Fix alignment issue with 64-bit atomics on 32 bit machines

5 files changed, 150 insertions, 8 deletions

diff --git a/pkg/tcpip/BUILD b/pkg/tcpip/BUILD
index f979d22f0..f71a805bf 100644
--- a/pkg/tcpip/BUILD
+++ b/pkg/tcpip/BUILD
@@ -18,6 +18,8 @@ go_template_instance(
 go_library(
     name = "tcpip",
     srcs = [
+        "aligned.go",
+        "aligned_unsafe.go",
         "errors.go",
         "sock_err_list.go",
         "socketops.go",
diff --git a/pkg/tcpip/aligned.go b/pkg/tcpip/aligned.go
new file mode 100644
index 000000000..ea22c101f
--- /dev/null
+++ b/pkg/tcpip/aligned.go
@@ -0,0 +1,61 @@
+// Copyright 2021 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 !arm,!386
+
+package tcpip
+
+import "sync/atomic"
+
+// AlignedAtomicInt64 is an atomic int64 that is guaranteed to be 64-bit
+// aligned, even on 32-bit systems. On 64-bit machines, it's just a regular
+// int64.
+//
+// See aligned_unsafe.go in this directory for justification.
+type AlignedAtomicInt64 struct {
+	value int64
+}
+
+func (aa *AlignedAtomicInt64) Load() int64 {
+	return atomic.LoadInt64(&aa.value)
+}
+
+func (aa *AlignedAtomicInt64) Store(v int64) {
+	atomic.StoreInt64(&aa.value, v)
+}
+
+func (aa *AlignedAtomicInt64) Add(v int64) int64 {
+	return atomic.AddInt64(&aa.value, v)
+}
+
+// AlignedAtomicUint64 is an atomic uint64 that is guaranteed to be 64-bit
+// aligned, even on 32-bit systems. On 64-bit machines, it's just a regular
+// uint64.
+//
+// See aligned_unsafe.go in this directory for justification.
+type AlignedAtomicUint64 struct {
+	value uint64
+}
+
+func (aa *AlignedAtomicUint64) Load() uint64 {
+	return atomic.LoadUint64(&aa.value)
+}
+
+func (aa *AlignedAtomicUint64) Store(v uint64) {
+	atomic.StoreUint64(&aa.value, v)
+}
+
+func (aa *AlignedAtomicUint64) Add(v uint64) uint64 {
+	return atomic.AddUint64(&aa.value, v)
+}
diff --git a/pkg/tcpip/aligned_unsafe.go b/pkg/tcpip/aligned_unsafe.go
new file mode 100644
index 000000000..35e869453
--- /dev/null
+++ b/pkg/tcpip/aligned_unsafe.go
@@ -0,0 +1,80 @@
+// Copyright 2021 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 arm 386
+
+package tcpip
+
+import (
+	"sync/atomic"
+	"unsafe"
+)
+
+// AlignedAtomicInt64 is an atomic int64 that is guaranteed to be 64-bit
+// aligned, even on 32-bit systems.
+//
+// Per https://golang.org/pkg/sync/atomic/#pkg-note-BUG:
+//
+// "On ARM, 386, and 32-bit MIPS, it is the caller's responsibility to arrange
+// for 64-bit alignment of 64-bit words accessed atomically. The first word in
+// a variable or in an allocated struct, array, or slice can be relied upon to
+// be 64-bit aligned."
+type AlignedAtomicInt64 struct {
+	value [15]byte
+}
+
+func (aa *AlignedAtomicInt64) ptr() *int64 {
+	return (*int64)(unsafe.Pointer((uintptr(unsafe.Pointer(&aa.value)) + 7) &^ 7))
+}
+
+func (aa *AlignedAtomicInt64) Load() int64 {
+	return atomic.LoadInt64(aa.ptr())
+}
+
+func (aa *AlignedAtomicInt64) Store(v int64) {
+	atomic.StoreInt64(aa.ptr(), v)
+}
+
+func (aa *AlignedAtomicInt64) Add(v int64) int64 {
+	return atomic.AddInt64(aa.ptr(), v)
+}
+
+// AlignedAtomicUint64 is an atomic uint64 that is guaranteed to be 64-bit
+// aligned, even on 32-bit systems.
+//
+// Per https://golang.org/pkg/sync/atomic/#pkg-note-BUG:
+//
+// "On ARM, 386, and 32-bit MIPS, it is the caller's responsibility to arrange
+// for 64-bit alignment of 64-bit words accessed atomically. The first word in
+// a variable or in an allocated struct, array, or slice can be relied upon to
+// be 64-bit aligned."
+type AlignedAtomicUint64 struct {
+	value [15]byte
+}
+
+func (aa *AlignedAtomicUint64) ptr() *uint64 {
+	return (*uint64)(unsafe.Pointer((uintptr(unsafe.Pointer(&aa.value)) + 7) &^ 7))
+}
+
+func (aa *AlignedAtomicUint64) Load() uint64 {
+	return atomic.LoadUint64(aa.ptr())
+}
+
+func (aa *AlignedAtomicUint64) Store(v uint64) {
+	atomic.StoreUint64(aa.ptr(), v)
+}
+
+func (aa *AlignedAtomicUint64) Add(v uint64) uint64 {
+	return atomic.AddUint64(aa.ptr(), v)
+}
diff --git a/pkg/tcpip/socketops.go b/pkg/tcpip/socketops.go
index dc37e61a4..9ce5f231b 100644
--- a/pkg/tcpip/socketops.go
+++ b/pkg/tcpip/socketops.go
@@ -205,7 +205,7 @@ type SocketOptions struct {
 	getSendBufferLimits GetSendBufferLimits `state:"manual"`
 
