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/* protsgix.c: PROTECTION (SIGNAL HANDLER) FOR UNIX
 *
 *  $Id: //info.ravenbrook.com/project/mps/version/1.116/code/protsgix.c#1 $
 *  Copyright (c) 2001-2014 Ravenbrook Limited.  See end of file for license.
 *
 * This implements protection exception handling using POSIX signals.
 * It is designed to run on any POSIX-compliant Unix, but currently is
 * only used on FreeBSD, as we have separate implementions for OS X
 * (see protxc.c) and Linux (see protli.c).
 *
 *
 * SOURCES
 *
 * .source.man: sigaction(2): FreeBSD System Calls Manual.
 *
 * .source.merge: A blend from primarily the FreeBSD version (protfri3.c)
 * and the OSF/1 (DIGITAL UNIX) version (proto1.c); influenced by other
 * Unix versions.
 */

#include "mpm.h"

#if !defined(MPS_OS_FR)
#error "protsgix.c is Unix-specific, currently for MPS_OS_FR"
#endif

#include <signal.h>    /* for many functions */
#include <sys/types.h> /* for getpid */
#include <unistd.h>    /* for getpid */

SRCID(protsgix, "$Id: //info.ravenbrook.com/project/mps/version/1.116/code/protsgix.c#1 $");


/* The previously-installed signal action, as returned by */
/* sigaction(3).  See ProtSetup. */

static struct sigaction sigNext;

/* sigHandle -- protection signal handler
 *
 *  This is the signal handler installed by ProtSetup to deal with
 *  protection faults.  It is installed on the PROT_SIGNAL (a macro
 *  defined according to the platform in config.h) signal.  It
 *  decodes the protection fault details from the signal context and
 *  passes them to ArenaAccess, which attempts to handle the fault and
 *  remove its cause.  If the fault is handled, then the handler
 *  returns and execution resumes.  If it isn't handled, then
 *  sigHandle does its best to pass the signal on to the previously
 *  installed signal handler (sigNext); which it does by signalling
 *  itself using kill(2).
 *
 *  .sigh.args: The sigaction manual page .source.man documents three
 *  different handler prototypes: ANSI C sa_handler, traditional BSD
 *  sa_handler, and POSIX SA_SIGINFO sa_sigaction.  The ANSI C
 *  prototype isn't powerful enough for us (can't get addresses), and
 *  the manual page deprecates the BSD sa_handler in favour of the
 *  POSIX SA_SIGINFO sa_sigaction.  In that prototype, the arguments
 *  are: signal number, pointer to signal info structure, pointer to
 *  signal context structure.
 *
 *  .sigh.context: We use the PROT_SIGINFO_GOOD macro to (usually) check
 *  the info->si_code.  The macro is platform dependent and defined in
 *  config.h.  We assume that info->si_addr is the fault address.  This
 *  assumption turns out to fail for PowerPC Darwin (we use protxcpp.c
 *  there).
 *
 *  .sigh.mode: The fault type (read/write) does not appear to be
 *  available to the signal handler (see mail archive).
 */

static void sigHandle(int sig, siginfo_t *info, void *context)  /* .sigh.args */
{
  int e;
  /* sigset renamed to asigset due to clash with global on Darwin. */
  sigset_t asigset, oldset;
  struct sigaction sa;
  
  UNUSED(context);
  AVER(sig == PROT_SIGNAL);

  /* .sigh.context */
  if(PROT_SIGINFO_GOOD(info)) {
    AccessSet mode;
    Addr base;

    mode = AccessREAD | AccessWRITE; /* .sigh.mode */

    /* We assume that the access is for one word at the address. */
    base = (Addr)info->si_addr;   /* .sigh.context */

    /* Offer each protection structure the opportunity to handle the */
    /* exception.  If it succeeds, then allow the mutator to continue. */
    if(ArenaAccess(base, mode, NULL))
      return;
  }

  /* The exception was not handled by any known protection structure, */
  /* so throw it to the previously installed handler. */

  e = sigaction(PROT_SIGNAL, &sigNext, &sa);
  AVER(e == 0);
  sigemptyset(&asigset);
  sigaddset(&asigset, PROT_SIGNAL);
  e = sigprocmask(SIG_UNBLOCK, &asigset, &oldset);
  AVER(e == 0);
  kill(getpid(), PROT_SIGNAL);
  e = sigprocmask(SIG_SETMASK, &oldset, NULL);
  AVER(e == 0);
  e = sigaction(PROT_SIGNAL, &sa, NULL);
  AVER(e == 0);
}


/*  ProtSetup -- global protection setup
 *
 *  Under Unix, the global setup involves installing a signal
 *  handler on PROT_SIGNAL to catch and handle page faults (see
 *  sigHandle).  The previous handler is recorded so that it can be
 *  reached from sigHandle if it fails to handle the fault.
 *
 *  NOTE: There are problems with this approach:
 *    1. we can't honor the sa_flags for the previous handler,
 *    2. what if this thread is suspended just after calling signal(3)?
 *       The sigNext variable will never be initialized!  */

void ProtSetup(void)
{
  struct sigaction sa;
  int result;

  sa.sa_sigaction = sigHandle;
  sigemptyset(&sa.sa_mask);
  sa.sa_flags = SA_SIGINFO;

  result = sigaction(PROT_SIGNAL, &sa, &sigNext);
  AVER(result == 0);
}


/* C. COPYRIGHT AND LICENSE
 *
 * Copyright (C) 2001-2014 Ravenbrook Limited <http://www.ravenbrook.com/>.
 * All rights reserved.  This is an open source license.  Contact
 * Ravenbrook for commercial licensing options.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 * 
 * 1. Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 * 
 * 2. Redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution.
 * 
 * 3. Redistributions in any form must be accompanied by information on how
 * to obtain complete source code for this software and any accompanying
 * software that uses this software.  The source code must either be
 * included in the distribution or be available for no more than the cost
 * of distribution plus a nominal fee, and must be freely redistributable
 * under reasonable conditions.  For an executable file, complete source
 * code means the source code for all modules it contains. It does not
 * include source code for modules or files that typically accompany the
 * major components of the operating system on which the executable file
 * runs.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
 * PURPOSE, OR NON-INFRINGEMENT, ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS AND CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
 * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
 * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */