File: careadlinkat.c

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/* Read symbolic links into a buffer without size limitation, relative to fd.

   Copyright (C) 2001, 2003-2004, 2007, 2009-2018 Free Software Foundation,
   Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */

/* Written by Paul Eggert, Bruno Haible, and Jim Meyering.  */

#include <config.h>

#include "careadlinkat.h"

#include <errno.h>
#include <limits.h>
#include <string.h>
#include <unistd.h>

/* Define this independently so that stdint.h is not a prerequisite.  */
#ifndef SIZE_MAX
# define SIZE_MAX ((size_t) -1)
#endif

#ifndef SSIZE_MAX
# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
#endif

#include "allocator.h"

/* Assuming the current directory is FD, get the symbolic link value
   of FILENAME as a null-terminated string and put it into a buffer.
   If FD is AT_FDCWD, FILENAME is interpreted relative to the current
   working directory, as in openat.

   If the link is small enough to fit into BUFFER put it there.
   BUFFER's size is BUFFER_SIZE, and BUFFER can be null
   if BUFFER_SIZE is zero.

   If the link is not small, put it into a dynamically allocated
   buffer managed by ALLOC.  It is the caller's responsibility to free
   the returned value if it is nonnull and is not BUFFER.  A null
   ALLOC stands for the standard allocator.

   The PREADLINKAT function specifies how to read links.  It operates
   like POSIX readlinkat()
   <http://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html>
   but can assume that its first argument is the same as FD.

   If successful, return the buffer address; otherwise return NULL and
   set errno.  */

char *
careadlinkat (int fd, char const *filename,
              char *buffer, size_t buffer_size,
              struct allocator const *alloc,
              ssize_t (*preadlinkat) (int, char const *, char *, size_t))
{
  char *buf;
  size_t buf_size;
  size_t buf_size_max =
    SSIZE_MAX < SIZE_MAX ? (size_t) SSIZE_MAX + 1 : SIZE_MAX;
  char stack_buf[1024];

  if (! alloc)
    alloc = &stdlib_allocator;

  if (! buffer_size)
    {
      /* Allocate the initial buffer on the stack.  This way, in the
         common case of a symlink of small size, we get away with a
         single small malloc() instead of a big malloc() followed by a
         shrinking realloc().  */
      buffer = stack_buf;
      buffer_size = sizeof stack_buf;
    }

  buf = buffer;
  buf_size = buffer_size;

  do
    {
      /* Attempt to read the link into the current buffer.  */
      ssize_t link_length = preadlinkat (fd, filename, buf, buf_size);
      size_t link_size;
      if (link_length < 0)
        {
          /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
             with errno == ERANGE if the buffer is too small.  */
          int readlinkat_errno = errno;
          if (readlinkat_errno != ERANGE)
            {
              if (buf != buffer)
                {
                  alloc->free (buf);
                  errno = readlinkat_errno;
                }
              return NULL;
            }
        }

      link_size = link_length;

      if (link_size < buf_size)
        {
          buf[link_size++] = '\0';

          if (buf == stack_buf)
            {
              char *b = (char *) alloc->allocate (link_size);
              buf_size = link_size;
              if (! b)
                break;
              memcpy (b, buf, link_size);
              buf = b;
            }
          else if (link_size < buf_size && buf != buffer && alloc->reallocate)
            {
              /* Shrink BUF before returning it.  */
              char *b = (char *) alloc->reallocate (buf, link_size);
              if (b)
                buf = b;
            }

          return buf;
        }

      if (buf != buffer)
        alloc->free (buf);

      if (buf_size <= buf_size_max / 2)
        buf_size *= 2;
      else if (buf_size < buf_size_max)
        buf_size = buf_size_max;
      else if (buf_size_max < SIZE_MAX)
        {
          errno = ENAMETOOLONG;
          return NULL;
        }
      else
        break;
      buf = (char *) alloc->allocate (buf_size);
    }
  while (buf);

  if (alloc->die)
    alloc->die (buf_size);
  errno = ENOMEM;
  return NULL;
}