 	// sendBufferSize determines the send buffer size for this socket.
-	sendBufferSize int64
+	sendBufferSize AlignedAtomicInt64
 
 	// mu protects the access to the below fields.
 	mu sync.Mutex `state:"nosave"`
@@ -595,7 +595,7 @@ func (so *SocketOptions) SetBindToDevice(bindToDevice int32) Error {
 
 // GetSendBufferSize gets value for SO_SNDBUF option.
 func (so *SocketOptions) GetSendBufferSize() int64 {
-	return atomic.LoadInt64(&so.sendBufferSize)
+	return so.sendBufferSize.Load()
 }
 
 // SetSendBufferSize sets value for SO_SNDBUF option. notify indicates if the
@@ -604,7 +604,7 @@ func (so *SocketOptions) SetSendBufferSize(sendBufferSize int64, notify bool) {
 	v := sendBufferSize
 
 	if !notify {
-		atomic.StoreInt64(&so.sendBufferSize, v)
+		so.sendBufferSize.Store(v)
 		return
 	}
 
@@ -630,5 +630,5 @@ func (so *SocketOptions) SetSendBufferSize(sendBufferSize int64, notify bool) {
 
 	// Notify endpoint about change in buffer size.
 	newSz := so.handler.OnSetSendBufferSize(v)
-	atomic.StoreInt64(&so.sendBufferSize, newSz)
+	so.sendBufferSize.Store(newSz)
 }
diff --git a/pkg/tcpip/tcpip.go b/pkg/tcpip/tcpip.go
index 87ea09a5e..a2935d695 100644
--- a/pkg/tcpip/tcpip.go
+++ b/pkg/tcpip/tcpip.go
@@ -37,7 +37,6 @@ import (
 	"reflect"
 	"strconv"
 	"strings"
-	"sync/atomic"
 	"time"
 
 	"gvisor.dev/gvisor/pkg/sync"
@@ -1204,7 +1203,7 @@ type NetworkProtocolNumber uint32
 
 // A StatCounter keeps track of a statistic.
 type StatCounter struct {
-	count uint64
+	count AlignedAtomicUint64
 }
 
 // Increment adds one to the counter.
@@ -1219,12 +1218,12 @@ func (s *StatCounter) Decrement() {
 
 // Value returns the current value of the counter.
 func (s *StatCounter) Value() uint64 {
-	return atomic.LoadUint64(&s.count)
+	return s.count.Load()
 }
 
 // IncrementBy increments the counter by v.
 func (s *StatCounter) IncrementBy(v uint64) {
-	atomic.AddUint64(&s.count, v)
+	s.count.Add(v)
 }
 
 func (s *StatCounter) String() string {