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, 2009-2025 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 /* 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 readlinkat to get the symbolic link value of FILE, relative to FD.
  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 readlinkat fails, malloc fails, or if the link value is longer
  49    than SSIZE_MAX, return NULL (caller may use errno to diagnose).
  50    However, failure to change directory during readlinkat will issue
  51    a diagnostic and exit.  */
  52
  53 char *
  54 areadlinkat_with_size (int fd, char const *file, size_t size)
  55 {
  56   /* Some buggy file systems report garbage in st_size.  Defend
  57      against them by ignoring outlandish st_size values in the initial
  58      memory allocation.  */
  59   size_t symlink_max = SYMLINK_MAX;
  60   size_t INITIAL_LIMIT_BOUND = 8 * 1024;
  61   size_t initial_limit = (symlink_max < INITIAL_LIMIT_BOUND
  62                           ? symlink_max + 1
  63                           : INITIAL_LIMIT_BOUND);
  64
  65   enum { stackbuf_size = 128 };
  66
  67   /* The initial buffer size for the link value.  */
  68   size_t buf_size = (size == 0 ? stackbuf_size
  69                      : size < initial_limit ? size + 1 : initial_limit);
  70
  71   while (1)
  72     {
  73       ssize_t r;
  74       size_t link_length;
  75       char stackbuf[stackbuf_size];
  76       char *buf = stackbuf;
  77       char *buffer = NULL;
  78
  79       if (! (size == 0 && buf_size == stackbuf_size))
  80         {
  81           buf = buffer = malloc (buf_size);
  82           if (!buffer)
  83             /* We can assume errno == ENOMEM here, since all platforms that have
  84                readlinkat() have a POSIX compliant malloc().  */
  85             return NULL;
  86         }
  87
  88       r = readlinkat (fd, file, buf, buf_size);
  89       link_length = r;
  90
  91       if (r < 0)
  92         {
  93           free (buffer);
  94           return NULL;
  95         }
  96
  97       if (link_length < buf_size)
  98         {
  99           buf[link_length] = 0;
 100           if (!buffer)
 101             {
 102               buffer = malloc (link_length + 1);
 103               if (buffer)
 104                 return memcpy (buffer, buf, link_length + 1);
 105             }
 106           else if (link_length + 1 < buf_size)
 107             {
 108               /* Shrink BUFFER before returning it.  */
 109               char *shrinked_buffer = realloc (buffer, link_length + 1);
 110               if (shrinked_buffer != NULL)
 111                 buffer = shrinked_buffer;
 112             }
 113           return buffer;
 114         }
 115
 116       free (buffer);
 117       if (buf_size <= MAXSIZE / 2)
 118         buf_size *= 2;
 119       else if (buf_size < MAXSIZE)
 120         buf_size = MAXSIZE;
 121       else
 122         {
 123           errno = ENOMEM;
 124           return NULL;
 125         }
 126     }
 127 }
 128
 129 #else /* !HAVE_READLINKAT */
 130
 131
 132 /* It is more efficient to change directories only once and call
 133    areadlink_with_size, rather than repeatedly call the replacement
 134    readlinkat.  */
 135
 136 # define AT_FUNC_NAME areadlinkat_with_size
 137 # define AT_FUNC_F1 areadlink_with_size
 138 # define AT_FUNC_POST_FILE_PARAM_DECLS , size_t size
 139 # define AT_FUNC_POST_FILE_ARGS        , size
 140 # define AT_FUNC_RESULT char *
 141 # define AT_FUNC_FAIL NULL
 142 # include "at-func.c"
 143 # undef AT_FUNC_NAME
 144 # undef AT_FUNC_F1
 145 # undef AT_FUNC_POST_FILE_PARAM_DECLS
 146 # undef AT_FUNC_POST_FILE_ARGS
 147 # undef AT_FUNC_RESULT
 148 # undef AT_FUNC_FAIL
 149
 150 #endif /* !HAVE_READLINKAT */