_size)
 113             {
 114               /* Shrink BUFFER before returning it.  */
 115               char *shrinked_buffer = realloc (buffer, link_length + 1);
 116               if (shrinked_buffer != NULL)
 117                 buffer = shrinked_buffer;
 118             }
 119           return buffer;
 120         }
 121
 122       free (buffer);
 123       if (buf_size <= MAXSIZE / 2)
 124         buf_size *= 2;
 125       else if (buf_size < MAXSIZE)
 126         buf_size = MAXSIZE;
 127       else
 128         {
 129           errno = ENOMEM;
 130           return NULL;
 131         }
 132     }
 133 }
 134
 135 #else /* !HAVE_READLINKAT */
 136
 137
 138 /* It is more efficient to change directories only once and call
 139    areadlink_with_size, rather than repeatedly call the replacement
 140    readlinkat.  */
 141
 142 # define AT_FUNC_NAME areadlinkat_with_size
 143 # define AT_FUNC_F1 areadlink_with_size
 144 # define AT_FUNC_POST_FILE_PARAM_DECLS , size_t size
 145 # define AT_FUNC_POST_FILE_ARGS        , size
 146 # define AT_FUNC_RESULT char *
 147 # define AT_FUNC_FAIL NULL
 148 # include "at-func.c"
 149 # undef AT_FUNC_NAME
 150 # undef AT_FUNC_F1
 151 # undef AT_FUNC_POST_FILE_PARAM_DECLS
 152 # undef AT_FUNC_POST_FILE_ARGS
 153 # undef AT_FUNC_RESULT
 154 # undef AT_FUNC_FAIL
 155
 156 #endif /* !HAVE_READLINKAT */