src/hotspot/os_cpu/bsd_x86/os_bsd_x86.cpp

/*
 * Copyright (c) 1999, 2025, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code 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
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

#include "asm/macroAssembler.hpp"
#include "classfile/vmSymbols.hpp"
#include "code/codeCache.hpp"
#include "code/vtableStubs.hpp"
#include "interpreter/interpreter.hpp"
#include "jvm.h"
#include "logging/log.hpp"
#include "memory/allocation.inline.hpp"
#include "os_bsd.hpp"
#include "os_posix.hpp"
#include "prims/jniFastGetField.hpp"
#include "prims/jvm_misc.hpp"
#include "runtime/arguments.hpp"
#include "runtime/frame.inline.hpp"
#include "runtime/interfaceSupport.inline.hpp"
#include "runtime/java.hpp"
#include "runtime/javaCalls.hpp"
#include "runtime/javaThread.hpp"
#include "runtime/mutexLocker.hpp"
#include "runtime/osThread.hpp"
#include "runtime/safepointMechanism.hpp"
#include "runtime/sharedRuntime.hpp"
#include "runtime/stubRoutines.hpp"
#include "runtime/timer.hpp"
#include "signals_posix.hpp"
#include "utilities/align.hpp"
#include "utilities/events.hpp"
#include "utilities/vmError.hpp"

// put OS-includes here
# include <sys/types.h>
# include <sys/mman.h>
# include <pthread.h>
# include <signal.h>
# include <errno.h>
# include <dlfcn.h>
# include <stdlib.h>
# include <stdio.h>
# include <unistd.h>
# include <sys/resource.h>
# include <sys/stat.h>
# include <sys/time.h>
# include <sys/utsname.h>
# include <sys/socket.h>
# include <sys/wait.h>
# include <pwd.h>
# include <poll.h>
#ifndef __OpenBSD__
# include <ucontext.h>
#endif

#if !defined(__APPLE__) && !defined(__NetBSD__)
# include <pthread_np.h>
#endif

// needed by current_stack_base_and_size() workaround for Mavericks
#if defined(__APPLE__)
# include <errno.h>
# include <sys/types.h>
# include <sys/sysctl.h>
# define DEFAULT_MAIN_THREAD_STACK_PAGES 2048
# define OS_X_10_9_0_KERNEL_MAJOR_VERSION 13
#endif

#ifdef AMD64
#define SPELL_REG_SP "rsp"
#define SPELL_REG_FP "rbp"
#define REG_BCP context_r13
#else
#define SPELL_REG_SP "esp"
#define SPELL_REG_FP "ebp"
#endif // AMD64

#ifdef __FreeBSD__
# define context_trapno uc_mcontext.mc_trapno
# ifdef AMD64
#  define context_pc uc_mcontext.mc_rip
#  define context_sp uc_mcontext.mc_rsp
#  define context_fp uc_mcontext.mc_rbp
#  define context_rip uc_mcontext.mc_rip
#  define context_rsp uc_mcontext.mc_rsp
#  define context_rbp uc_mcontext.mc_rbp
#  define context_rax uc_mcontext.mc_rax
#  define context_rbx uc_mcontext.mc_rbx
#  define context_rcx uc_mcontext.mc_rcx
#  define context_rdx uc_mcontext.mc_rdx
#  define context_rsi uc_mcontext.mc_rsi
#  define context_rdi uc_mcontext.mc_rdi
#  define context_r8  uc_mcontext.mc_r8
#  define context_r9  uc_mcontext.mc_r9
#  define context_r10 uc_mcontext.mc_r10
#  define context_r11 uc_mcontext.mc_r11
#  define context_r12 uc_mcontext.mc_r12
#  define context_r13 uc_mcontext.mc_r13
#  define context_r14 uc_mcontext.mc_r14
#  define context_r15 uc_mcontext.mc_r15
#  define context_flags uc_mcontext.mc_flags
#  define context_err uc_mcontext.mc_err
# else
#  define context_pc uc_mcontext.mc_eip
#  define context_sp uc_mcontext.mc_esp
#  define context_fp uc_mcontext.mc_ebp
#  define context_eip uc_mcontext.mc_eip
#  define context_esp uc_mcontext.mc_esp
#  define context_eax uc_mcontext.mc_eax
#  define context_ebx uc_mcontext.mc_ebx
#  define context_ecx uc_mcontext.mc_ecx
#  define context_edx uc_mcontext.mc_edx
#  define context_ebp uc_mcontext.mc_ebp
#  define context_esi uc_mcontext.mc_esi
#  define context_edi uc_mcontext.mc_edi
#  define context_eflags uc_mcontext.mc_eflags
#  define context_trapno uc_mcontext.mc_trapno
# endif
#endif

#ifdef __APPLE__
# if __DARWIN_UNIX03 && (MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_5)
  // 10.5 UNIX03 member name prefixes
  #define DU3_PREFIX(s, m) __ ## s.__ ## m
# else
  #define DU3_PREFIX(s, m) s ## . ## m
# endif

