lib/careadlinkat.c

   1 /* Read symbolic links into a buffer without size limitation, relative to fd.
   2
   3    Copyright (C) 2001, 2003-2004, 2007, 2009-2018 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 <https://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 #include <unistd.h>
  29
  30 /* Define this independently so that stdint.h is not a prerequisite.  */
  31 #ifndef SIZE_MAX
  32 # define SIZE_MAX ((size_t) -1)
  33 #endif
  34
  35 #ifndef SSIZE_MAX
  36 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
  37 #endif
  38
  39 #include "allocator.h"
  40
  41 /* Assuming the current directory is FD, get the symbolic link value
  42    of FILENAME as a null-terminated string and put it into a buffer.
  43    If FD is AT_FDCWD, FILENAME is interpreted relative to the current
  44    working directory, as in openat.
  45
  46    If the link is small enough to fit into BUFFER put it there.
  47    BUFFER's size is BUFFER_SIZE, and BUFFER can be null
  48    if BUFFER_SIZE is zero.
  49
  50    If the link is not small, put it into a dynamically allocated
  51    buffer managed by ALLOC.  It is the caller's responsibility to free
  52    the returned value if it is nonnull and is not BUFFER.  A null
  53    ALLOC stands for the standard allocator.
  54
  55    The PREADLINKAT function specifies how to read links.  It operates
  56    like POSIX readlinkat()
  57    <http://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html>
  58    but can assume that its first argument is the same as FD.
  59
  60    If successful, return the buffer address; otherwise return NULL and
  61    set errno.  */
  62
  63 char *
  64 careadlinkat (int fd, char const *filename,
  65               char *buffer, size_t buffer_size,
  66               struct allocator const *alloc,
  67               ssize_t (*preadlinkat) (int, char const *, char *, size_t))
  68 {
  69   char *buf;
  70   size_t buf_size;
  71   size_t buf_size_max =
  72     SSIZE_MAX < SIZE_MAX ? (size_t) SSIZE_MAX + 1 : SIZE_MAX;
  73   char stack_buf[1024];
  74
  75   if (! alloc)
  76     alloc = &stdlib_allocator;
  77
  78   if (! buffer_size)
  79     {
  80       /* Allocate the initial buffer on the stack.  This way, in the
  81          common case of a symlink of small size, we get away with a
  82          single small malloc() instead of a big malloc() followed by a
  83          shrinking realloc().  */
  84       buffer = stack_buf;
  85       buffer_size = sizeof stack_buf;
  86     }
  87
  88   buf = buffer;
  89   buf_size = buffer_size;
  90
  91   do
  92     {
  93       /* Attempt to read the link into the current buffer.  */
  94       ssize_t link_length = preadlinkat (fd, filename, buf, buf_size);
  95       size_t link_size;
  96       if (link_length < 0)
  97         {
  98           /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
  99              with errno == ERANGE if the buffer is too small.  */
 100           int readlinkat_errno = errno;
 101           if (readlinkat_errno != ERANGE)
 102             {
 103               if (buf != buffer)
 104                 {
 105                   alloc->free (buf);
 106                   errno = readlinkat_errno;
 107                 }
 108               return NULL;
 109             }
 110         }
 111
 112       link_size = link_length;
 113
 114       if (link_size < buf_size)
 115         {
 116           buf[link_size++] = '\0';
 117
 118           if (buf == stack_buf)
 119             {
 120               char *b = (char *) alloc->allocate (link_size);
 121               buf_size = link_size;
 122               if (! b)
 123                 break;
 124               memcpy (b, buf, link_size);
 125               buf = b;
 126             }
 127           else if (link_size < buf_size && buf != buffer && alloc->reallocate)
 128             {
 129               /* Shrink BUF before returning it.  */
 130               char *b = (char *) alloc->reallocate (buf, link_size);
 131               if (b)
 132                 buf = b;
 133             }
 134
 135           return buf;
 136         }
 137
 138       if (buf != buffer)
 139         alloc->free (buf);
 140
 141       if (buf_size <= buf_size_max / 2)
 142         buf_size *= 2;
 143       else if (buf_size < buf_size_max)
 144         buf_size = buf_size_max;
 145       else if (buf_size_max < SIZE_MAX)
 146         {
 147           errno = ENAMETOOLONG;
 148           return NULL;
 149         }
 150       else
 151         break;
 152       buf = (char *) alloc->allocate (buf_size);
 153     }
 154   while (buf);
 155
 156   if (alloc->die)
 157     alloc->die (buf_size);
 158   errno = ENOMEM;
 159   return NULL;
 160 }