Check in gVisor.

PiperOrigin-RevId: 194583126
Change-Id: Ica1d8821a90f74e7e745962d71801c598c652463

10 files changed, 1597 insertions, 0 deletions

diff --git a/pkg/sentry/platform/ptrace/BUILD b/pkg/sentry/platform/ptrace/BUILD
new file mode 100644
index 000000000..16b0b3c69
--- /dev/null
+++ b/pkg/sentry/platform/ptrace/BUILD
@@ -0,0 +1,31 @@
+package(licenses = ["notice"])  # Apache 2.0
+
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "ptrace",
+    srcs = [
+        "ptrace.go",
+        "ptrace_unsafe.go",
+        "stub_amd64.s",
+        "stub_unsafe.go",
+        "subprocess.go",
+        "subprocess_amd64.go",
+        "subprocess_linux.go",
+        "subprocess_linux_amd64_unsafe.go",
+        "subprocess_unsafe.go",
+    ],
+    importpath = "gvisor.googlesource.com/gvisor/pkg/sentry/platform/ptrace",
+    visibility = ["//:sandbox"],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/sentry/arch",
+        "//pkg/sentry/platform",
+        "//pkg/sentry/platform/filemem",
+        "//pkg/sentry/platform/interrupt",
+        "//pkg/sentry/platform/procid",
+        "//pkg/sentry/platform/safecopy",
+        "//pkg/sentry/usermem",
+        "@org_golang_x_sys//unix:go_default_library",
+    ],
+)
diff --git a/pkg/sentry/platform/ptrace/ptrace.go b/pkg/sentry/platform/ptrace/ptrace.go
new file mode 100644
index 000000000..05f8b1d05
--- /dev/null
+++ b/pkg/sentry/platform/ptrace/ptrace.go
@@ -0,0 +1,242 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package ptrace provides a ptrace-based implementation of the platform
+// interface. This is useful for development and testing purposes primarily,
+// and runs on stock kernels without special permissions.
+//
+// In a nutshell, it works as follows:
+//
+// The creation of a new address space creates a new child processes with a
+// single thread which is traced by a single goroutine.
+//
+// A context is just a collection of temporary variables. Calling Switch on a
+// context does the following:
+//
+//	Locks the runtime thread.
+//
+//	Looks up a traced subprocess thread for the current runtime thread. If
+//	none exists, the dedicated goroutine is asked to create a new stopped
+//	thread in the subprocess. This stopped subprocess thread is then traced
+//	by the current thread and this information is stored for subsequent
+//	switches.
+//
+//	The context is then bound with information about the subprocess thread
+//	so that the context may be appropriately interrupted via a signal.
+//
+//	The requested operation is performed in the traced subprocess thread
+//	(e.g. set registers, execute, return).
+//
+// FIXME: This package is currently sloppy with cleanup.
+//
+// Lock order:
+//
+// subprocess.mu
+//   context.mu
+package ptrace
+
+import (
+	"sync"
+
+	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/platform"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/platform/filemem"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/platform/interrupt"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
+)
+
+var (
+	// stubStart is the link address for our stub, and determines the
+	// maximum user address. This is valid only after a call to stubInit.
+	//
+	// We attempt to link the stub here, and adjust downward as needed.
+	stubStart uintptr = 0x7fffffff0000
+
+	// stubEnd is the first byte past the end of the stub, as with
+	// stubStart this is valid only after a call to stubInit.
+	stubEnd uintptr
+
+	// stubInitialized controls one-time stub initialization.
+	stubInitialized sync.Once
+)
+
+type context struct {
+	// signalInfo is the signal info, if and when a signal is received.
+	signalInfo arch.SignalInfo
+
+	// interrupt is the interrupt context.
+	interrupt interrupt.Forwarder
+
+	// mu protects the following fields.
+	mu sync.Mutex
+
+	// If lastFaultSP is non-nil, the last context switch was due to a fault
+	// received while executing lastFaultSP. Only context.Switch may set
+	// lastFaultSP to a non-nil value.
+	lastFaultSP *subprocess
+
+	// lastFaultAddr is the last faulting address; this is only meaningful if
+	// lastFaultSP is non-nil.
+	lastFaultAddr usermem.Addr
+
+	// lastFaultIP is the address of the last faulting instruction;
+	// this is also only meaningful if lastFaultSP is non-nil.
+	lastFaultIP usermem.Addr
+}
+
+// Switch runs the provided context in the given address space.
+func (c *context) Switch(as platform.AddressSpace, ac arch.Context, cpu int32) (*arch.SignalInfo, usermem.AccessType, error) {
+	s := as.(*subprocess)
+	isSyscall := s.switchToApp(c, ac)
+
+	var faultSP *subprocess
+	var faultAddr usermem.Addr
+	var faultIP usermem.Addr
+	if !isSyscall && linux.Signal(c.signalInfo.Signo) == linux.SIGSEGV {
+		faultSP = s
+		faultAddr = usermem.Addr(c.signalInfo.Addr())
+		faultIP = usermem.Addr(ac.IP())
+	}
+
+	// Update the context to reflect the outcome of this context switch.
+	c.mu.Lock()
+	lastFaultSP := c.lastFaultSP
+	lastFaultAddr := c.lastFaultAddr
+	lastFaultIP := c.lastFaultIP
+	// At this point, c may not yet be in s.contexts, so c.lastFaultSP won't be
+	// updated by s.Unmap(). This is fine; we only need to synchronize with
+	// calls to s.Unmap() that occur after the handling of this fault.
+	c.lastFaultSP = faultSP
+	c.lastFaultAddr = faultAddr
+	c.lastFaultIP = faultIP
+	c.mu.Unlock()
+
+	// Update subprocesses to reflect the outcome of this context switch.
+	if lastFaultSP != faultSP {
+		if lastFaultSP != nil {
+			lastFaultSP.mu.Lock()
+			delete(lastFaultSP.contexts, c)
+			lastFaultSP.mu.Unlock()
+		}
+		if faultSP != nil {
+			faultSP.mu.Lock()
+			faultSP.contexts[c] = struct{}{}
+			faultSP.mu.Unlock()
+		}
+	}
+
+	if isSyscall {
+		return nil, usermem.NoAccess, nil
+	}
+	if faultSP == nil {
+		// Non-fault signal.
+		return &c.signalInfo, usermem.NoAccess, platform.ErrContextSignal
+	}
+
+	// Got a page fault. Ideally, we'd get real fault type here, but ptrace
+	// doesn't expose this information. Instead, we use a simple heuristic:
+	//
+	// It was an instruction fault iff the faulting addr == instruction
+	// pointer.
+	//
+	// It was a write fault if the fault is immediately repeated.