# ifdef AMD64
#  define context_pc context_rip
#  define context_sp context_rsp
#  define context_fp context_rbp
#  define context_rip uc_mcontext->DU3_PREFIX(ss,rip)
#  define context_rsp uc_mcontext->DU3_PREFIX(ss,rsp)
#  define context_rax uc_mcontext->DU3_PREFIX(ss,rax)
#  define context_rbx uc_mcontext->DU3_PREFIX(ss,rbx)
#  define context_rcx uc_mcontext->DU3_PREFIX(ss,rcx)
#  define context_rdx uc_mcontext->DU3_PREFIX(ss,rdx)
#  define context_rbp uc_mcontext->DU3_PREFIX(ss,rbp)
#  define context_rsi uc_mcontext->DU3_PREFIX(ss,rsi)
#  define context_rdi uc_mcontext->DU3_PREFIX(ss,rdi)
#  define context_r8  uc_mcontext->DU3_PREFIX(ss,r8)
#  define context_r9  uc_mcontext->DU3_PREFIX(ss,r9)
#  define context_r10 uc_mcontext->DU3_PREFIX(ss,r10)
#  define context_r11 uc_mcontext->DU3_PREFIX(ss,r11)
#  define context_r12 uc_mcontext->DU3_PREFIX(ss,r12)
#  define context_r13 uc_mcontext->DU3_PREFIX(ss,r13)
#  define context_r14 uc_mcontext->DU3_PREFIX(ss,r14)
#  define context_r15 uc_mcontext->DU3_PREFIX(ss,r15)
#  define context_flags uc_mcontext->DU3_PREFIX(ss,rflags)
#  define context_trapno uc_mcontext->DU3_PREFIX(es,trapno)
#  define context_err uc_mcontext->DU3_PREFIX(es,err)
# else
#  define context_pc context_eip
#  define context_sp context_esp
#  define context_fp context_ebp
#  define context_eip uc_mcontext->DU3_PREFIX(ss,eip)
#  define context_esp uc_mcontext->DU3_PREFIX(ss,esp)
#  define context_eax uc_mcontext->DU3_PREFIX(ss,eax)
#  define context_ebx uc_mcontext->DU3_PREFIX(ss,ebx)
#  define context_ecx uc_mcontext->DU3_PREFIX(ss,ecx)
#  define context_edx uc_mcontext->DU3_PREFIX(ss,edx)
#  define context_ebp uc_mcontext->DU3_PREFIX(ss,ebp)
#  define context_esi uc_mcontext->DU3_PREFIX(ss,esi)
#  define context_edi uc_mcontext->DU3_PREFIX(ss,edi)
#  define context_eflags uc_mcontext->DU3_PREFIX(ss,eflags)
#  define context_trapno uc_mcontext->DU3_PREFIX(es,trapno)
# endif
#endif

#ifdef __OpenBSD__
# define context_trapno sc_trapno
# ifdef AMD64
#  define context_pc sc_rip
#  define context_sp sc_rsp
#  define context_fp sc_rbp
#  define context_rip sc_rip
#  define context_rsp sc_rsp
#  define context_rbp sc_rbp
#  define context_rax sc_rax
#  define context_rbx sc_rbx
#  define context_rcx sc_rcx
#  define context_rdx sc_rdx
#  define context_rsi sc_rsi
#  define context_rdi sc_rdi
#  define context_r8  sc_r8
#  define context_r9  sc_r9
#  define context_r10 sc_r10
#  define context_r11 sc_r11
#  define context_r12 sc_r12
#  define context_r13 sc_r13
#  define context_r14 sc_r14
#  define context_r15 sc_r15
#  define context_flags sc_rflags
#  define context_err sc_err
# else
#  define context_pc sc_eip
#  define context_sp sc_esp
#  define context_fp sc_ebp
#  define context_eip sc_eip
#  define context_esp sc_esp
#  define context_eax sc_eax
#  define context_ebx sc_ebx
#  define context_ecx sc_ecx
#  define context_edx sc_edx
#  define context_ebp sc_ebp
#  define context_esi sc_esi
#  define context_edi sc_edi
#  define context_eflags sc_eflags
#  define context_trapno sc_trapno
# endif
#endif

