Add NETLINK_KOBJECT_UEVENT socket support

NETLINK_KOBJECT_UEVENT sockets send udev-style messages for device events.
gVisor doesn't have any device events, so our sockets don't need to do anything
once created.

systemd's device manager needs to be able to create one of these sockets. It
also wants to install a BPF filter on the socket. Since we'll never send any
messages, the filter would never be invoked, thus we just fake it out.

Fixes #1117
Updates #1119

PiperOrigin-RevId: 278405893

5 files changed, 131 insertions, 0 deletions

diff --git a/pkg/sentry/socket/netlink/provider.go b/pkg/sentry/socket/netlink/provider.go
index 689cad997..be005df24 100644
--- a/pkg/sentry/socket/netlink/provider.go
+++ b/pkg/sentry/socket/netlink/provider.go
@@ -30,6 +30,13 @@ type Protocol interface {
 	// Protocol returns the Linux netlink protocol value.
 	Protocol() int
 
+	// CanSend returns true if this protocol may ever send messages.
+	//
+	// TODO(gvisor.dev/issue/1119): This is a workaround to allow
+	// advertising support for otherwise unimplemented features on sockets
+	// that will never send messages, thus making those features no-ops.
+	CanSend() bool
+
 	// ProcessMessage processes a single message from userspace.
 	//
 	// If err == nil, any messages added to ms will be sent back to the
diff --git a/pkg/sentry/socket/netlink/route/protocol.go b/pkg/sentry/socket/netlink/route/protocol.go
index cc70ac237..6b4a0ecf4 100644
--- a/pkg/sentry/socket/netlink/route/protocol.go
+++ b/pkg/sentry/socket/netlink/route/protocol.go
@@ -61,6 +61,11 @@ func (p *Protocol) Protocol() int {
 	return linux.NETLINK_ROUTE
 }
 
+// CanSend implements netlink.Protocol.CanSend.
+func (p *Protocol) CanSend() bool {
+	return true
+}
+
 // dumpLinks handles RTM_GETLINK + NLM_F_DUMP requests.
 func (p *Protocol) dumpLinks(ctx context.Context, hdr linux.NetlinkMessageHeader, data []byte, ms *netlink.MessageSet) *syserr.Error {
 	// NLM_F_DUMP + RTM_GETLINK messages are supposed to include an
diff --git a/pkg/sentry/socket/netlink/socket.go b/pkg/sentry/socket/netlink/socket.go
index 05dac4f0a..4a1b87a9a 100644
--- a/pkg/sentry/socket/netlink/socket.go
+++ b/pkg/sentry/socket/netlink/socket.go
@@ -54,6 +54,8 @@ const (
 	maxSendBufferSize = 4 << 20 // 4MB
 )
 
+var errNoFilter = syserr.New("no filter attached", linux.ENOENT)
+
 // netlinkSocketDevice is the netlink socket virtual device.
 var netlinkSocketDevice = device.NewAnonDevice()
 
@@ -108,6 +110,12 @@ type Socket struct {
 
 	// passcred indicates if this socket wants SCM credentials.
 	passcred bool
+
+	// filter indicates that this socket has a BPF filter "installed".
+	//
+	// TODO(gvisor.dev/issue/1119): We don't actually support filtering,
+	// this is just bookkeeping for tracking add/remove.
+	filter bool
 }
 
 var _ socket.Socket = (*Socket)(nil)
@@ -400,6 +408,40 @@ func (s *Socket) SetSockOpt(t *kernel.Task, level int, name int, opt []byte) *sy
 			s.mu.Unlock()
 			return nil
 
+		case linux.SO_ATTACH_FILTER:
+			// TODO(gvisor.dev/issue/1119): We don't actually
+			// support filtering. If this socket can't ever send
+			// messages, then there is nothing to filter and we can
+			// advertise support. Otherwise, be conservative and
+			// return an error.
+			if s.protocol.CanSend() {
+				socket.SetSockOptEmitUnimplementedEvent(t, name)
+				return syserr.ErrProtocolNotAvailable
+			}
+
+			s.mu.Lock()
+			s.filter = true
+			s.mu.Unlock()
+			return nil
+
+		case linux.SO_DETACH_FILTER:
+			// TODO(gvisor.dev/issue/1119): See above.
+			if s.protocol.CanSend() {
+				socket.SetSockOptEmitUnimplementedEvent(t, name)
+				return syserr.ErrProtocolNotAvailable
+			}
+
+			s.mu.Lock()
+			filter := s.filter
+			s.filter = false
+			s.mu.Unlock()
+
+			if !filter {
+				return errNoFilter
+			}
+
+			return nil
+
 		default:
 			socket.SetSockOptEmitUnimplementedEvent(t, name)
 		}
diff --git a/pkg/sentry/socket/netlink/uevent/BUILD b/pkg/sentry/socket/netlink/uevent/BUILD
new file mode 100644
index 000000000..0777f3baf
--- /dev/null
+++ b/pkg/sentry/socket/netlink/uevent/BUILD
@@ -0,0 +1,17 @@
+load("//tools/go_stateify:defs.bzl", "go_library")
+
+package(licenses = ["notice"])
+
+go_library(
+    name = "uevent",
+    srcs = ["protocol.go"],
+    importpath = "gvisor.dev/gvisor/pkg/sentry/socket/netlink/uevent",
+    visibility = ["//pkg/sentry:internal"],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/sentry/context",
+        "//pkg/sentry/kernel",
+        "//pkg/sentry/socket/netlink",
+        "//pkg/syserr",
+    ],
+)
diff --git a/pkg/sentry/socket/netlink/uevent/protocol.go b/pkg/sentry/socket/netlink/uevent/protocol.go
new file mode 100644
index 000000000..b5d7808d7
--- /dev/null
+++ b/pkg/sentry/socket/netlink/uevent/protocol.go
@@ -0,0 +1,60 @@
+// Copyright 2019 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 uevent provides a NETLINK_KOBJECT_UEVENT socket protocol.
+//
+// NETLINK_KOBJECT_UEVENT sockets send udev-style device events. gVisor does
+// not support any device events, so these sockets never send any messages.
+package uevent
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/socket/netlink"
+	"gvisor.dev/gvisor/pkg/syserr"
+)
+
+// Protocol implements netlink.Protocol.
+//
+// +stateify savable
+type Protocol struct{}
+
+var _ netlink.Protocol = (*Protocol)(nil)
+
+// NewProtocol creates a NETLINK_KOBJECT_UEVENT netlink.Protocol.
+func NewProtocol(t *kernel.Task) (netlink.Protocol, *syserr.Error) {
+	return &Protocol{}, nil
+}
+
+// Protocol implements netlink.Protocol.Protocol.
+func (p *Protocol) Protocol() int {
+	return linux.NETLINK_KOBJECT_UEVENT
+}
+
+// CanSend implements netlink.Protocol.CanSend.
+func (p *Protocol) CanSend() bool {
+	return false
+}
+
+// ProcessMessage implements netlink.Protocol.ProcessMessage.
+func (p *Protocol) ProcessMessage(ctx context.Context, hdr linux.NetlinkMessageHeader, data []byte, ms *netlink.MessageSet) *syserr.Error {
+	// Silently ignore all messages.
+	return nil
+}
+
+// init registers the NETLINK_KOBJECT_UEVENT provider.
+func init() {
+	netlink.RegisterProvider(linux.NETLINK_KOBJECT_UEVENT, NewProtocol)
+}