+	at := usermem.Read
+	if faultAddr == faultIP {
+		at.Execute = true
+	}
+	if lastFaultSP == faultSP &&
+		lastFaultAddr == faultAddr &&
+		lastFaultIP == faultIP {
+		at.Write = true
+	}
+	return &c.signalInfo, at, platform.ErrContextSignal
+}
+
+// Interrupt interrupts the running guest application associated with this context.
+func (c *context) Interrupt() {
+	c.interrupt.NotifyInterrupt()
+}
+
+// PTrace represents a collection of ptrace subprocesses.
+type PTrace struct {
+	platform.MMapMinAddr
+	platform.NoCPUPreemptionDetection
+	*filemem.FileMem
+}
+
+// New returns a new ptrace-based implementation of the platform interface.
+func New() (*PTrace, error) {
+	stubInitialized.Do(func() {
+		// Initialize the stub.
+		stubInit()
+
+		// Create the master process for the global pool. This must be
+		// done before initializing any other processes.
+		master, err := newSubprocess(createStub)
+		if err != nil {
+			// Should never happen.
+			panic("unable to initialize ptrace master: " + err.Error())
+		}
+
+		// Set the master on the globalPool.
+		globalPool.master = master
+	})
+
+	fm, err := filemem.New("ptrace-memory")
+	if err != nil {
+		return nil, err
+	}
+
+	return &PTrace{FileMem: fm}, nil
+}
+
+// SupportsAddressSpaceIO implements platform.Platform.SupportsAddressSpaceIO.
+func (*PTrace) SupportsAddressSpaceIO() bool {
+	return false
+}
+
+// CooperativelySchedulesAddressSpace implements platform.Platform.CooperativelySchedulesAddressSpace.
+func (*PTrace) CooperativelySchedulesAddressSpace() bool {
+	return false
+}
+
+// MapUnit implements platform.Platform.MapUnit.
+func (*PTrace) MapUnit() uint64 {
+	// The host kernel manages page tables and arbitrary-sized mappings
+	// have effectively the same cost.
+	return 0
+}
+
+// MaxUserAddress returns the first address that may not be used by user
+// applications.
+func (*PTrace) MaxUserAddress() usermem.Addr {
+	return usermem.Addr(stubStart)
+}
+
+// NewAddressSpace returns a new subprocess.
+func (p *PTrace) NewAddressSpace(_ interface{}) (platform.AddressSpace, <-chan struct{}, error) {
+	as, err := newSubprocess(globalPool.master.createStub)
+	return as, nil, err
+}
+
+// NewContext returns an interruptible context.
+func (*PTrace) NewContext() platform.Context {
+	return &context{}
+}
+
+// Memory returns the platform memory used to do allocations.
+func (p *PTrace) Memory() platform.Memory {
+	return p.FileMem
+}
diff --git a/pkg/sentry/platform/ptrace/ptrace_unsafe.go b/pkg/sentry/platform/ptrace/ptrace_unsafe.go
new file mode 100644
index 000000000..b55b2795a
--- /dev/null
+++ b/pkg/sentry/platform/ptrace/ptrace_unsafe.go
@@ -0,0 +1,166 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ptrace
+
+import (
+	"syscall"
+	"unsafe"
+
+	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
+)
+
+// GETREGSET/SETREGSET register set types.
+//
+// See include/uapi/linux/elf.h.
+const (
+	// _NT_PRFPREG is for x86 floating-point state without using xsave.
+	_NT_PRFPREG = 0x2
+
+	// _NT_X86_XSTATE is for x86 extended state using xsave.
+	_NT_X86_XSTATE = 0x202
+)
+
+// fpRegSet returns the GETREGSET/SETREGSET register set type to be used.
+func fpRegSet(useXsave bool) uintptr {
+	if useXsave {
+		return _NT_X86_XSTATE
+	}
+	return _NT_PRFPREG
+}
+
+// getRegs sets the regular register set.
+func (t *thread) getRegs(regs *syscall.PtraceRegs) error {
+	_, _, errno := syscall.RawSyscall6(
+		syscall.SYS_PTRACE,
+		syscall.PTRACE_GETREGS,
+		uintptr(t.tid),
+		0,
+		uintptr(unsafe.Pointer(regs)),
+		0, 0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+// setRegs sets the regular register set.
+func (t *thread) setRegs(regs *syscall.PtraceRegs) error {
+	_, _, errno := syscall.RawSyscall6(
+		syscall.SYS_PTRACE,
+		syscall.PTRACE_SETREGS,
+		uintptr(t.tid),
+		0,
+		uintptr(unsafe.Pointer(regs)),
+		0, 0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+// getFPRegs gets the floating-point data via the GETREGSET ptrace syscall.
+func (t *thread) getFPRegs(fpState *arch.FloatingPointData, fpLen uint64, useXsave bool) error {
+	iovec := syscall.Iovec{
+		Base: (*byte)(fpState),
+		Len:  fpLen,
+	}
+	_, _, errno := syscall.RawSyscall6(
+		syscall.SYS_PTRACE,
+		syscall.PTRACE_GETREGSET,
+		uintptr(t.tid),
+		fpRegSet(useXsave),
+		uintptr(unsafe.Pointer(&iovec)),
+		0, 0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+// setFPRegs sets the floating-point data via the SETREGSET ptrace syscall.
+func (t *thread) setFPRegs(fpState *arch.FloatingPointData, fpLen uint64, useXsave bool) error {
+	iovec := syscall.Iovec{
+		Base: (*byte)(fpState),
+		Len:  fpLen,
+	}
+	_, _, errno := syscall.RawSyscall6(
+		syscall.SYS_PTRACE,
+		syscall.PTRACE_SETREGSET,
+		uintptr(t.tid),
+		fpRegSet(useXsave),
+		uintptr(unsafe.Pointer(&iovec)),
+		0, 0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+// getSignalInfo retrieves information about the signal that caused the stop.
+func (t *thread) getSignalInfo(si *arch.SignalInfo) error {
+	_, _, errno := syscall.RawSyscall6(
+		syscall.SYS_PTRACE,
+		syscall.PTRACE_GETSIGINFO,
+		uintptr(t.tid),
+		0,
+		uintptr(unsafe.Pointer(si)),
+		0, 0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+// clone creates a new thread from this one.
+//
+// The returned thread will be stopped and available for any system thread to
+// call attach on it.
+//
+// Precondition: the OS thread must be locked and own t.