#ifdef __NetBSD__
# define context_trapno uc_mcontext.__gregs[_REG_TRAPNO]
# ifdef AMD64
#  define __register_t __greg_t
#  define context_pc uc_mcontext.__gregs[_REG_RIP]
#  define context_sp uc_mcontext.__gregs[_REG_URSP]
#  define context_fp uc_mcontext.__gregs[_REG_RBP]
#  define context_rip uc_mcontext.__gregs[_REG_RIP]
#  define context_rsp uc_mcontext.__gregs[_REG_URSP]
#  define context_rax uc_mcontext.__gregs[_REG_RAX]
#  define context_rbx uc_mcontext.__gregs[_REG_RBX]
#  define context_rcx uc_mcontext.__gregs[_REG_RCX]
#  define context_rdx uc_mcontext.__gregs[_REG_RDX]
#  define context_rbp uc_mcontext.__gregs[_REG_RBP]
#  define context_rsi uc_mcontext.__gregs[_REG_RSI]
#  define context_rdi uc_mcontext.__gregs[_REG_RDI]
#  define context_r8  uc_mcontext.__gregs[_REG_R8]
#  define context_r9  uc_mcontext.__gregs[_REG_R9]
#  define context_r10 uc_mcontext.__gregs[_REG_R10]
#  define context_r11 uc_mcontext.__gregs[_REG_R11]
#  define context_r12 uc_mcontext.__gregs[_REG_R12]
#  define context_r13 uc_mcontext.__gregs[_REG_R13]
#  define context_r14 uc_mcontext.__gregs[_REG_R14]
#  define context_r15 uc_mcontext.__gregs[_REG_R15]
#  define context_flags uc_mcontext.__gregs[_REG_RFL]
#  define context_err uc_mcontext.__gregs[_REG_ERR]
# else
#  define context_pc uc_mcontext.__gregs[_REG_EIP]
#  define context_sp uc_mcontext.__gregs[_REG_UESP]
#  define context_fp uc_mcontext.__gregs[_REG_EBP]
#  define context_eip uc_mcontext.__gregs[_REG_EIP]
#  define context_esp uc_mcontext.__gregs[_REG_UESP]
#  define context_eax uc_mcontext.__gregs[_REG_EAX]
#  define context_ebx uc_mcontext.__gregs[_REG_EBX]
#  define context_ecx uc_mcontext.__gregs[_REG_ECX]
#  define context_edx uc_mcontext.__gregs[_REG_EDX]
#  define context_ebp uc_mcontext.__gregs[_REG_EBP]
#  define context_esi uc_mcontext.__gregs[_REG_ESI]
#  define context_edi uc_mcontext.__gregs[_REG_EDI]
#  define context_eflags uc_mcontext.__gregs[_REG_EFL]
#  define context_trapno uc_mcontext.__gregs[_REG_TRAPNO]
# endif
#endif

address os::current_stack_pointer() {
#if defined(__clang__) || defined(__llvm__)
  void *esp;
  __asm__("mov %%" SPELL_REG_SP ", %0":"=r"(esp));
  return (address) esp;
#else
  register void *esp __asm__ (SPELL_REG_SP);
  return (address) esp;
#endif
}

char* os::non_memory_address_word() {
  // Must never look like an address returned by reserve_memory,
  // even in its subfields (as defined by the CPU immediate fields,
  // if the CPU splits constants across multiple instructions).

  return (char*) -1;
}

address os::Posix::ucontext_get_pc(const ucontext_t * uc) {
  return (address)uc->context_pc;
}

void os::Posix::ucontext_set_pc(ucontext_t * uc, address pc) {
  uc->context_pc = (intptr_t)pc ;
}

intptr_t* os::Bsd::ucontext_get_sp(const ucontext_t * uc) {
  return (intptr_t*)uc->context_sp;
}

intptr_t* os::Bsd::ucontext_get_fp(const ucontext_t * uc) {
  return (intptr_t*)uc->context_fp;
}

address os::fetch_frame_from_context(const void* ucVoid,
                    intptr_t** ret_sp, intptr_t** ret_fp) {

  address  epc;
  const ucontext_t* uc = (const ucontext_t*)ucVoid;

  if (uc != nullptr) {
    epc = os::Posix::ucontext_get_pc(uc);
    if (ret_sp) *ret_sp = os::Bsd::ucontext_get_sp(uc);
    if (ret_fp) *ret_fp = os::Bsd::ucontext_get_fp(uc);
  } else {
    epc = nullptr;
    if (ret_sp) *ret_sp = (intptr_t *)nullptr;
    if (ret_fp) *ret_fp = (intptr_t *)nullptr;
  }

  return epc;
}

frame os::fetch_frame_from_context(const void* ucVoid) {
  intptr_t* sp;
  intptr_t* fp;
  address epc = fetch_frame_from_context(ucVoid, &sp, &fp);
  if (!is_readable_pointer(epc)) {
    // Try to recover from calling into bad memory
    // Assume new frame has not been set up, the same as
    // compiled frame stack bang
    return fetch_compiled_frame_from_context(ucVoid);
  }
  return frame(sp, fp, epc);
}

frame os::fetch_compiled_frame_from_context(const void* ucVoid) {
  const ucontext_t* uc = (const ucontext_t*)ucVoid;
  frame fr = os::fetch_frame_from_context(uc);
  // in compiled code, the stack banging is performed just after the return pc
  // has been pushed on the stack
  return frame(fr.sp() + 1, fr.fp(), (address)*(fr.sp()));
}

intptr_t* os::fetch_bcp_from_context(const void* ucVoid) {
  assert(ucVoid != nullptr, "invariant");
  const ucontext_t* uc = (const ucontext_t*)ucVoid;
  assert(os::Posix::ucontext_is_interpreter(uc), "invariant");
  return reinterpret_cast<intptr_t*>(uc->REG_BCP);
}

