lib/careadlinkat.c

   1 /* Read symbolic links into a buffer without size limitation, relative to fd.
   2
   3    Copyright (C) 2001, 2003-2004, 2007, 2009-2013 Free Software Foundation,
   4    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 <http://www.gnu.org/licenses/>.  */
  18
  19 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering.  */
  20
  21 #include <config.h>
  22
  23 #include "careadlinkat.h"
  24
  25 #include <errno.h>
  26 #include <limits.h>
  27 #include <string.h>
  28
  29 /* Define this independently so that stdint.h is not a prerequisite.  */
  30 #ifndef SIZE_MAX
  31 # define SIZE_MAX ((size_t) -1)
  32 #endif
  33
  34 #ifndef SSIZE_MAX
  35 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
  36 #endif
  37
  38 #include "allocator.h"
  39
  40 /* Assuming the current directory is FD, get the symbolic link value
  41    of FILENAME as a null-terminated string and put it into a buffer.
  42    If FD is AT_FDCWD, FILENAME is interpreted relative to the current
  43    working directory, as in openat.
  44
  45    If the link is small enough to fit into BUFFER put it there.
  46    BUFFER's size is BUFFER_SIZE, and BUFFER can be null
  47    if BUFFER_SIZE is zero.
  48
  49    If the link is not small, put it into a dynamically allocated
  50    buffer managed by ALLOC.  It is the caller's responsibility to free
  51    the returned value if it is nonnull and is not BUFFER.  A null
  52    ALLOC stands for the standard allocator.
  53
  54    The PREADLINKAT function specifies how to read links.  It operates
  55    like POSIX readlinkat()
  56    <http://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html>
  57    but can assume that its first argument is the same as FD.
  58
  59    If successful, return the buffer address; otherwise return NULL and
  60    set errno.  */
  61
  62 char *
  63 careadlinkat (int fd, char const *filename,
  64               char *buffer, size_t buffer_size,
  65               struct allocator const *alloc,
  66               ssize_t (*preadlinkat) (int, char const *, char *, size_t))
  67 {
  68   char *buf;
  69   size_t buf_size;
  70   size_t buf_size_max =
  71     SSIZE_MAX < SIZE_MAX ? (size_t) SSIZE_MAX + 1 : SIZE_MAX;
  72   char stack_buf[1024];
  73
  74   if (! alloc)
  75     alloc = &stdlib_allocator;
  76
  77   if (! buffer_size)
  78     {
  79       /* Allocate the initial buffer on the stack.  This way, in the
  80          common case of a symlink of small size, we get away with a
  81          single small malloc() instead of a big malloc() followed by a
  82          shrinking realloc().  */
  83       buffer = stack_buf;
  84       buffer_size = sizeof stack_buf;
  85     }
  86
  87   buf = buffer;
  88   buf_size = buffer_size;
  89
  90   do
  91     {
  92       /* Attempt to read the link into the current buffer.  */
  93       ssize_t link_length = preadlinkat (fd, filename, buf, buf_size);
  94       size_t link_size;
  95       if (link_length < 0)
  96         {
  97           /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
  98              with errno == ERANGE if the buffer is too small.  */
  99           int readlinkat_errno = errno;
 100           if (readlinkat_errno != ERANGE)
 101             {
 102               if (buf != buffer)
 103                 {
 104                   alloc->free (buf);
 105                   errno = readlinkat_errno;
 106                 }
 107               return NULL;
 108             }
 109         }
 110
 111       link_size = link_length;
 112
 113       if (link_size < buf_size)
 114         {
 115           buf[link_size++] = '\0';
 116
 117           if (buf == stack_buf)
 118             {
 119               char *b = (char *) alloc->allocate (link_size);
 120               buf_size = link_size;
 121               if (! b)
 122                 break;
 123               memcpy (b, buf, link_size);
 124               buf = b;
 125             }
 126           else if (link_size < buf_size && buf != buffer && alloc->reallocate)
 127             {
 128               /* Shrink BUF before returning it.  */
 129               char *b = (char *) alloc->reallocate (buf, link_size);
 130               if (b)
 131                 buf = b;
 132             }
 133
 134           return buf;
 135         }
 136
 137       if (buf != buffer)
 138         alloc->free (buf);
 139
 140       if (buf_size <= buf_size_max / 2)
 141         buf_size *= 2;
 142       else if (buf_size < buf_size_max)
 143         buf_size = buf_size_max;
 144       else if (buf_size_max < SIZE_MAX)
 145         {
 146           errno = ENAMETOOLONG;
 147           return NULL;
 148         }
 149       else
 150         break;
 151       buf = (char *) alloc->allocate (buf_size);
 152     }
 153   while (buf);
 154
 155   if (alloc->die)
 156     alloc->die (buf_size);
 157   errno = ENOMEM;
 158   return NULL;
 159 }