+func (t *thread) clone(initRegs *syscall.PtraceRegs) (*thread, error) {
+	r, ok := usermem.Addr(initRegs.Rsp).RoundUp()
+	if !ok {
+		return nil, syscall.EINVAL
+	}
+	rval, err := t.syscallIgnoreInterrupt(
+		initRegs,
+		syscall.SYS_CLONE,
+		arch.SyscallArgument{Value: uintptr(
+			syscall.CLONE_FILES |
+				syscall.CLONE_FS |
+				syscall.CLONE_SIGHAND |
+				syscall.CLONE_THREAD |
+				syscall.CLONE_PTRACE |
+				syscall.CLONE_VM)},
+		// The stack pointer is just made up, but we have it be
+		// something sensible so the kernel doesn't think we're
+		// up to no good. Which we are.
+		arch.SyscallArgument{Value: uintptr(r)},
+		arch.SyscallArgument{},
+		arch.SyscallArgument{},
+		// We use these registers initially, but really they
+		// could be anything. We're going to stop immediately.
+		arch.SyscallArgument{Value: uintptr(unsafe.Pointer(initRegs))})
+	if err != nil {
+		return nil, err
+	}
+
+	return &thread{
+		tgid: t.tgid,
+		tid:  int32(rval),
+		cpu:  ^uint32(0),
+	}, nil
+}
diff --git a/pkg/sentry/platform/ptrace/stub_amd64.s b/pkg/sentry/platform/ptrace/stub_amd64.s
new file mode 100644
index 000000000..9bf87b6f6
--- /dev/null
+++ b/pkg/sentry/platform/ptrace/stub_amd64.s
@@ -0,0 +1,114 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "funcdata.h"
+#include "textflag.h"
+
+#define SYS_GETPID		39
+#define SYS_EXIT		60
+#define SYS_KILL		62
+#define SYS_GETPPID		110
+#define SYS_PRCTL		157
+
+#define SIGKILL			9
+#define SIGSTOP			19
+
+#define PR_SET_PDEATHSIG	1
+
+// stub bootstraps the child and sends itself SIGSTOP to wait for attach.
+//
+// R15 contains the expected PPID. R15 is used instead of a more typical DI
+// since syscalls will clobber DI and createStub wants to pass a new PPID to
+// grandchildren.
+//
+// This should not be used outside the context of a new ptrace child (as the
+// function is otherwise a bunch of nonsense).
+TEXT ·stub(SB),NOSPLIT,$0
+begin:
+	// N.B. This loop only executes in the context of a single-threaded
+	// fork child.
+
+	MOVQ $SYS_PRCTL, AX
+	MOVQ $PR_SET_PDEATHSIG, DI
+	MOVQ $SIGKILL, SI
+	SYSCALL
+
+	CMPQ AX, $0
+	JNE error
+
+	// If the parent already died before we called PR_SET_DEATHSIG then
+	// we'll have an unexpected PPID.
+	MOVQ $SYS_GETPPID, AX
+	SYSCALL
+
+	CMPQ AX, $0
+	JL error
+
+	CMPQ AX, R15
+	JNE parent_dead
+
+	MOVQ $SYS_GETPID, AX
+	SYSCALL
+
+	CMPQ AX, $0
+	JL error
+
+	// SIGSTOP to wait for attach.
+	//
+	// The SYSCALL instruction will be used for future syscall injection by
+	// thread.syscall.
+	MOVQ AX, DI
+	MOVQ $SYS_KILL, AX
+	MOVQ $SIGSTOP, SI
+	SYSCALL
+
+	// The tracer may "detach" and/or allow code execution here in three cases:
+	//
+	// 1. New (traced) stub threads are explicitly detached by the
+	// goroutine in newSubprocess. However, they are detached while in
+	// group-stop, so they do not execute code here.
+	//
+	// 2. If a tracer thread exits, it implicitly detaches from the stub,
+	// potentially allowing code execution here. However, the Go runtime
+	// never exits individual threads, so this case never occurs.
+	//
+	// 3. subprocess.createStub clones a new stub process that is untraced,
+	// thus executing this code. We setup the PDEATHSIG before SIGSTOPing
+	// ourselves for attach by the tracer.
+	//
+	// R15 has been updated with the expected PPID.
+	JMP begin
+
+error:
+	// Exit with -errno.
+	MOVQ AX, DI
+	NEGQ DI
+	MOVQ $SYS_EXIT, AX
+	SYSCALL
+	HLT
+
+parent_dead:
+	MOVQ $SYS_EXIT, AX
+	MOVQ $1, DI
+	SYSCALL
+	HLT
+
+// stubCall calls the stub function at the given address with the given PPID.
+//
+// This is a distinct function because stub, above, may be mapped at any
+// arbitrary location, and stub has a specific binary API (see above).
+TEXT ·stubCall(SB),NOSPLIT,$0-16
+	MOVQ addr+0(FP), AX
+	MOVQ pid+8(FP), R15
+	JMP AX
diff --git a/pkg/sentry/platform/ptrace/stub_unsafe.go b/pkg/sentry/platform/ptrace/stub_unsafe.go
new file mode 100644
index 000000000..c868a2d68
--- /dev/null
+++ b/pkg/sentry/platform/ptrace/stub_unsafe.go
@@ -0,0 +1,98 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ptrace
+
+import (
+	"reflect"
+	"syscall"
+	"unsafe"
+
+	"gvisor.googlesource.com/gvisor/pkg/sentry/platform/safecopy"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
+)
+
+// stub is defined in arch-specific assembly.
+func stub()
+
+// stubCall calls the stub at the given address with the given pid.
+func stubCall(addr, pid uintptr)
+
+// unsafeSlice returns a slice for the given address and length.
+func unsafeSlice(addr uintptr, length int) (slice []byte) {
+	sh := (*reflect.SliceHeader)(unsafe.Pointer(&slice))
+	sh.Data = addr
+	sh.Len = length
+	sh.Cap = length
+	return
+}
+
+// stubInit initializes the stub.
+func stubInit() {
+	// Grab the existing stub.
+	stubBegin := reflect.ValueOf(stub).Pointer()
+	stubLen := int(safecopy.FindEndAddress(stubBegin) - stubBegin)
+	stubSlice := unsafeSlice(stubBegin, stubLen)
+	mapLen := uintptr(stubLen)
+	if offset := mapLen % usermem.PageSize; offset != 0 {
+		mapLen += usermem.PageSize - offset
+	}
+
+	for stubStart > 0 {
+		// Map the target address for the stub.
+		//
+		// We don't use FIXED here because we don't want to unmap
+		// something that may have been there already. We just walk
+		// down the address space until we find a place where the stub
+		// can be placed.
+		addr, _, errno := syscall.RawSyscall6(
+			syscall.SYS_MMAP,
+			stubStart,
+			mapLen,
+			syscall.PROT_WRITE|syscall.PROT_READ,
+			syscall.MAP_PRIVATE|syscall.MAP_ANONYMOUS,
+			0 /* fd */, 0 /* offset */)
+		if addr != stubStart || errno != 0 {
+			if addr != 0 {
+				// Unmap the region we've mapped accidentally.
