lib/careadlinkat.c

   1 /* Read symbolic links into a buffer without size limitation, relative to fd.
   2
   3    Copyright (C) 2001, 2003-2004, 2007, 2009-2012 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 <stdlib.h>
  28 #include <string.h>
  29 #include <unistd.h>
  30
  31 /* Define this independently so that stdint.h is not a prerequisite.  */
  32 #ifndef SIZE_MAX
  33 # define SIZE_MAX ((size_t) -1)
  34 #endif
  35
  36 #ifndef SSIZE_MAX
  37 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
  38 #endif
  39
  40 #include "allocator.h"
  41
  42 /* Get the symbolic link value of FILENAME and put it into BUFFER, with
  43    size BUFFER_SIZE.  This function acts like readlink  but has
  44    readlinkat's signature.  */
  45 ssize_t
  46 careadlinkatcwd (int fd, char const *filename, char *buffer,
  47                  size_t buffer_size)
  48 {
  49   /* FD must be AT_FDCWD here, otherwise the caller is using this
  50      function in contexts for which it was not meant for.  */
  51   if (fd != AT_FDCWD)
  52     abort ();
  53   return readlink (filename, buffer, buffer_size);
  54 }
  55
  56 /* Assuming the current directory is FD, get the symbolic link value
  57    of FILENAME as a null-terminated string and put it into a buffer.
  58    If FD is AT_FDCWD, FILENAME is interpreted relative to the current
  59    working directory, as in openat.
  60
  61    If the link is small enough to fit into BUFFER put it there.
  62    BUFFER's size is BUFFER_SIZE, and BUFFER can be null
  63    if BUFFER_SIZE is zero.
  64
  65    If the link is not small, put it into a dynamically allocated
  66    buffer managed by ALLOC.  It is the caller's responsibility to free
  67    the returned value if it is nonnull and is not BUFFER.  A null
  68    ALLOC stands for the standard allocator.
  69
  70    The PREADLINKAT function specifies how to read links.  It operates
  71    like POSIX readlinkat()
  72    <http://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html>
  73    but can assume that its first argument is the same as FD.
  74
  75    If successful, return the buffer address; otherwise return NULL and
  76    set errno.  */
  77
  78 char *
  79 careadlinkat (int fd, char const *filename,
  80               char *buffer, size_t buffer_size,
  81               struct allocator const *alloc,
  82               ssize_t (*preadlinkat) (int, char const *, char *, size_t))
  83 {
  84   char *buf;
  85   size_t buf_size;
  86   size_t buf_size_max =
  87     SSIZE_MAX < SIZE_MAX ? (size_t) SSIZE_MAX + 1 : SIZE_MAX;
  88   char stack_buf[1024];
  89
  90   if (! alloc)
  91     alloc = &stdlib_allocator;
  92
  93   if (! buffer_size)
  94     {
  95       /* Allocate the initial buffer on the stack.  This way, in the
  96          common case of a symlink of small size, we get away with a
  97          single small malloc() instead of a big malloc() followed by a
  98          shrinking realloc().  */
  99       buffer = stack_buf;
 100       buffer_size = sizeof stack_buf;
 101     }
 102
 103   buf = buffer;
 104   buf_size = buffer_size;
 105
 106   do
 107     {
 108       /* Attempt to read the link into the current buffer.  */
 109       ssize_t link_length = preadlinkat (fd, filename, buf, buf_size);
 110       size_t link_size;
 111       if (link_length < 0)
 112         {
 113           /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
 114              with errno == ERANGE if the buffer is too small.  */
 115           int readlinkat_errno = errno;
 116           if (readlinkat_errno != ERANGE)
 117             {
 118               if (buf != buffer)
 119                 {
 120                   alloc->free (buf);
 121                   errno = readlinkat_errno;
 122                 }
 123               return NULL;
 124             }
 125         }
 126
 127       link_size = link_length;
 128
 129       if (link_size < buf_size)
 130         {
 131           buf[link_size++] = '\0';
 132
 133           if (buf == stack_buf)
 134             {
 135               char *b = (char *) alloc->allocate (link_size);
 136               buf_size = link_size;
 137               if (! b)
 138                 break;
 139               memcpy (b, buf, link_size);
 140               buf = b;
 141             }
 142           else if (link_size < buf_size && buf != buffer && alloc->reallocate)
 143             {
 144               /* Shrink BUF before returning it.  */
 145               char *b = (char *) alloc->reallocate (buf, link_size);
 146               if (b)
 147                 buf = b;
 148             }
 149
 150           return buf;
 151         }
 152
 153       if (buf != buffer)
 154         alloc->free (buf);
 155
 156       if (buf_size <= buf_size_max / 2)
 157         buf_size *= 2;
 158       else if (buf_size < buf_size_max)
 159         buf_size = buf_size_max;
 160       else if (buf_size_max < SIZE_MAX)
 161         {
 162           errno = ENAMETOOLONG;
 163           return NULL;
 164         }
 165       else
 166         break;
 167       buf = (char *) alloc->allocate (buf_size);
 168     }
 169   while (buf);
 170
 171   if (alloc->die)
 172     alloc->die (buf_size);
 173   errno = ENOMEM;
 174   return NULL;
 175 }