// By default, gcc always save frame pointer (%ebp/%rbp) on stack. It may get
// turned off by -fomit-frame-pointer,
frame os::get_sender_for_C_frame(frame* fr) {
  return frame(fr->sender_sp(), fr->link(), fr->sender_pc());
}

static intptr_t* _get_previous_fp() {
#if defined(__clang__) || defined(__llvm__)
  intptr_t **ebp;
  __asm__("mov %%" SPELL_REG_FP ", %0":"=r"(ebp));
#else
  register intptr_t **ebp __asm__ (SPELL_REG_FP);
#endif
  // ebp is for this frame (_get_previous_fp). We want the ebp for the
  // caller of os::current_frame*(), so go up two frames. However, for
  // optimized builds, _get_previous_fp() will be inlined, so only go
  // up 1 frame in that case.
#ifdef _NMT_NOINLINE_
  return **(intptr_t***)ebp;
#else
  return *ebp;
#endif
}


frame os::current_frame() {
  intptr_t* fp = _get_previous_fp();
  frame myframe((intptr_t*)os::current_stack_pointer(),
                (intptr_t*)fp,
                CAST_FROM_FN_PTR(address, os::current_frame));
  if (os::is_first_C_frame(&myframe)) {
    // stack is not walkable
    return frame();
  } else {
    return os::get_sender_for_C_frame(&myframe);
  }
}

// From IA32 System Programming Guide
enum {
  trap_page_fault = 0xE
};

bool PosixSignals::pd_hotspot_signal_handler(int sig, siginfo_t* info,
                                             ucontext_t* uc, JavaThread* thread) {
  // decide if this trap can be handled by a stub
  address stub = nullptr;

  address pc          = nullptr;

  //%note os_trap_1
  if (info != nullptr && uc != nullptr && thread != nullptr) {
    pc = (address) os::Posix::ucontext_get_pc(uc);

    // Handle ALL stack overflow variations here
    if (sig == SIGSEGV || sig == SIGBUS) {
      address addr = (address) info->si_addr;

      // check if fault address is within thread stack
      if (thread->is_in_full_stack(addr)) {
        // stack overflow
        if (os::Posix::handle_stack_overflow(thread, addr, pc, uc, &stub)) {
          return true; // continue
        }
      }
    }

    if ((sig == SIGSEGV || sig == SIGBUS) && VM_Version::is_cpuinfo_segv_addr(pc)) {
      // Verify that OS save/restore AVX registers.
      stub = VM_Version::cpuinfo_cont_addr();
    }

    if ((sig == SIGSEGV || sig == SIGBUS) && VM_Version::is_cpuinfo_segv_addr_apx(pc)) {
      // Verify that OS save/restore APX registers.
      stub = VM_Version::cpuinfo_cont_addr_apx();
      VM_Version::clear_apx_test_state();
    }

    // We test if stub is already set (by the stack overflow code
    // above) so it is not overwritten by the code that follows. This
    // check is not required on other platforms, because on other
    // platforms we check for SIGSEGV only or SIGBUS only, where here
    // we have to check for both SIGSEGV and SIGBUS.
    if (thread->thread_state() == _thread_in_Java && stub == nullptr) {
      // Java thread running in Java code => find exception handler if any
      // a fault inside compiled code, the interpreter, or a stub

      if ((sig == SIGSEGV || sig == SIGBUS) && SafepointMechanism::is_poll_address((address)info->si_addr)) {
        stub = SharedRuntime::get_poll_stub(pc);
#if defined(__APPLE__)
      // 32-bit Darwin reports a SIGBUS for nearly all memory access exceptions.
      // 64-bit Darwin may also use a SIGBUS (seen with compressed oops).
      // Catching SIGBUS here prevents the implicit SIGBUS null check below from
      // being called, so only do so if the implicit null check is not necessary.
      } else if (sig == SIGBUS && !MacroAssembler::uses_implicit_null_check(info->si_addr)) {
#else
      } else if (sig == SIGBUS /* && info->si_code == BUS_OBJERR */) {
#endif
        // BugId 4454115: A read from a MappedByteBuffer can fault
        // here if the underlying file has been truncated.
        // Do not crash the VM in such a case.
        CodeBlob* cb = CodeCache::find_blob(pc);
        nmethod* nm = (cb != nullptr) ? cb->as_nmethod_or_null() : nullptr;
        bool is_unsafe_memory_access = thread->doing_unsafe_access() && UnsafeMemoryAccess::contains_pc(pc);
        if ((nm != nullptr && nm->has_unsafe_access()) || is_unsafe_memory_access) {
          address next_pc = Assembler::locate_next_instruction(pc);
          if (is_unsafe_memory_access) {
            next_pc = UnsafeMemoryAccess::page_error_continue_pc(pc);
          }
          stub = SharedRuntime::handle_unsafe_access(thread, next_pc);
        }
      } else
#ifdef AMD64
      if (sig == SIGFPE &&
          (info->si_code == FPE_INTDIV || info->si_code == FPE_FLTDIV
           // Workaround for macOS ARM incorrectly reporting FPE_FLTINV for "div by 0"
           // instead of the expected FPE_FLTDIV when running x86_64 binary under Rosetta emulation
           MACOS_ONLY(|| (VM_Version::is_cpu_emulated() && info->si_code == FPE_FLTINV)))) {
        stub =
          SharedRuntime::
          continuation_for_implicit_exception(thread,
                                              pc,
                                              SharedRuntime::
                                              IMPLICIT_DIVIDE_BY_ZERO);
#ifdef __APPLE__
      } else if (sig == SIGFPE && info->si_code == FPE_NOOP) {
        int op = pc[0];

        // Skip REX
        if ((pc[0] & 0xf0) == 0x40) {
          op = pc[1];
        } else {
          op = pc[0];
        }

        // Check for IDIV
        if (op == 0xF7) {
          stub = SharedRuntime::continuation_for_implicit_exception(thread, pc, SharedRuntime:: IMPLICIT_DIVIDE_BY_ZERO);
        } else {
          // TODO: handle more cases if we are using other x86 instructions
          //   that can generate SIGFPE signal.
          tty->print_cr("unknown opcode 0x%X with SIGFPE.", op);
          fatal("please update this code.");
        }
#endif /* __APPLE__ */

#else
      if (sig == SIGFPE /* && info->si_code == FPE_INTDIV */) {
        // HACK: si_code does not work on bsd 2.2.12-20!!!
        int op = pc[0];
        if (op == 0xDB) {
          // FIST
          // TODO: The encoding of D2I in x86_32.ad can cause an exception
          // prior to the fist instruction if there was an invalid operation
          // pending. We want to dismiss that exception. From the win_32
          // side it also seems that if it really was the fist causing
          // the exception that we do the d2i by hand with different
          // rounding. Seems kind of weird.
          // NOTE: that we take the exception at the NEXT floating point instruction.
          assert(pc[0] == 0xDB, "not a FIST opcode");
          assert(pc[1] == 0x14, "not a FIST opcode");
          assert(pc[2] == 0x24, "not a FIST opcode");
          return true;
        } else if (op == 0xF7) {
          // IDIV
          stub = SharedRuntime::continuation_for_implicit_exception(thread, pc, SharedRuntime::IMPLICIT_DIVIDE_BY_ZERO);
        } else {
          // TODO: handle more cases if we are using other x86 instructions
          //   that can generate SIGFPE signal on bsd.
          tty->print_cr("unknown opcode 0x%X with SIGFPE.", op);
          fatal("please update this code.");
        }
#endif // AMD64
      } else if ((sig == SIGSEGV || sig == SIGBUS) &&
                 MacroAssembler::uses_implicit_null_check(info->si_addr)) {
          // Determination of interpreter/vtable stub/compiled code null exception
          stub = SharedRuntime::continuation_for_implicit_exception(thread, pc, SharedRuntime::IMPLICIT_NULL);
      }
    } else if ((thread->thread_state() == _thread_in_vm ||
                thread->thread_state() == _thread_in_native) &&
               sig == SIGBUS && /* info->si_code == BUS_OBJERR && */
               thread->doing_unsafe_access()) {
        address next_pc = Assembler::locate_next_instruction(pc);
        if (UnsafeMemoryAccess::contains_pc(pc)) {
          next_pc = UnsafeMemoryAccess::page_error_continue_pc(pc);
        }
        stub = SharedRuntime::handle_unsafe_access(thread, next_pc);
    }

    // jni_fast_Get<Primitive>Field can trap at certain pc's if a GC kicks in
    // and the heap gets shrunk before the field access.
    if ((sig == SIGSEGV) || (sig == SIGBUS)) {
      address addr = JNI_FastGetField::find_slowcase_pc(pc);
      if (addr != (address)-1) {
        stub = addr;
      }
    }
  }