+				syscall.RawSyscall(syscall.SYS_MUNMAP, addr, mapLen, 0)
+			}
+
+			// Attempt to begin at a lower address.
+			stubStart -= uintptr(usermem.PageSize)
+			continue
+		}
+
+		// Copy the stub to the address.
+		targetSlice := unsafeSlice(addr, stubLen)
+		copy(targetSlice, stubSlice)
+
+		// Make the stub executable.
+		if _, _, errno := syscall.RawSyscall(
+			syscall.SYS_MPROTECT,
+			stubStart,
+			mapLen,
+			syscall.PROT_EXEC|syscall.PROT_READ); errno != 0 {
+			panic("mprotect failed: " + errno.Error())
+		}
+
+		// Set the end.
+		stubEnd = stubStart + mapLen
+		return
+	}
+
+	// This will happen only if we exhaust the entire address
+	// space, and it will take a long, long time.
+	panic("failed to map stub")
+}
diff --git a/pkg/sentry/platform/ptrace/subprocess.go b/pkg/sentry/platform/ptrace/subprocess.go
new file mode 100644
index 000000000..0d6a38f15
--- /dev/null
+++ b/pkg/sentry/platform/ptrace/subprocess.go
@@ -0,0 +1,559 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ptrace
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+	"sync"
+	"syscall"
+
+	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/platform"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/platform/procid"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
+)
+
+// globalPool exists to solve two distinct problems:
+//
+// 1) Subprocesses can't always be killed properly (see Release).
+//
+// 2) Any seccomp filters that have been installed will apply to subprocesses
+// created here. Therefore we use the intermediary (master), which is created
+// on initialization of the platform.
+var globalPool struct {
+	mu        sync.Mutex
+	master    *subprocess
+	available []*subprocess
+}
+
+// thread is a traced thread; it is a thread identifier.
+//
+// This is a convenience type for defining ptrace operations.
+type thread struct {
+	tgid int32
+	tid  int32
+	cpu  uint32
+}
+
+// threadPool is a collection of threads.
+type threadPool struct {
+	// mu protects below.
+	mu sync.Mutex
+
+	// threads is the collection of threads.
+	//
+	// This map is indexed by system TID (the calling thread); which will
+	// be the tracer for the given *thread, and therefore capable of using
+	// relevant ptrace calls.
+	threads map[int32]*thread
+}
+
+// lookupOrCreate looks up a given thread or creates one.
+//
+// newThread will generally be subprocess.newThread.
+//
+// Precondition: the runtime OS thread must be locked.
+func (tp *threadPool) lookupOrCreate(currentTID int32, newThread func() *thread) *thread {
+	tp.mu.Lock()
+	t, ok := tp.threads[currentTID]
+	if !ok {
+		// Before creating a new thread, see if we can find a thread
+		// whose system tid has disappeared.
+		//
+		// TODO: Other parts of this package depend on
+		// threads never exiting.
+		for origTID, t := range tp.threads {
+			// Signal zero is an easy existence check.
+			if err := syscall.Tgkill(syscall.Getpid(), int(origTID), 0); err != nil {
+				// This thread has been abandoned; reuse it.
+				delete(tp.threads, origTID)
+				tp.threads[currentTID] = t
+				tp.mu.Unlock()
+				return t
+			}
+		}
+
+		// Create a new thread.
+		t = newThread()
+		tp.threads[currentTID] = t
+	}
+	tp.mu.Unlock()
+	return t
+}
+
+// subprocess is a collection of threads being traced.
+type subprocess struct {
+	platform.NoAddressSpaceIO
+
+	// initRegs are the initial registers for the first thread.
+	//
+	// These are used for the register set for system calls.
+	initRegs syscall.PtraceRegs
+
+	// requests is used to signal creation of new threads.
+	requests chan chan *thread
+
+	// sysemuThreads are reserved for emulation.
+	sysemuThreads threadPool
+
+	// syscallThreads are reserved for syscalls (except clone, which is
+	// handled in the dedicated goroutine corresponding to requests above).
+	syscallThreads threadPool
+
+	// mu protects the following fields.
+	mu sync.Mutex
+
+	// contexts is the set of contexts for which it's possible that
+	// context.lastFaultSP == this subprocess.
+	contexts map[*context]struct{}
+}
+
+// newSubprocess returns a useable subprocess.
+//
+// This will either be a newly created subprocess, or one from the global pool.
+// The create function will be called in the latter case, which is guaranteed
+// to happen with the runtime thread locked.
+func newSubprocess(create func() (*thread, error)) (*subprocess, error) {
+	// See Release.
+	globalPool.mu.Lock()
+	if len(globalPool.available) > 0 {
+		sp := globalPool.available[len(globalPool.available)-1]
+		globalPool.available = globalPool.available[:len(globalPool.available)-1]
+		globalPool.mu.Unlock()
+		return sp, nil
+	}
+	globalPool.mu.Unlock()
+
+	// The following goroutine is responsible for creating the first traced
+	// thread, and responding to requests to make additional threads in the
+	// traced process. The process will be killed and reaped when the
+	// request channel is closed, which happens in Release below.
+	var initRegs syscall.PtraceRegs
+	errChan := make(chan error)
+	requests := make(chan chan *thread)
+	go func() { // S/R-SAFE: Platform-related.
+		runtime.LockOSThread()
+		defer runtime.UnlockOSThread()
+
+		// Initialize the first thread.
+		firstThread, err := create()
+		if err != nil {
+			errChan <- err
+			return
+		}
+
+		// Grab registers.
+		//
+		// Note that we adjust the current register RIP value to be
+		// just before the current system call executed. This depends
+		// on the definition of the stub itself.
+		if err := firstThread.getRegs(&initRegs); err != nil {
+			panic(fmt.Sprintf("ptrace get regs failed: %v", err))
+		}
+		initRegs.Rip -= initRegsRipAdjustment
+
+		// Ready to handle requests.
+		errChan <- nil
+
+		// Wait for requests to create threads.
+		for r := range requests {
+			t, err := firstThread.clone(&initRegs)
+			if err != nil {
+				// Should not happen: not recoverable.
+				panic(fmt.Sprintf("error initializing first thread: %v", err))
+			}
+
+			// Since the new thread was created with
+			// clone(CLONE_PTRACE), it will begin execution with
+			// SIGSTOP pending and with this thread as its tracer.
+			// (Hopefully nobody tgkilled it with a signal <
+			// SIGSTOP before the SIGSTOP was delivered, in which
+			// case that signal would be delivered before SIGSTOP.)
+			if sig := t.wait(); sig != syscall.SIGSTOP {
+				panic(fmt.Sprintf("error waiting for new clone: expected SIGSTOP, got %v", sig))
+			}
+
+			// Detach the thread without suppressing the SIGSTOP,
+			// causing it to enter group-stop.
+			if _, _, errno := syscall.RawSyscall6(syscall.SYS_PTRACE, syscall.PTRACE_DETACH, uintptr(t.tid), 0, uintptr(syscall.SIGSTOP), 0, 0); errno != 0 {
+				panic(fmt.Sprintf("can't detach new clone: %v", errno))
+			}
+
+			// Return the thread.
+			r <- t
+		}
+
+		// Requests should never be closed.
+		panic("unreachable")
+	}()
+
+	// Wait until error or readiness.
+	if err := <-errChan; err != nil {
+		return nil, err
+	}
+
+	// Ready.
+	sp := &subprocess{
+		initRegs: initRegs,
+		requests: requests,
+		sysemuThreads: threadPool{
+			threads: make(map[int32]*thread),
+		},
+		syscallThreads: threadPool{
+			threads: make(map[int32]*thread),
+		},
+		contexts: make(map[*context]struct{}),
+	}
+
+	sp.unmap()
+	return sp, nil
+}
+
+// unmap unmaps non-stub regions of the process.
+//
+// This will panic on failure (which should never happen).
+func (s *subprocess) unmap() {
+	s.Unmap(0, uint64(stubStart))
+	if maximumUserAddress != stubEnd {
+		s.Unmap(usermem.Addr(stubEnd), uint64(maximumUserAddress-stubEnd))
+	}
+}
+
+// Release kills the subprocess.
+//
+// Just kidding! We can't safely co-ordinate the detaching of all the
+// tracees (since the tracers are random runtime threads, and the process
+// won't exit until tracers have been notifier).
+//
+// Therefore we simply unmap everything in the subprocess and return it to the
+// globalPool. This has the added benefit of reducing creation time for new
+// subprocesses.
+func (s *subprocess) Release() error {
+	go func() { // S/R-SAFE: Platform.
+		s.unmap()
+		globalPool.mu.Lock()
+		globalPool.available = append(globalPool.available, s)
+		globalPool.mu.Unlock()
+	}()
+	return nil
+}
+
+// newThread creates a new traced thread.
+//
+// Precondition: the OS thread must be locked.
+func (s *subprocess) newThread() *thread {
+	// Ask the first thread to create a new one.
+	r := make(chan *thread)
+	s.requests <- r
+	t := <-r
+
+	// Attach the subprocess to this one.
+	t.attach()
+
+	// Return the new thread, which is now bound.
+	return t
+}
+
+// attach attachs to the thread.
+func (t *thread) attach() {
+	if _, _, errno := syscall.RawSyscall(syscall.SYS_PTRACE, syscall.PTRACE_ATTACH, uintptr(t.tid), 0); errno != 0 {
+		panic(fmt.Sprintf("unable to attach: %v", errno))
+	}
+
+	// PTRACE_ATTACH sends SIGSTOP, and wakes the tracee if it was already
+	// stopped from the SIGSTOP queued by CLONE_PTRACE (see inner loop of
+	// newSubprocess), so we always expect to see signal-delivery-stop with
+	// SIGSTOP.
+	if sig := t.wait(); sig != syscall.SIGSTOP {
+		panic(fmt.Sprintf("wait failed: expected SIGSTOP, got %v", sig))
+	}
+
+	// Initialize options.
+	t.init()
+}
+
+// wait waits for a stop event.
+func (t *thread) wait() syscall.Signal {
+	var status syscall.WaitStatus
+
+	for {
+		r, err := syscall.Wait4(int(t.tid), &status, syscall.WALL|syscall.WUNTRACED, nil)
+		if err == syscall.EINTR || err == syscall.EAGAIN {
+			// Wait was interrupted; wait again.
+			continue
+		} else if err != nil {
+			panic(fmt.Sprintf("ptrace wait failed: %v", err))
+		}
+		if int(r) != int(t.tid) {
+			panic(fmt.Sprintf("ptrace wait returned %v, expected %v", r, t.tid))
+		}
+		if !status.Stopped() {
+			panic(fmt.Sprintf("ptrace status unexpected: got %v, wanted stopped", status))
+		}
+		if status.StopSignal() == 0 {
+			continue // Spurious stop.
+		}
+		return status.StopSignal()
+	}
+}
+
+// init initializes trace options.
+func (t *thread) init() {
+	// Set our TRACESYSGOOD option to differeniate real SIGTRAP.
+	_, _, errno := syscall.RawSyscall6(
+		syscall.SYS_PTRACE,
+		syscall.PTRACE_SETOPTIONS,
+		uintptr(t.tid),
+		0,
+		syscall.PTRACE_O_TRACESYSGOOD,
+		0, 0)
+	if errno != 0 {
+		panic(fmt.Sprintf("ptrace set options failed: %v", errno))
+	}
+}
+
+// syscall executes a system call cycle in the traced context.
+//
+// This is _not_ for use by application system calls, rather it is for use when
+// a system call must be injected into the remote context (e.g. mmap, munmap).
+// Note that clones are handled separately.
+func (t *thread) syscall(regs *syscall.PtraceRegs) (uintptr, error) {
+	// Set registers.
+	if err := t.setRegs(regs); err != nil {
+		panic(fmt.Sprintf("ptrace set regs failed: %v", err))
+	}
+
+	for {
+		// Execute the syscall instruction.
+		if _, _, errno := syscall.RawSyscall(syscall.SYS_PTRACE, syscall.PTRACE_SYSCALL, uintptr(t.tid), 0); errno != 0 {
+			panic(fmt.Sprintf("ptrace syscall-enter failed: %v", errno))
+		}
+
+		sig := t.wait()
+		if sig == (0x80 | syscall.SIGTRAP) {
+			// Reached syscall-enter-stop.
+			break
+		} else {
+			// Some other signal caused a thread stop; ignore.
+			continue
+		}
+	}
+
+	// Complete the actual system call.
+	if _, _, errno := syscall.RawSyscall(syscall.SYS_PTRACE, syscall.PTRACE_SYSCALL, uintptr(t.tid), 0); errno != 0 {
+		panic(fmt.Sprintf("ptrace syscall-enter failed: %v", errno))
+	}
+
+	// Wait for syscall-exit-stop. "[Signal-delivery-stop] never happens
+	// between syscall-enter-stop and syscall-exit-stop; it happens *after*
+	// syscall-exit-stop.)" - ptrace(2), "Syscall-stops"
+	if sig := t.wait(); sig != (0x80 | syscall.SIGTRAP) {
+		panic(fmt.Sprintf("wait failed: expected SIGTRAP, got %v [%d]", sig, sig))
+	}
+
+	// Grab registers.
+	if err := t.getRegs(regs); err != nil {
+		panic(fmt.Sprintf("ptrace get regs failed: %v", err))
+	}
+
+	return syscallReturnValue(regs)
+}
+
+// syscallIgnoreInterrupt ignores interrupts on the system call thread and
+// restarts the syscall if the kernel indicates that should happen.
+func (t *thread) syscallIgnoreInterrupt(
+	initRegs *syscall.PtraceRegs,
+	sysno uintptr,
+	args ...arch.SyscallArgument) (uintptr, error) {
+	for {
+		regs := createSyscallRegs(initRegs, sysno, args...)
+		rval, err := t.syscall(&regs)
+		switch err {
+		case ERESTARTSYS:
+			continue
+		case ERESTARTNOINTR:
+			continue
+		case ERESTARTNOHAND:
+			continue
+		default:
+			return rval, err
+		}
+	}
+}
+
+// NotifyInterrupt implements interrupt.Receiver.NotifyInterrupt.
+func (t *thread) NotifyInterrupt() {
+	syscall.Tgkill(int(t.tgid), int(t.tid), syscall.Signal(platform.SignalInterrupt))
+}
+
+// switchToApp is called from the main SwitchToApp entrypoint.
+//
+// This function returns true on a system call, false on a signal.
+func (s *subprocess) switchToApp(c *context, ac arch.Context) bool {
+	regs := &ac.StateData().Regs
+	s.resetSysemuRegs(regs)
+
+	// Extract floating point state.
+	fpState := ac.FloatingPointData()
+	fpLen, _ := ac.FeatureSet().ExtendedStateSize()
+	useXsave := ac.FeatureSet().UseXsave()
+
+	// Lock the thread for ptrace operations.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	// Grab our thread from the pool.
+	currentTID := int32(procid.Current())
+	t := s.sysemuThreads.lookupOrCreate(currentTID, s.newThread)
+
+	// Check for interrupts, and ensure that future interrupts will signal t.
+	if !c.interrupt.Enable(t) {
+		// Pending interrupt; simulate.
+		c.signalInfo = arch.SignalInfo{Signo: int32(platform.SignalInterrupt)}
+		return false
+	}
+	defer c.interrupt.Disable()
+
+	// Ensure that the CPU set is bound appropriately; this makes the
+	// emulation below several times faster, presumably by avoiding
+	// interprocessor wakeups and by simplifying the schedule.
+	t.bind()
+
+	// Set registers.
+	if err := t.setRegs(regs); err != nil {
+		panic(fmt.Sprintf("ptrace set regs failed: %v", err))
+	}
+	if err := t.setFPRegs(fpState, uint64(fpLen), useXsave); err != nil {
+		panic(fmt.Sprintf("ptrace set fpregs failed: %v", err))
+	}
+
+	for {
+		// Start running until the next system call.
+		if isSingleStepping(regs) {
+			if _, _, errno := syscall.RawSyscall(
+				syscall.SYS_PTRACE,
+				syscall.PTRACE_SYSEMU_SINGLESTEP,
+				uintptr(t.tid), 0); errno != 0 {
+				panic(fmt.Sprintf("ptrace sysemu failed: %v", errno))
+			}
+		} else {
+			if _, _, errno := syscall.RawSyscall(
+				syscall.SYS_PTRACE,
+				syscall.PTRACE_SYSEMU,
+				uintptr(t.tid), 0); errno != 0 {
+				panic(fmt.Sprintf("ptrace sysemu failed: %v", errno))
+			}
+		}
+
+		// Wait for the syscall-enter stop.
+		sig := t.wait()
+
+		// Refresh all registers.
+		if err := t.getRegs(regs); err != nil {
+			panic(fmt.Sprintf("ptrace get regs failed: %v", err))
+		}
+		if err := t.getFPRegs(fpState, uint64(fpLen), useXsave); err != nil {
+			panic(fmt.Sprintf("ptrace get fpregs failed: %v", err))
+		}
+
+		// Is it a system call?
+		if sig == (0x80 | syscall.SIGTRAP) {
+			// Ensure registers are sane.
+			updateSyscallRegs(regs)
+			return true
+		}
+
+		if sig == syscall.SIGSTOP {
+			// SIGSTOP was delivered to another thread in the same thread
+			// group, which initiated another group stop. Just ignore it.
+			continue
+		}
+
+		// Grab signal information.
+		if err := t.getSignalInfo(&c.signalInfo); err != nil {
+			// Should never happen.
+			panic(fmt.Sprintf("ptrace get signal info failed: %v", err))
+		}
+
+		// We have a signal. We verify however, that the signal was
+		// either delivered from the kernel or from this process. We
+		// don't respect other signals.
+		if c.signalInfo.Code > 0 {
+			return false // kernel.
+		} else if c.signalInfo.Code <= 0 && c.signalInfo.Pid() == int32(os.Getpid()) {
+			return false // this process.
+		}
+	}
+}
+
+// syscall executes the given system call without handling interruptions.
+func (s *subprocess) syscall(sysno uintptr, args ...arch.SyscallArgument) (uintptr, error) {
+	// Grab a thread.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+	currentTID := int32(procid.Current())
+	t := s.syscallThreads.lookupOrCreate(currentTID, s.newThread)
+
+	return t.syscallIgnoreInterrupt(&s.initRegs, sysno, args...)
+}
+
+// MapFile implements platform.AddressSpace.MapFile.
+func (s *subprocess) MapFile(addr usermem.Addr, fd int, fr platform.FileRange, at usermem.AccessType, precommit bool) error {
+	var flags int
+	if precommit {
+		flags |= syscall.MAP_POPULATE
+	}
+	_, err := s.syscall(
+		syscall.SYS_MMAP,
+		arch.SyscallArgument{Value: uintptr(addr)},
+		arch.SyscallArgument{Value: uintptr(fr.Length())},
+		arch.SyscallArgument{Value: uintptr(at.Prot())},
+		arch.SyscallArgument{Value: uintptr(flags | syscall.MAP_SHARED | syscall.MAP_FIXED)},
+		arch.SyscallArgument{Value: uintptr(fd)},
+		arch.SyscallArgument{Value: uintptr(fr.Start)})
+	return err
+}
+
+// Unmap implements platform.AddressSpace.Unmap.
+func (s *subprocess) Unmap(addr usermem.Addr, length uint64) {
+	ar, ok := addr.ToRange(length)
+	if !ok {
+		panic(fmt.Sprintf("addr %#x + length %#x overflows", addr, length))
+	}
+	s.mu.Lock()
+	for c := range s.contexts {
+		c.mu.Lock()
+		if c.lastFaultSP == s && ar.Contains(c.lastFaultAddr) {
+			// Forget the last fault so that if c faults again, the fault isn't
+			// incorrectly reported as a write fault. If this is being called
+			// due to munmap() of the corresponding vma, handling of the second
+			// fault will fail anyway.
+			c.lastFaultSP = nil
+			delete(s.contexts, c)
+		}
+		c.mu.Unlock()
+	}
+	s.mu.Unlock()
+	_, err := s.syscall(
+		syscall.SYS_MUNMAP,
+		arch.SyscallArgument{Value: uintptr(addr)},
+		arch.SyscallArgument{Value: uintptr(length)})
+	if err != nil {
+		// We never expect this to happen.
+		panic(fmt.Sprintf("munmap(%x, %x)) failed: %v", addr, length, err))
+	}
+}
diff --git a/pkg/sentry/platform/ptrace/subprocess_amd64.go b/pkg/sentry/platform/ptrace/subprocess_amd64.go
new file mode 100644
index 000000000..8211215df
--- /dev/null
+++ b/pkg/sentry/platform/ptrace/subprocess_amd64.go
@@ -0,0 +1,104 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build amd64
+
+package ptrace
+
+import (
+	"syscall"
+
+	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
+)
+
+const (
+	// maximumUserAddress is the largest possible user address.
+	maximumUserAddress = 0x7ffffffff000
+
+	// initRegsRipAdjustment is the size of the syscall instruction.
+	initRegsRipAdjustment = 2
+)
+
+// Linux kernel errnos which "should never be seen by user programs", but will
+// be revealed to ptrace syscall exit tracing.
+//
+// These constants are used in subprocess.go.
+const (
+	ERESTARTSYS    = syscall.Errno(512)
+	ERESTARTNOINTR = syscall.Errno(513)
+	ERESTARTNOHAND = syscall.Errno(514)
+)
+
+// resetSysemuRegs sets up emulation registers.
+//
+// This should be called prior to calling sysemu.
+func (s *subprocess) resetSysemuRegs(regs *syscall.PtraceRegs) {
+	regs.Cs = s.initRegs.Cs
+	regs.Ss = s.initRegs.Ss
+	regs.Ds = s.initRegs.Ds
+	regs.Es = s.initRegs.Es
+	regs.Fs = s.initRegs.Fs
+	regs.Gs = s.initRegs.Gs
+}
+
+// createSyscallRegs sets up syscall registers.
+//
+// This should be called to generate registers for a system call.
+func createSyscallRegs(initRegs *syscall.PtraceRegs, sysno uintptr, args ...arch.SyscallArgument) syscall.PtraceRegs {
+	// Copy initial registers (RIP, segments, etc.).
+	regs := *initRegs
+
+	// Set our syscall number.
+	regs.Rax = uint64(sysno)
+	if len(args) >= 1 {
+		regs.Rdi = args[0].Uint64()
+	}
+	if len(args) >= 2 {
+		regs.Rsi = args[1].Uint64()
+	}
+	if len(args) >= 3 {
+		regs.Rdx = args[2].Uint64()
+	}
+	if len(args) >= 4 {
+		regs.R10 = args[3].Uint64()
+	}
+	if len(args) >= 5 {
+		regs.R8 = args[4].Uint64()
+	}
+	if len(args) >= 6 {
+		regs.R9 = args[5].Uint64()
+	}
+
+	return regs
+}
+
+// isSingleStepping determines if the registers indicate single-stepping.
+func isSingleStepping(regs *syscall.PtraceRegs) bool {
+	return (regs.Eflags & arch.X86TrapFlag) != 0
+}
+
+// updateSyscallRegs updates registers after finishing sysemu.
+func updateSyscallRegs(regs *syscall.PtraceRegs) {
+	// Ptrace puts -ENOSYS in rax on syscall-enter-stop.
+	regs.Rax = regs.Orig_rax
+}
+
+// syscallReturnValue extracts a sensible return from registers.
+func syscallReturnValue(regs *syscall.PtraceRegs) (uintptr, error) {
+	rval := int64(regs.Rax)
+	if rval < 0 {
+		return 0, syscall.Errno(-rval)
+	}
+	return uintptr(rval), nil
+}
diff --git a/pkg/sentry/platform/ptrace/subprocess_linux.go b/pkg/sentry/platform/ptrace/subprocess_linux.go
new file mode 100644
index 000000000..227dd4882
--- /dev/null
+++ b/pkg/sentry/platform/ptrace/subprocess_linux.go
@@ -0,0 +1,146 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build linux
+
+package ptrace
+
+import (
+	"fmt"
+	"syscall"
+
+	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/platform/procid"
+)
+
+// createStub creates a fresh stub processes.
+//
+// Precondition: the runtime OS thread must be locked.
+func createStub() (*thread, error) {
+	// Declare all variables up front in order to ensure that there's no
+	// need for allocations between beforeFork & afterFork.
+	var (
+		pid   uintptr
+		ppid  uintptr
+		errno syscall.Errno
+	)
+
+	// Remember the current ppid for the pdeathsig race.
+	ppid, _, _ = syscall.RawSyscall(syscall.SYS_GETPID, 0, 0, 0)
+
+	// Among other things, beforeFork masks all signals.
+	beforeFork()
+	pid, _, errno = syscall.RawSyscall6(syscall.SYS_CLONE, uintptr(syscall.SIGCHLD)|syscall.CLONE_FILES, 0, 0, 0, 0, 0)
+	if errno != 0 {
+		afterFork()
+		return nil, errno
+	}
+
+	// Is this the parent?
+	if pid != 0 {
+		// Among other things, restore signal mask.
+		afterFork()
+
+		// Initialize the first thread.
+		t := &thread{
+			tgid: int32(pid),
+			tid:  int32(pid),
+			cpu:  ^uint32(0),
+		}
+		if sig := t.wait(); sig != syscall.SIGSTOP {
+			return nil, fmt.Errorf("wait failed: expected SIGSTOP, got %v", sig)
+		}
+		t.attach()
+
+		return t, nil
+	}
+
+	// afterForkInChild resets all signals to their default dispositions
+	// and restores the signal mask to its pre-fork state.
+	afterForkInChild()
+
+	// Explicitly unmask all signals to ensure that the tracer can see
+	// them.
+	errno = unmaskAllSignals()
+	if errno != 0 {
+		syscall.RawSyscall(syscall.SYS_EXIT, uintptr(errno), 0, 0)
+	}
+
+	// Call the stub; should not return.
+	stubCall(stubStart, ppid)
+	panic("unreachable")
+}
+
+// createStub creates a stub processes as a child of an existing subprocesses.
+//
+// Precondition: the runtime OS thread must be locked.
+func (s *subprocess) createStub() (*thread, error) {
+	// There's no need to lock the runtime thread here, as this can only be
+	// called from a context that is already locked.
