8 files changed, 60 insertions, 21 deletions
diff --git a/pkg/sentry/arch/BUILD b/pkg/sentry/arch/BUILD
index 34c0a867d..e27f21e5e 100644
--- a/pkg/sentry/arch/BUILD
+++ b/pkg/sentry/arch/BUILD
@@ -14,6 +14,7 @@ go_library(
         "arch_state_aarch64.go",
         "arch_state_x86.go",
         "arch_x86.go",
+        "arch_x86_impl.go",
         "auxv.go",
         "signal.go",
         "signal_act.go",
diff --git a/pkg/sentry/arch/arch_aarch64.go b/pkg/sentry/arch/arch_aarch64.go
index 53039465d..5053393c1 100644
--- a/pkg/sentry/arch/arch_aarch64.go
+++ b/pkg/sentry/arch/arch_aarch64.go
@@ -40,6 +40,9 @@ const (
 	fpsimdContextSize = 0x210
 )
 
+// ARMTrapFlag is the mask for the trap flag.
+const ARMTrapFlag = uint64(1) << 21
+
 // aarch64FPState is aarch64 floating point state.
 type aarch64FPState []byte
 
diff --git a/pkg/sentry/arch/arch_amd64.s b/pkg/sentry/arch/arch_amd64.s
index bd61402cf..6c10336e7 100644
--- a/pkg/sentry/arch/arch_amd64.s
+++ b/pkg/sentry/arch/arch_amd64.s
@@ -26,10 +26,11 @@
 //
 // func initX86FPState(data *FloatingPointData, useXsave bool)
 //
-// We need to clear out and initialize an empty fp state area since the sentry
-// may have left sensitive information in the floating point registers.
+// We need to clear out and initialize an empty fp state area since the sentry,
+// or any previous loader, may have left sensitive information in the floating
+// point registers.
 //
-// Preconditions: data is zeroed
+// Preconditions: data is zeroed.
 TEXT ·initX86FPState(SB), $24-16
 	// Save MXCSR (callee-save)
 	STMXCSR	mxcsr-8(SP)
diff --git a/pkg/sentry/arch/arch_state_x86.go b/pkg/sentry/arch/arch_state_x86.go
index d388ee9cf..e35c9214a 100644
--- a/pkg/sentry/arch/arch_state_x86.go
+++ b/pkg/sentry/arch/arch_state_x86.go
@@ -43,8 +43,8 @@ func (e ErrFloatingPoint) Error() string {
 // and SSE state, so this is the equivalent XSTATE_BV value.
 const fxsaveBV uint64 = cpuid.XSAVEFeatureX87 | cpuid.XSAVEFeatureSSE
 
-// afterLoad is invoked by stateify.
-func (s *State) afterLoad() {
+// afterLoadFPState is invoked by afterLoad.
+func (s *State) afterLoadFPState() {
 	old := s.x86FPState
 
 	// Recreate the slice. This is done to ensure that it is aligned
diff --git a/pkg/sentry/arch/arch_x86.go b/pkg/sentry/arch/arch_x86.go
index a18093155..88b40a9d1 100644
--- a/pkg/sentry/arch/arch_x86.go
+++ b/pkg/sentry/arch/arch_x86.go
@@ -114,6 +114,10 @@ func newX86FPStateSlice() []byte {
 	size, align := cpuid.HostFeatureSet().ExtendedStateSize()
 	capacity := size
 	// Always use at least 4096 bytes.
+	//
+	// For the KVM platform, this state is a fixed 4096 bytes, so make sure
+	// that the underlying array is at _least_ that size otherwise we will
+	// corrupt random memory. This is not a pleasant thing to debug.
 	if capacity < 4096 {
 		capacity = 4096
 	}
@@ -151,21 +155,6 @@ func NewFloatingPointData() *FloatingPointData {
 	return (*FloatingPointData)(&(newX86FPState()[0]))
 }
 
-// State contains the common architecture bits for X86 (the build tag of this
-// file ensures it's only built on x86).
-//
-// +stateify savable
-type State struct {
-	// The system registers.
-	Regs syscall.PtraceRegs `state:".(syscallPtraceRegs)"`
-
-	// Our floating point state.
-	x86FPState `state:"wait"`
-
-	// FeatureSet is a pointer to the currently active feature set.
-	FeatureSet *cpuid.FeatureSet
-}
-
 // Proto returns a protobuf representation of the system registers in State.
 func (s State) Proto() *rpb.Registers {
 	regs := &rpb.AMD64Registers{
diff --git a/pkg/sentry/arch/arch_x86_impl.go b/pkg/sentry/arch/arch_x86_impl.go
new file mode 100644
index 000000000..04ac283c6
--- /dev/null
+++ b/pkg/sentry/arch/arch_x86_impl.go
@@ -0,0 +1,43 @@
+// Copyright 2020 The gVisor Authors.
+//
+// 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 i386
+
+package arch
+
+import (
+	"syscall"
+
+	"gvisor.dev/gvisor/pkg/cpuid"
+)
+
+// State contains the common architecture bits for X86 (the build tag of this
+// file ensures it's only built on x86).
+//
+// +stateify savable
+type State struct {
+	// The system registers.
+	Regs syscall.PtraceRegs `state:".(syscallPtraceRegs)"`
+
+	// Our floating point state.
+	x86FPState `state:"wait"`
+
+	// FeatureSet is a pointer to the currently active feature set.
+	FeatureSet *cpuid.FeatureSet
+}
+
+// afterLoad is invoked by stateify.
+func (s *State) afterLoad() {
+	s.afterLoadFPState()
+}
diff --git a/pkg/sentry/arch/signal_amd64.go b/pkg/sentry/arch/signal_amd64.go
index 81b92bb43..6fb756f0e 100644
--- a/pkg/sentry/arch/signal_amd64.go
+++ b/pkg/sentry/arch/signal_amd64.go
@@ -55,7 +55,7 @@ type SignalContext64 struct {
 	Trapno  uint64
 	Oldmask linux.SignalSet
 	Cr2     uint64
-	// Pointer to a struct _fpstate.
+	// Pointer to a struct _fpstate. See b/33003106#comment8.
 	Fpstate  uint64
 	Reserved [8]uint64
 }
diff --git a/pkg/sentry/arch/signal_arm64.go b/pkg/sentry/arch/signal_arm64.go
index 28615f97f..0c1db4b13 100644
--- a/pkg/sentry/arch/signal_arm64.go
+++ b/pkg/sentry/arch/signal_arm64.go
@@ -40,6 +40,8 @@ type aarch64Ctx struct {
 	Size  uint32
 }
 
+// FpsimdContext is equivalent to struct fpsimd_context on arm64
+// (arch/arm64/include/uapi/asm/sigcontext.h).
 type FpsimdContext struct {
 	Head  aarch64Ctx
 	Fpsr  uint32