#ifndef AMD64
  // Execution protection violation
  //
  // This should be kept as the last step in the triage.  We don't
  // have a dedicated trap number for a no-execute fault, so be
  // conservative and allow other handlers the first shot.
  //
  // Note: We don't test that info->si_code == SEGV_ACCERR here.
  // this si_code is so generic that it is almost meaningless; and
  // the si_code for this condition may change in the future.
  // Furthermore, a false-positive should be harmless.
  if (UnguardOnExecutionViolation > 0 &&
      stub == nullptr &&
      (sig == SIGSEGV || sig == SIGBUS) &&
      uc->context_trapno == trap_page_fault) {
    size_t page_size = os::vm_page_size();
    address addr = (address) info->si_addr;
    address pc = os::Posix::ucontext_get_pc(uc);
    // Make sure the pc and the faulting address are sane.
    //
    // If an instruction spans a page boundary, and the page containing
    // the beginning of the instruction is executable but the following
    // page is not, the pc and the faulting address might be slightly
    // different - we still want to unguard the 2nd page in this case.
    //
    // 15 bytes seems to be a (very) safe value for max instruction size.
    bool pc_is_near_addr =
      (pointer_delta((void*) addr, (void*) pc, sizeof(char)) < 15);
    bool instr_spans_page_boundary =
      (align_down((intptr_t) pc ^ (intptr_t) addr,
                       (intptr_t) page_size) > 0);

    if (pc == addr || (pc_is_near_addr && instr_spans_page_boundary)) {
      static volatile address last_addr =
        (address) os::non_memory_address_word();

      // In conservative mode, don't unguard unless the address is in the VM
      if (addr != last_addr &&
          (UnguardOnExecutionViolation > 1 || os::address_is_in_vm(addr))) {

        // Set memory to RWX and retry
        address page_start = align_down(addr, page_size);
        bool res = os::protect_memory((char*) page_start, page_size,
                                      os::MEM_PROT_RWX);

        log_debug(os)("Execution protection violation "
                      "at " INTPTR_FORMAT
                      ", unguarding " INTPTR_FORMAT ": %s, errno=%d", p2i(addr),
                      p2i(page_start), (res ? "success" : "failed"), errno);
        stub = pc;

        // Set last_addr so if we fault again at the same address, we don't end
        // up in an endless loop.
        //
        // There are two potential complications here.  Two threads trapping at
        // the same address at the same time could cause one of the threads to
        // think it already unguarded, and abort the VM.  Likely very rare.
        //
        // The other race involves two threads alternately trapping at
        // different addresses and failing to unguard the page, resulting in
        // an endless loop.  This condition is probably even more unlikely than
        // the first.
        //
        // Although both cases could be avoided by using locks or thread local
        // last_addr, these solutions are unnecessary complication: this
        // handler is a best-effort safety net, not a complete solution.  It is
        // disabled by default and should only be used as a workaround in case
        // we missed any no-execute-unsafe VM code.

        last_addr = addr;
      }
    }
  }
#endif // !AMD64

  if (stub != nullptr) {
    // save all thread context in case we need to restore it
    if (thread != nullptr) thread->set_saved_exception_pc(pc);

    os::Posix::ucontext_set_pc(uc, stub);
    return true;
  }

  return false;
}

// From solaris_i486.s ported to bsd_i486.s
extern "C" void fixcw();

void os::Bsd::init_thread_fpu_state(void) {
#ifndef AMD64
  // Set fpu to 53 bit precision. This happens too early to use a stub.
  fixcw();
#endif // !AMD64
}

juint os::cpu_microcode_revision() {
  juint result = 0;
  char data[8];
  size_t sz = sizeof(data);
  int ret = sysctlbyname("machdep.cpu.microcode_version", data, &sz, nullptr, 0);
  if (ret == 0) {
    if (sz == 4) result = *((juint*)data);
    if (sz == 8) result = *((juint*)data + 1); // upper 32-bits
  }
  return result;
}

////////////////////////////////////////////////////////////////////////////////
// thread stack

// Minimum usable stack sizes required to get to user code. Space for
// HotSpot guard pages is added later.
size_t os::_compiler_thread_min_stack_allowed = 48 * K;
size_t os::_java_thread_min_stack_allowed = 48 * K;
#ifdef _LP64
size_t os::_vm_internal_thread_min_stack_allowed = 64 * K;
#else
size_t os::_vm_internal_thread_min_stack_allowed = (48 DEBUG_ONLY(+ 4)) * K;
#endif // _LP64

#ifndef AMD64
#ifdef __GNUC__
#define GET_GS() ({int gs; __asm__ volatile("movw %%gs, %w0":"=q"(gs)); gs&0xffff;})
#endif
#endif // AMD64

// return default stack size for thr_type
size_t os::Posix::default_stack_size(os::ThreadType thr_type) {
  // default stack size (compiler thread needs larger stack)
#ifdef AMD64
  size_t s = (thr_type == os::compiler_thread ? 4 * M : 1 * M);
#else
  size_t s = (thr_type == os::compiler_thread ? 2 * M : 512 * K);
#endif // AMD64
  return s;
}


