Move CtxIPCNamespace to kernel/ipc package.

CtxIPCNamespace is needed by mqfs package to be able to retreive an
IPCNamespace using ctx.Value. As ctx.Value compares keys as interfaces,
we need to use type kernel.contextID in package mqfs, which is not
possible due to circular depenedency, so move it to kernel/ipc instead.

Updates #136

pkg/sentry/kernel/BUILD

@@ -216,6 +216,7 @@ go_library(
     visibility = ["//:sandbox"],
     deps = [
         ":uncaught_signal_go_proto",
+        "//pkg/sentry/kernel/ipc",
         "//pkg/abi",
         "//pkg/abi/linux",
         "//pkg/abi/linux/errno",

pkg/sentry/kernel/context.go

@@ -16,6 +16,7 @@ package kernel
 
 import (
 	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/ipc"
 )
 
 // contextID is the kernel package's type for context.Context.Value keys.
@@ -37,9 +38,6 @@ const (
 
 	// CtxUTSNamespace is a Context.Value key for a UTSNamespace.
 	CtxUTSNamespace
-
-	// CtxIPCNamespace is a Context.Value key for a IPCNamespace.
-	CtxIPCNamespace
 )
 
 // ContextCanTrace returns true if ctx is permitted to trace t, in the same sense
@@ -82,7 +80,7 @@ func UTSNamespaceFromContext(ctx context.Context) *UTSNamespace {
 // or nil if there is no such IPC namespace. It takes a reference on the
 // namespace.
 func IPCNamespaceFromContext(ctx context.Context) *IPCNamespace {
-	if v := ctx.Value(CtxIPCNamespace); v != nil {
+	if v := ctx.Value(ipc.CtxIPCNamespace); v != nil {
 		return v.(*IPCNamespace)
 	}
 	return nil

pkg/sentry/kernel/ipc/BUILD

@@ -7,6 +7,7 @@ go_library(
     srcs = [
         "object.go",
         "registry.go",
+        "ns.go",
     ],
     visibility = ["//pkg/sentry:internal"],
     deps = [

pkg/sentry/kernel/ipc/ns.go

@@ -0,0 +1,22 @@
+// 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.
+
+package ipc
+
+type contextID int
+
+// CtxIPCNamespace is the context.Value key used to retreive an IPC namespace.
+// We define it here because it's needed in several packages, and is not
+// possible to use otherwise without causing a circular depenedency.
+const CtxIPCNamespace contextID = iota

pkg/sentry/kernel/kernel.go

@@ -59,6 +59,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/epoll"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/futex"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/ipc"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/sched"
 	ktime "gvisor.dev/gvisor/pkg/sentry/kernel/time"
 	"gvisor.dev/gvisor/pkg/sentry/limits"
@@ -861,7 +862,7 @@ func (ctx *createProcessContext) Value(key interface{}) interface{} {
 		return ctx.args.PIDNamespace
 	case CtxUTSNamespace:
 		return ctx.args.UTSNamespace
-	case CtxIPCNamespace:
+	case ipc.CtxIPCNamespace:
 		ipcns := ctx.args.IPCNamespace
 		ipcns.IncRef()
 		return ipcns
@@ -1689,7 +1690,7 @@ func (ctx supervisorContext) Value(key interface{}) interface{} {
 		return ctx.k.tasks.Root
 	case CtxUTSNamespace:
 		return ctx.k.rootUTSNamespace
-	case CtxIPCNamespace:
+	case ipc.CtxIPCNamespace:
 		ipcns := ctx.k.rootIPCNamespace
 		ipcns.IncRef()
 		return ipcns

pkg/sentry/kernel/task_context.go

@@ -23,6 +23,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/fs"
 	"gvisor.dev/gvisor/pkg/sentry/inet"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/ipc"
 	ktime "gvisor.dev/gvisor/pkg/sentry/kernel/time"
 	"gvisor.dev/gvisor/pkg/sentry/limits"
 	"gvisor.dev/gvisor/pkg/sentry/pgalloc"
@@ -73,7 +74,7 @@ func (t *Task) contextValue(key interface{}, isTaskGoroutine bool) interface{} {
 			defer t.mu.Unlock()
 		}
 		return t.utsns
-	case CtxIPCNamespace:
+	case ipc.CtxIPCNamespace:
 		if !isTaskGoroutine {
 			t.mu.Lock()
 			defer t.mu.Unlock()