/* Stack overflow handling.

   Copyright (C) 2002, 2004, 2006, 2008 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 <http://www.gnu.org/licenses/>.  */

/* Written by Paul Eggert.  */

/* NOTES:

   A program that uses alloca, dynamic arrays, or large local
   variables may extend the stack by more than a page at a time.  If
   so, when the stack overflows the operating system may not detect
   the overflow until the program uses the array, and this module may
   incorrectly report a program error instead of a stack overflow.

   To avoid this problem, allocate only small objects on the stack; a
   program should be OK if it limits single allocations to a page or
   less.  Allocate larger arrays in static storage, or on the heap
   (e.g., with malloc).  Yes, this is a pain, but we don't know of any
   better solution that is portable.

   No attempt has been made to deal with multithreaded applications.  */

#include <config.h>

#ifndef __attribute__
# if __GNUC__ < 3
#  define __attribute__(x)
# endif
#endif

#include "gettext.h"
#define _(msgid) gettext (msgid)

#include <errno.h>
#ifndef ENOTSUP
# define ENOTSUP EINVAL
#endif

#include <signal.h>
#if ! HAVE_STACK_T && ! defined stack_t
typedef struct sigaltstack stack_t;
#endif

#include <stdlib.h>
#include <string.h>

/* Posix 2001 declares ucontext_t in <ucontext.h>, Posix 200x in
   <signal.h>.  */
#if HAVE_UCONTEXT_H
# include <ucontext.h>
#endif

#include <unistd.h>
#ifndef STDERR_FILENO
# define STDERR_FILENO 2
#endif

#include "c-stack.h"
#include "exitfail.h"

#if defined SA_ONSTACK && defined SA_SIGINFO
# define SIGACTION_WORKS 1
#else
# define SIGACTION_WORKS 0
#endif

extern char *program_name;

/* The user-specified action to take when a SEGV-related program error
   or stack overflow occurs.  */
static void (* volatile segv_action) (int);

/* Translated messages for program errors and stack overflow.  Do not
   translate them in the signal handler, since gettext is not
   async-signal-safe.  */
static char const * volatile program_error_message;
static char const * volatile stack_overflow_message;

/* Output an error message, then exit with status EXIT_FAILURE if it
   appears to have been a stack overflow, or with a core dump
   otherwise.  This function is async-signal-safe.  */

static void die (int) __attribute__ ((noreturn));
static void
die (int signo)
{
  char const *message;
  segv_action (signo);
  message = signo ? program_error_message : stack_overflow_message;
  write (STDERR_FILENO, program_name, strlen (program_name));
  write (STDERR_FILENO, ": ", 2);
  write (STDERR_FILENO, message, strlen (message));
  write (STDERR_FILENO, "\n", 1);
  if (! signo)
    _exit (exit_failure);
  raise (signo);
  abort ();
}

#if HAVE_SIGALTSTACK && HAVE_DECL_SIGALTSTACK

/* Direction of the C runtime stack.  This function is
   async-signal-safe.  */

# if STACK_DIRECTION
#  define find_stack_direction(ptr) STACK_DIRECTION
# else
static int
find_stack_direction (char const *addr)
{
  char dummy;
  return ! addr ? find_stack_direction (&dummy) : addr < &dummy ? 1 : -1;
}
# endif

/* Storage for the alternate signal stack.  */
static union
{
  char buffer[SIGSTKSZ];

  /* These other members are for proper alignment.  There's no
     standard way to guarantee stack alignment, but this seems enough
     in practice.  */
  long double ld;
  long l;
  void *p;
} alternate_signal_stack;

# if SIGACTION_WORKS

/* Handle a segmentation violation and exit.  This function is
   async-signal-safe.  */

static void segv_handler (int, siginfo_t *, void *) __attribute__((noreturn));
static void
segv_handler (int signo, siginfo_t *info,
	      void *context __attribute__ ((unused)))
{
  /* Clear SIGNO if it seems to have been a stack overflow.  */
  if (0 < info->si_code)
    {
#  if ! HAVE_XSI_STACK_OVERFLOW_HEURISTIC
      /* We can't easily determine whether it is a stack overflow; so
	 assume that the rest of our program is perfect (!) and that
	 this segmentation violation is a stack overflow.  */
      signo = 0;
#  else
      /* If the faulting address is within the stack, or within one
	 page of the stack end, assume that it is a stack
	 overflow.  */
      ucontext_t const *user_context = context;
      char const *stack_base = user_context->uc_stack.ss_sp;
      size_t stack_size = user_context->uc_stack.ss_size;
      char const *faulting_address = info->si_addr;
      size_t s = faulting_address - stack_base;
      size_t page_size = sysconf (_SC_PAGESIZE);
      if (find_stack_direction (NULL) < 0)
	s += page_size;
      if (s < stack_size + page_size)
	signo = 0;

#   if DEBUG
      {
	char buf[1024];
	sprintf (buf,
		 "segv_handler fault=%p base=%p size=%lx page=%lx signo=%d\n",
		 faulting_address, stack_base, (unsigned long) stack_size,
		 (unsigned long) page_size, signo);
	write (STDERR_FILENO, buf, strlen (buf));
      }
#   endif
#  endif
    }

  die (signo);
}
# endif

static void
null_action (int signo __attribute__ ((unused)))
{
}

/* Set up ACTION so that it is invoked on C stack overflow.  Return -1
   (setting errno) if this cannot be done.

   When ACTION is called, it is passed an argument equal to SIGSEGV
   for a segmentation violation that does not appear related to stack
   overflow, and is passed zero otherwise.  On many platforms it is
   hard to tell; when in doubt, zero is passed.

   A null ACTION acts like an action that does nothing.

   ACTION must be async-signal-safe.  ACTION together with its callees
   must not require more than SIGSTKSZ bytes of stack space.  */

int
c_stack_action (void (*action) (int))
{
  int r;
  stack_t st;
  struct sigaction act;
  st.ss_flags = 0;
  st.ss_sp = alternate_signal_stack.buffer;
  st.ss_size = sizeof alternate_signal_stack.buffer;
  r = sigaltstack (&st, NULL);
  if (r != 0)
    return r;

  segv_action = action ? action : null_action;
  program_error_message = _("program error");
  stack_overflow_message = _("stack overflow");

  sigemptyset (&act.sa_mask);

# if SIGACTION_WORKS
  /* POSIX 1003.1-2001 says SA_RESETHAND implies SA_NODEFER, but
     this is not true on Solaris 8 at least.  It doesn't hurt to use
     SA_NODEFER here, so leave it in.  */
  act.sa_flags = SA_NODEFER | SA_ONSTACK | SA_RESETHAND | SA_SIGINFO;
  act.sa_sigaction = segv_handler;
# else
  act.sa_flags = SA_NODEFER | SA_RESETHAND;
  act.sa_handler = die;
# endif

  return sigaction (SIGSEGV, &act, NULL);
}

#else /* ! (HAVE_SIGALTSTACK && HAVE_DECL_SIGALTSTACK) */

int
c_stack_action (void (*action) (int)  __attribute__ ((unused)))
{
  errno = ENOTSUP;
  return -1;
}

#endif