auxv.c

/*******************************************************************************
 * Copyright IBM Corp. and others 2013
 *
 * This program and the accompanying materials are made available under
 * the terms of the Eclipse Public License 2.0 which accompanies this
 * distribution and is available at https://www.eclipse.org/legal/epl-2.0/
 * or the Apache License, Version 2.0 which accompanies this distribution and
 * is available at https://www.apache.org/licenses/LICENSE-2.0.
 *
 * This Source Code may also be made available under the following
 * Secondary Licenses when the conditions for such availability set
 * forth in the Eclipse Public License, v. 2.0 are satisfied: GNU
 * General Public License, version 2 with the GNU Classpath
 * Exception [1] and GNU General Public License, version 2 with the
 * OpenJDK Assembly Exception [2].
 *
 * [1] https://www.gnu.org/software/classpath/license.html
 * [2] https://openjdk.org/legal/assembly-exception.html
 *
 * SPDX-License-Identifier: EPL-2.0 OR Apache-2.0 OR GPL-2.0-only WITH Classpath-exception-2.0 OR GPL-2.0-only WITH OpenJDK-assembly-exception-1.0
 *******************************************************************************/

/* If your system is old enough that it doesn't have /proc/self/auxv then you'll
 * suffer the consequences if you're an ingenious fool and dlopen this library
 * after you've moved the env via a call to setenv!  Just don't do it.  */

#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <elf.h>
#include <link.h>
#include <pthread.h>
#include <auxv.h>
#include <errno.h>
#define __set_errno(the_errno)	*__errno_location() = the_errno

static char **saved_environ = NULL;

/* The user space application may perform a setenv which would effectively move
 * the env and therefore the assumed position of the auxv following the env
 * wouldn't be valid.  The constructor for this library saves the original
 * position of the env so that it can find the auxv if it needs to use the
 * fallback method, i.e., no /proc/self/auxv.  */
void __attribute__ ((__constructor__)) __attribute__ ((__visibility__ ("hidden")))
libauxv_init (void)
{
  saved_environ = environ;
}

/* Scan to the end of the saved environ to find the aux vector.  */
static ElfW(auxv_t) * __attribute__ ((__noinline__))
__auxv_fallback (void)
{
  char **result = saved_environ;
  /* Scan over the env vector looking for the ending NULL */
  for (; *result != NULL; ++result)
    {
    }
  /* Bump the pointer one more step, which should be the auxv. */
  return (ElfW (auxv_t) *) ++result;
}

volatile static ElfW(auxv_t) *auxv_buf = NULL;

/* pthread_once won't return a ret val from the init routine so we have to
 * do this ourselves.  This needs to be process global so that regardless of
 * which thread ends up calling into fetch_auxv(), if the initialization has
 * failed, the init_errno will be visible to all other threads and they can set
 * their thread local errno from it.  This gets cleared everytime a thread makes
 * it into fetch_auxv() and successfully allocates auxv_buf.  */
static int init_errno = 0;

static pthread_once_t auxv_once_control = PTHREAD_ONCE_INIT;

/* Always return 0 in an error case because this is always called via
 * pthread_once and the return value isn't a condition of successful
 * fetch_auxv().  The init_errno indicates a successful fetch_auxv().  */
static ElfW(auxv_t) *
fetch_auxv()
{
  extern int getpagesize (void);
  ElfW(auxv_t) *auxv_temp = (ElfW(auxv_t) *)auxv_buf;
  int auxv_f;
  size_t page_size = getpagesize();
  ssize_t bytes;

  /* If the /proc/self/auxv file has not been copied into the heap
     yet, then do it */

  if(auxv_temp == NULL)
    {
      auxv_f = open("/proc/self/auxv", O_RDONLY);

      if(auxv_f == -1)
	{
	  /* Older kernels did not support /proc/<PID>/auxv. But the auvx
	  table does exist in the process address space following the
	  env table. So try scanning over the environment table to find
	  the auxv. */
	  if (errno == ENOENT)
	    {
	      /* Hopefully we successfully captured the __environ with the
	       * constructor before the user was able to move the env with a
	       * call to setenv.  This is most likely only possible if they call
	       * libauxv via dlopen().  If that were to happen the behavior is
	       * undefined.  */
	      auxv_temp = __auxv_fallback();
	    }
	  else
	    {
	      init_errno = errno;
	      perror("Error opening file /proc/self/auxv for reading.");
	      return 0;
	    }
	}
      else
	{
	  auxv_temp = (ElfW(auxv_t) *)malloc(page_size);
	  if (!auxv_temp)
	    {
	      init_errno = errno;
	      perror("Allocation of space for auxv failed.");
	      return 0;
	    }
	  bytes = read (auxv_f, (void*)auxv_temp, page_size);

	  if (bytes <=0)
	    {
	      free (auxv_temp);
	      auxv_temp = NULL;
	      close (auxv_f);
	      init_errno = errno;
	      perror("Error /proc/self/auxv read failed");
	      return 0;
	    }

	  /* We have the auxv buffered and the file handle is no longer
	     needed.  Close the file. */
	  if ( close (auxv_f) )
	    {
	      init_errno = errno;
	      perror("Error close failed");
	    }
	}
      init_errno=0; /* Clear it out each time through so failures can be retried.  */
      auxv_buf = auxv_temp;
    }
  return (ElfW(auxv_t) *)auxv_temp;
}


int prefetch_auxv(void)
{
  ElfW(auxv_t) *auxv_temp = (ElfW(auxv_t) *)auxv_buf;

  if(auxv_temp == NULL)
    {
      if (pthread_once (&auxv_once_control, (void (*)(void))fetch_auxv))
	{
	  perror("Error pthread_once(fetch_auxv()) failed");
	  /* pthread_once's errno should be valid.  */
	  return -1;
	}
      /* Assuming that pthread_once is blocking, this is valid.  */
      auxv_temp = (ElfW(auxv_t) *)auxv_buf;
    }

  /* Since pthread_once doesn't give a return value from fetch_auxv() we have
   * to see if the fetch_auxv() set our process global errno to indicate a
   * problem.  This may have happened in another thread as well, and if so,
   * we should propogate it to this thread's local errno and fail.  */

  if(auxv_temp == NULL)
    {
      __set_errno(init_errno);
      return -1;
    }

  return 0;
}

ElfW(auxv_t) *
get_auxv(void)
{
  return (ElfW(auxv_t) *)auxv_buf;
}

ElfW(Addr)
query_auxv(ElfW(Addr) type)
{
  ElfW(auxv_t) *auxv_temp;
  long i = 0;

  /* If the /proc/self/auxv file has not been manually copied into the heap
     yet, then do it */

  auxv_temp = (ElfW(auxv_t) *)auxv_buf;

  if(auxv_temp == NULL)
    {
      if (pthread_once (&auxv_once_control, (void (*)(void))fetch_auxv))
	{
	  perror("Error pthread_once(fetch_auxv()) failed");
	  /* pthread_once's errno should be valid.  */
	  return -1;
	}
      /* Assuming that pthread_once is blocking, this is valid.  */
      auxv_temp = (ElfW(auxv_t) *)auxv_buf;
    }

  /* Since pthread_once doesn't give a return value from fetch_auxv() we have
   * to see if the fetch_auxv() set our process global errno to indicate a
   * problem.  This may have happened in another thread as well, and if so,
   * we should propogate it to this thread's local errno and fail.  */

  if(auxv_temp == NULL)
    {
      __set_errno(init_errno);
      return -1;
    }

  i = 0;
  do
    {
      if(auxv_temp[i].a_type == type)
	{
	  return auxv_temp[i].a_un.a_val;
	}
      i++;
    } while (auxv_temp[i].a_type != AT_NULL);

  return 0;
}