// Java thread:
//
//   Low memory addresses
//    +------------------------+
//    |                        |\  Java thread created by VM does not have glibc
//    |    glibc guard page    | - guard, attached Java thread usually has
//    |                        |/  1 glibc guard page.
// P1 +------------------------+ Thread::stack_base() - Thread::stack_size()
//    |                        |\
//    |  HotSpot Guard Pages   | - red, yellow and reserved pages
//    |                        |/
//    +------------------------+ StackOverflow::stack_reserved_zone_base()
//    |                        |\
//    |      Normal Stack      | -
//    |                        |/
// P2 +------------------------+ Thread::stack_base()
//
// Non-Java thread:
//
//   Low memory addresses
//    +------------------------+
//    |                        |\
//    |  glibc guard page      | - usually 1 page
//    |                        |/
// P1 +------------------------+ Thread::stack_base() - Thread::stack_size()
//    |                        |\
//    |      Normal Stack      | -
//    |                        |/
// P2 +------------------------+ Thread::stack_base()
//
// ** P1 (aka bottom) and size are the address and stack size
//    returned from pthread_attr_getstack().
// ** P2 (aka stack top or base) = P1 + size

void os::current_stack_base_and_size(address* base, size_t* size) {
  address bottom;
#ifdef __APPLE__
  pthread_t self = pthread_self();
  *base = (address) pthread_get_stackaddr_np(self);
  *size = pthread_get_stacksize_np(self);
  // workaround for OS X 10.9.0 (Mavericks)
  // pthread_get_stacksize_np returns 128 pages even though the actual size is 2048 pages
  if (pthread_main_np() == 1) {
    // At least on Mac OS 10.12 we have observed stack sizes not aligned
    // to pages boundaries. This can be provoked by e.g. setrlimit() (ulimit -s xxxx in the
    // shell). Apparently Mac OS actually rounds upwards to next multiple of page size,
    // however, we round downwards here to be on the safe side.
    *size = align_down(*size, getpagesize());

    if ((*size) < (DEFAULT_MAIN_THREAD_STACK_PAGES * (size_t)getpagesize())) {
      char kern_osrelease[256];
      size_t kern_osrelease_size = sizeof(kern_osrelease);
      int ret = sysctlbyname("kern.osrelease", kern_osrelease, &kern_osrelease_size, nullptr, 0);
      if (ret == 0) {
        // get the major number, atoi will ignore the minor amd micro portions of the version string
        if (atoi(kern_osrelease) >= OS_X_10_9_0_KERNEL_MAJOR_VERSION) {
          *size = (DEFAULT_MAIN_THREAD_STACK_PAGES*getpagesize());
        }
      }
    }
  }
  bottom = *base - *size;
#elif defined(__OpenBSD__)
  stack_t ss;
  int rslt = pthread_stackseg_np(pthread_self(), &ss);

  if (rslt != 0)
    fatal("pthread_stackseg_np failed with error = %d", rslt);

  *base = (address) ss.ss_sp;
  *size = ss.ss_size;
  bottom = *base - *size;
#else
  pthread_attr_t attr;

  int rslt = pthread_attr_init(&attr);

  // JVM needs to know exact stack location, abort if it fails
  if (rslt != 0)
    fatal("pthread_attr_init failed with error = %d", rslt);

  rslt = pthread_attr_get_np(pthread_self(), &attr);

  if (rslt != 0)
    fatal("pthread_attr_get_np failed with error = %d", rslt);

  if (pthread_attr_getstackaddr(&attr, (void **)&bottom) != 0 ||
      pthread_attr_getstacksize(&attr, size) != 0) {
    fatal("Can not locate current stack attributes!");
  }

  *base = bottom + *size;

  pthread_attr_destroy(&attr);
#endif
  assert(os::current_stack_pointer() >= bottom &&
         os::current_stack_pointer() < *base, "just checking");
}

/////////////////////////////////////////////////////////////////////////////
// helper functions for fatal error handler

void os::print_context(outputStream *st, const void *context) {
  if (context == nullptr) return;

  const ucontext_t *uc = (const ucontext_t*)context;

