Add socket filesystem and global disconnected socket mount for VFS2.

A socket mount where anonymous sockets will reside is added to the
VirtualFilesystem. Socketfs is built on top of kernfs.

Updates #1476, #1478, #1484, #1485.

PiperOrigin-RevId: 304095251

4 files changed, 114 insertions, 0 deletions

diff --git a/pkg/sentry/fsimpl/sockfs/BUILD b/pkg/sentry/fsimpl/sockfs/BUILD
new file mode 100644
index 000000000..790d50e65
--- /dev/null
+++ b/pkg/sentry/fsimpl/sockfs/BUILD
@@ -0,0 +1,16 @@
+load("//tools:defs.bzl", "go_library")
+
+licenses(["notice"])
+
+go_library(
+    name = "sockfs",
+    srcs = ["sockfs.go"],
+    visibility = ["//pkg/sentry:internal"],
+    deps = [
+        "//pkg/context",
+        "//pkg/sentry/fsimpl/kernfs",
+        "//pkg/sentry/kernel/auth",
+        "//pkg/sentry/vfs",
+        "//pkg/syserror",
+    ],
+)
diff --git a/pkg/sentry/fsimpl/sockfs/sockfs.go b/pkg/sentry/fsimpl/sockfs/sockfs.go
new file mode 100644
index 000000000..c13511de2
--- /dev/null
+++ b/pkg/sentry/fsimpl/sockfs/sockfs.go
@@ -0,0 +1,73 @@
+// 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.
+
+// Package sockfs provides a filesystem implementation for anonymous sockets.
+package sockfs
+
+import (
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/kernfs"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// NewFilesystem creates a new sockfs filesystem.
+//
+// Note that there should only ever be one instance of sockfs.Filesystem,
+// backing a global socket mount.
+func NewFilesystem(vfsObj *vfs.VirtualFilesystem) *vfs.Filesystem {
+	fs, _, err := filesystemType{}.GetFilesystem(nil, vfsObj, nil, "", vfs.GetFilesystemOptions{})
+	if err != nil {
+		panic("failed to create sockfs filesystem")
+	}
+	return fs
+}
+
+// filesystemType implements vfs.FilesystemType.
+type filesystemType struct{}
+
+// GetFilesystem implements FilesystemType.GetFilesystem.
+func (fsType filesystemType) GetFilesystem(_ context.Context, vfsObj *vfs.VirtualFilesystem, _ *auth.Credentials, _ string, _ vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
+	fs := &filesystem{}
+	fs.Init(vfsObj, fsType)
+	return fs.VFSFilesystem(), nil, nil
+}
+
+// Name implements FilesystemType.Name.
+//
+// Note that registering sockfs is unnecessary, except for the fact that it
+// will not show up under /proc/filesystems as a result. This is a very minor
+// discrepancy from Linux.
+func (filesystemType) Name() string {
+	return "sockfs"
+}
+
+// filesystem implements vfs.FilesystemImpl.
+type filesystem struct {
+	kernfs.Filesystem
+}
+
+// inode implements kernfs.Inode.
+type inode struct {
+	kernfs.InodeNotDirectory
+	kernfs.InodeNotSymlink
+	kernfs.InodeAttrs
+	kernfs.InodeNoopRefCount
+}
+
+// Open implements kernfs.Inode.Open.
+func (i *inode) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
+	return nil, syserror.ENXIO
+}
diff --git a/pkg/sentry/kernel/BUILD b/pkg/sentry/kernel/BUILD
index beba29a09..bb7e3cbc3 100644
--- a/pkg/sentry/kernel/BUILD
+++ b/pkg/sentry/kernel/BUILD
@@ -169,6 +169,7 @@ go_library(
         "//pkg/sentry/fs/lock",
         "//pkg/sentry/fs/timerfd",
         "//pkg/sentry/fsbridge",
+        "//pkg/sentry/fsimpl/sockfs",
         "//pkg/sentry/hostcpu",
         "//pkg/sentry/inet",
         "//pkg/sentry/kernel/auth",
diff --git a/pkg/sentry/kernel/kernel.go b/pkg/sentry/kernel/kernel.go
index 6feda8fa1..0a448b57c 100644
--- a/pkg/sentry/kernel/kernel.go
+++ b/pkg/sentry/kernel/kernel.go
@@ -50,6 +50,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/fs"
 	"gvisor.dev/gvisor/pkg/sentry/fs/timerfd"
 	"gvisor.dev/gvisor/pkg/sentry/fsbridge"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/sockfs"
 	"gvisor.dev/gvisor/pkg/sentry/hostcpu"
 	"gvisor.dev/gvisor/pkg/sentry/inet"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
@@ -225,6 +226,11 @@ type Kernel struct {
 	// by extMu.
 	nextSocketEntry uint64
 
+	// socketMount is a disconnected vfs.Mount, not included in k.vfs,
+	// representing a sockfs.filesystem. socketMount is used to back
+	// VirtualDentries representing anonymous sockets.
+	socketMount *vfs.Mount
+
 	// deviceRegistry is used to save/restore device.SimpleDevices.
 	deviceRegistry struct{} `state:".(*device.Registry)"`
 
@@ -348,6 +354,19 @@ func (k *Kernel) Init(args InitKernelArgs) error {
 	k.monotonicClock = &timekeeperClock{tk: args.Timekeeper, c: sentrytime.Monotonic}
 	k.futexes = futex.NewManager()
 	k.netlinkPorts = port.New()
+	if VFS2Enabled {
+		if err := k.vfs.Init(); err != nil {
+			return fmt.Errorf("failed to initialize VFS: %v", err)
+		}
+		fs := sockfs.NewFilesystem(&k.vfs)
+		// NewDisconnectedMount will take an additional reference on fs.
+		defer fs.DecRef()
+		sm, err := k.vfs.NewDisconnectedMount(fs, nil, &vfs.MountOptions{})
+		if err != nil {
+			return fmt.Errorf("failed to initialize socket mount: %v", err)
+		}
+		k.socketMount = sm
+	}
 	return nil
 }
 
@@ -1452,6 +1471,11 @@ func (k *Kernel) ListSockets() []*SocketEntry {
 	return socks
 }
 
+// SocketMount returns the global socket mount.
+func (k *Kernel) SocketMount() *vfs.Mount {
+	return k.socketMount
+}
+
 // supervisorContext is a privileged context.
 type supervisorContext struct {
 	context.NoopSleeper