+	currentTID := int32(procid.Current())
+	t := s.syscallThreads.lookupOrCreate(currentTID, s.newThread)
+
+	// Pass the expected PPID to the child via R15.
+	regs := s.initRegs
+	regs.R15 = uint64(t.tgid)
+
+	// Call fork in a subprocess.
+	//
+	// The new child must set up PDEATHSIG to ensure it dies if this
+	// process dies. Since this process could die at any time, this cannot
+	// be done via instrumentation from here.
+	//
+	// Instead, we create the child untraced, which will do the PDEATHSIG
+	// setup and then SIGSTOP itself for our attach below.
+	pid, err := t.syscallIgnoreInterrupt(
+		&regs,
+		syscall.SYS_CLONE,
+		arch.SyscallArgument{Value: uintptr(syscall.SIGCHLD | syscall.CLONE_FILES)},
+		arch.SyscallArgument{Value: 0},
+		arch.SyscallArgument{Value: 0},
+		arch.SyscallArgument{Value: 0},
+		arch.SyscallArgument{Value: 0},
+		arch.SyscallArgument{Value: 0})
+	if err != nil {
+		return nil, err
+	}
+
+	// Wait for child to enter group-stop, so we don't stop its
+	// bootstrapping work with t.attach below.
+	//
+	// We unfortunately don't have a handy part of memory to write the wait
+	// status. If the wait succeeds, we'll assume that it was the SIGSTOP.
+	// If the child actually exited, the attach below will fail.
+	_, err = t.syscallIgnoreInterrupt(
+		&s.initRegs,
+		syscall.SYS_WAIT4,
+		arch.SyscallArgument{Value: uintptr(pid)},
+		arch.SyscallArgument{Value: 0},
+		arch.SyscallArgument{Value: syscall.WUNTRACED},
+		arch.SyscallArgument{Value: 0},
+		arch.SyscallArgument{Value: 0},
+		arch.SyscallArgument{Value: 0})
+	if err != nil {
+		return nil, err
+	}
+
+	childT := &thread{
+		tgid: int32(pid),
+		tid:  int32(pid),
+		cpu:  ^uint32(0),
+	}
+	childT.attach()
+
+	return childT, nil
+}
diff --git a/pkg/sentry/platform/ptrace/subprocess_linux_amd64_unsafe.go b/pkg/sentry/platform/ptrace/subprocess_linux_amd64_unsafe.go
new file mode 100644
index 000000000..697431472
--- /dev/null
+++ b/pkg/sentry/platform/ptrace/subprocess_linux_amd64_unsafe.go
@@ -0,0 +1,109 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build amd64 linux
+
+package ptrace
+
+import (
+	"sync"
+	"sync/atomic"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
+)
+
+// maskPool contains reusable CPU masks for setting affinity. Unfortunately,
+// runtime.NumCPU doesn't actually record the number of CPUs on the system, it
+// just records the number of CPUs available in the scheduler affinity set at
+// startup. This may a) change over time and b) gives a number far lower than
+// the maximum indexable CPU. To prevent lots of allocation in the hot path, we
+// use a pool to store large masks that we can reuse during bind.
+var maskPool = sync.Pool{
+	New: func() interface{} {
+		const maxCPUs = 1024 // Not a hard limit; see below.
+		return make([]uintptr, maxCPUs/64)
+	},
+}
+
+// unmaskAllSignals unmasks all signals on the current thread.
+//
+//go:nosplit
+func unmaskAllSignals() syscall.Errno {
+	var set linux.SignalSet
+	_, _, errno := syscall.RawSyscall6(syscall.SYS_RT_SIGPROCMASK, linux.SIG_SETMASK, uintptr(unsafe.Pointer(&set)), 0, linux.SignalSetSize, 0, 0)
+	return errno
+}
+
+// getCPU gets the current CPU.
+//
+// Precondition: the current runtime thread should be locked.
+func getCPU() (uint32, error) {
+	var cpu uintptr
+	if _, _, errno := syscall.RawSyscall(
+		unix.SYS_GETCPU,
+		uintptr(unsafe.Pointer(&cpu)),
+		0, 0); errno != 0 {
+		return 0, errno
+	}
+	return uint32(cpu), nil
+}
+
+// setCPU sets the CPU affinity.
+func (t *thread) setCPU(cpu uint32) error {
+	mask := maskPool.Get().([]uintptr)
+	n := int(cpu / 64)
+	v := uintptr(1 << uintptr(cpu%64))
+	if n >= len(mask) {
+		// See maskPool note above. We've actually exceeded the number
+		// of available cores. Grow the mask and return it.
+		mask = make([]uintptr, n+1)
+	}
+	mask[n] |= v
+	if _, _, errno := syscall.RawSyscall(
+		unix.SYS_SCHED_SETAFFINITY,
+		uintptr(t.tid),
+		uintptr(len(mask)*8),
+		uintptr(unsafe.Pointer(&mask[0]))); errno != 0 {
+		return errno
+	}
+	mask[n] &^= v
+	maskPool.Put(mask)
+	return nil
+}
+
+// bind attempts to ensure that the thread is on the same CPU as the current
+// thread. This provides no guarantees as it is fundamentally a racy operation:
+// CPU sets may change and we may be rescheduled in the middle of this
+// operation. As a result, no failures are reported.
+//
+// Precondition: the current runtime thread should be locked.
+func (t *thread) bind() {
+	currentCPU, err := getCPU()
+	if err != nil {
+		return
+	}
+	if oldCPU := atomic.SwapUint32(&t.cpu, currentCPU); oldCPU != currentCPU {
+		// Set the affinity on the thread and save the CPU for next
+		// round; we don't expect CPUs to bounce around too frequently.
+		//
+		// (It's worth noting that we could move CPUs between this point
+		// and when the tracee finishes executing. But that would be
+		// roughly the status quo anyways -- we're just maximizing our
+		// chances of colocation, not guaranteeing it.)
+		t.setCPU(currentCPU)
+	}
+}
diff --git a/pkg/sentry/platform/ptrace/subprocess_unsafe.go b/pkg/sentry/platform/ptrace/subprocess_unsafe.go
new file mode 100644
index 000000000..fe41641d3
--- /dev/null
+++ b/pkg/sentry/platform/ptrace/subprocess_unsafe.go
@@ -0,0 +1,28 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ptrace
+
+import (
+	_ "unsafe" // required for go:linkname.
+)
+
+//go:linkname beforeFork syscall.runtime_BeforeFork
+func beforeFork()
+
+//go:linkname afterFork syscall.runtime_AfterFork
+func afterFork()
+
+//go:linkname afterForkInChild syscall.runtime_AfterForkInChild
+func afterForkInChild()