  st->print_cr("Registers:");
#ifdef AMD64
  st->print(  "RAX=" INTPTR_FORMAT, (intptr_t)uc->context_rax);
  st->print(", RBX=" INTPTR_FORMAT, (intptr_t)uc->context_rbx);
  st->print(", RCX=" INTPTR_FORMAT, (intptr_t)uc->context_rcx);
  st->print(", RDX=" INTPTR_FORMAT, (intptr_t)uc->context_rdx);
  st->cr();
  st->print(  "RSP=" INTPTR_FORMAT, (intptr_t)uc->context_rsp);
  st->print(", RBP=" INTPTR_FORMAT, (intptr_t)uc->context_rbp);
  st->print(", RSI=" INTPTR_FORMAT, (intptr_t)uc->context_rsi);
  st->print(", RDI=" INTPTR_FORMAT, (intptr_t)uc->context_rdi);
  st->cr();
  st->print(  "R8 =" INTPTR_FORMAT, (intptr_t)uc->context_r8);
  st->print(", R9 =" INTPTR_FORMAT, (intptr_t)uc->context_r9);
  st->print(", R10=" INTPTR_FORMAT, (intptr_t)uc->context_r10);
  st->print(", R11=" INTPTR_FORMAT, (intptr_t)uc->context_r11);
  st->cr();
  st->print(  "R12=" INTPTR_FORMAT, (intptr_t)uc->context_r12);
  st->print(", R13=" INTPTR_FORMAT, (intptr_t)uc->context_r13);
  st->print(", R14=" INTPTR_FORMAT, (intptr_t)uc->context_r14);
  st->print(", R15=" INTPTR_FORMAT, (intptr_t)uc->context_r15);
  st->cr();
  st->print(  "RIP=" INTPTR_FORMAT, (intptr_t)uc->context_rip);
  st->print(", EFLAGS=" INTPTR_FORMAT, (intptr_t)uc->context_flags);
  st->print(", ERR=" INTPTR_FORMAT, (intptr_t)uc->context_err);
  st->cr();
  st->print("  TRAPNO=" INTPTR_FORMAT, (intptr_t)uc->context_trapno);
#else
  st->print(  "EAX=" INTPTR_FORMAT, (intptr_t)uc->context_eax);
  st->print(", EBX=" INTPTR_FORMAT, (intptr_t)uc->context_ebx);
  st->print(", ECX=" INTPTR_FORMAT, (intptr_t)uc->context_ecx);
  st->print(", EDX=" INTPTR_FORMAT, (intptr_t)uc->context_edx);
  st->cr();
  st->print(  "ESP=" INTPTR_FORMAT, (intptr_t)uc->context_esp);
  st->print(", EBP=" INTPTR_FORMAT, (intptr_t)uc->context_ebp);
  st->print(", ESI=" INTPTR_FORMAT, (intptr_t)uc->context_esi);
  st->print(", EDI=" INTPTR_FORMAT, (intptr_t)uc->context_edi);
  st->cr();
  st->print(  "EIP=" INTPTR_FORMAT, (intptr_t)uc->context_eip);
  st->print(", EFLAGS=" INTPTR_FORMAT, (intptr_t)uc->context_eflags);
#endif // AMD64
  st->cr();
  st->cr();
}

void os::print_register_info(outputStream *st, const void *context, int& continuation) {
  const int register_count = AMD64_ONLY(16) NOT_AMD64(8);
  int n = continuation;
  assert(n >= 0 && n <= register_count, "Invalid continuation value");
  if (context == nullptr || n == register_count) {
    return;
  }

  const ucontext_t *uc = (const ucontext_t*)context;
  while (n < register_count) {
    // Update continuation with next index before printing location
    continuation = n + 1;
# define CASE_PRINT_REG(n, str, id) case n: st->print(str); print_location(st, uc->context_##id);
  switch (n) {
#ifdef AMD64
    CASE_PRINT_REG( 0, "RAX=", rax); break;
    CASE_PRINT_REG( 1, "RBX=", rbx); break;
    CASE_PRINT_REG( 2, "RCX=", rcx); break;
    CASE_PRINT_REG( 3, "RDX=", rdx); break;
    CASE_PRINT_REG( 4, "RSP=", rsp); break;
    CASE_PRINT_REG( 5, "RBP=", rbp); break;
    CASE_PRINT_REG( 6, "RSI=", rsi); break;
    CASE_PRINT_REG( 7, "RDI=", rdi); break;
    CASE_PRINT_REG( 8, "R8 =", r8); break;
    CASE_PRINT_REG( 9, "R9 =", r9); break;
    CASE_PRINT_REG(10, "R10=", r10); break;
    CASE_PRINT_REG(11, "R11=", r11); break;
    CASE_PRINT_REG(12, "R12=", r12); break;
    CASE_PRINT_REG(13, "R13=", r13); break;
    CASE_PRINT_REG(14, "R14=", r14); break;
    CASE_PRINT_REG(15, "R15=", r15); break;
#else
    CASE_PRINT_REG(0, "EAX=", eax); break;
    CASE_PRINT_REG(1, "EBX=", ebx); break;
    CASE_PRINT_REG(2, "ECX=", ecx); break;
    CASE_PRINT_REG(3, "EDX=", edx); break;
    CASE_PRINT_REG(4, "ESP=", esp); break;
    CASE_PRINT_REG(5, "EBP=", ebp); break;
    CASE_PRINT_REG(6, "ESI=", esi); break;
    CASE_PRINT_REG(7, "EDI=", edi); break;
#endif // AMD64
    }
# undef CASE_PRINT_REG
    ++n;
  }
}

void os::setup_fpu() {
#ifndef AMD64
  address fpu_cntrl = StubRoutines::addr_fpu_cntrl_wrd_std();
  __asm__ volatile (  "fldcw (%0)" :
                      : "r" (fpu_cntrl) : "memory");
#endif // !AMD64
}

#ifndef PRODUCT
void os::verify_stack_alignment() {
}
#endif

int os::extra_bang_size_in_bytes() {
  // JDK-8050147 requires the full cache line bang for x86.
  return VM_Version::L1_line_size();
}