Remove weak references from unix sockets.

The abstract socket namespace no longer holds any references on sockets.
Instead, TryIncRef() is used when a socket is being retrieved in
BoundEndpoint(). Abstract sockets are now responsible for removing themselves
from the namespace they are in, when they are destroyed.

Updates #1486.

PiperOrigin-RevId: 327064173

5 files changed, 87 insertions, 33 deletions

diff --git a/pkg/sentry/kernel/BUILD b/pkg/sentry/kernel/BUILD
index f6886a758..5416a310d 100644
--- a/pkg/sentry/kernel/BUILD
+++ b/pkg/sentry/kernel/BUILD
@@ -163,6 +163,7 @@ go_library(
         "//pkg/log",
         "//pkg/metric",
         "//pkg/refs",
+        "//pkg/refs_vfs2",
         "//pkg/safemem",
         "//pkg/secio",
         "//pkg/sentry/arch",
diff --git a/pkg/sentry/kernel/abstract_socket_namespace.go b/pkg/sentry/kernel/abstract_socket_namespace.go
index 52ed5cea2..1b9721534 100644
--- a/pkg/sentry/kernel/abstract_socket_namespace.go
+++ b/pkg/sentry/kernel/abstract_socket_namespace.go
@@ -15,29 +15,21 @@
 package kernel
 
 import (
+	"fmt"
 	"syscall"
 
 	"gvisor.dev/gvisor/pkg/context"
-	"gvisor.dev/gvisor/pkg/refs"
+	"gvisor.dev/gvisor/pkg/refs_vfs2"
 	"gvisor.dev/gvisor/pkg/sentry/socket/unix/transport"
 	"gvisor.dev/gvisor/pkg/sync"
 )
 
 // +stateify savable
 type abstractEndpoint struct {
-	ep   transport.BoundEndpoint
-	wr   *refs.WeakRef
-	name string
-	ns   *AbstractSocketNamespace
-}
-
-// WeakRefGone implements refs.WeakRefUser.WeakRefGone.
-func (e *abstractEndpoint) WeakRefGone(context.Context) {
-	e.ns.mu.Lock()
-	if e.ns.endpoints[e.name].ep == e.ep {
-		delete(e.ns.endpoints, e.name)
-	}
-	e.ns.mu.Unlock()
+	ep     transport.BoundEndpoint
+	socket refs_vfs2.RefCounter
+	name   string
+	ns     *AbstractSocketNamespace
 }
 
 // AbstractSocketNamespace is used to implement the Linux abstract socket functionality.
@@ -46,7 +38,11 @@ func (e *abstractEndpoint) WeakRefGone(context.Context) {
 type AbstractSocketNamespace struct {
 	mu sync.Mutex `state:"nosave"`
 
-	// Keeps mapping from name to endpoint.
+	// Keeps a mapping from name to endpoint. AbstractSocketNamespace does not hold
+	// any references on any sockets that it contains; when retrieving a socket,
+	// TryIncRef() must be called in case the socket is concurrently being
+	// destroyed. It is the responsibility of the socket to remove itself from the
+	// abstract socket namespace when it is destroyed.
 	endpoints map[string]abstractEndpoint
 }
 
@@ -58,15 +54,15 @@ func NewAbstractSocketNamespace() *AbstractSocketNamespace {
 }
 
 // A boundEndpoint wraps a transport.BoundEndpoint to maintain a reference on
-// its backing object.
+// its backing socket.
 type boundEndpoint struct {
 	transport.BoundEndpoint
-	rc refs.RefCounter
+	socket refs_vfs2.RefCounter
 }
 
 // Release implements transport.BoundEndpoint.Release.
 func (e *boundEndpoint) Release(ctx context.Context) {
-	e.rc.DecRef(ctx)
+	e.socket.DecRef(ctx)
 	e.BoundEndpoint.Release(ctx)
 }
 
@@ -81,32 +77,59 @@ func (a *AbstractSocketNamespace) BoundEndpoint(name string) transport.BoundEndp
 		return nil
 	}
 
-	rc := ep.wr.Get()
-	if rc == nil {
-		delete(a.endpoints, name)
+	if !ep.socket.TryIncRef() {
+		// The socket has reached zero references and is being destroyed.
 		return nil
 	}
 
-	return &boundEndpoint{ep.ep, rc}
+	return &boundEndpoint{ep.ep, ep.socket}
 }
 
 // Bind binds the given socket.
 //
-// When the last reference managed by rc is dropped, ep may be removed from the
+// When the last reference managed by socket is dropped, ep may be removed from the
 // namespace.
-func (a *AbstractSocketNamespace) Bind(ctx context.Context, name string, ep transport.BoundEndpoint, rc refs.RefCounter) error {
+func (a *AbstractSocketNamespace) Bind(ctx context.Context, name string, ep transport.BoundEndpoint, socket refs_vfs2.RefCounter) error {
 	a.mu.Lock()
 	defer a.mu.Unlock()
 
+	// Check if there is already a socket (which has not yet been destroyed) bound at name.
 	if ep, ok := a.endpoints[name]; ok {
-		if rc := ep.wr.Get(); rc != nil {
-			rc.DecRef(ctx)
+		if ep.socket.TryIncRef() {
+			ep.socket.DecRef(ctx)
 			return syscall.EADDRINUSE
 		}
 	}
 
 	ae := abstractEndpoint{ep: ep, name: name, ns: a}
-	ae.wr = refs.NewWeakRef(rc, &ae)
+	ae.socket = socket
 	a.endpoints[name] = ae
 	return nil
 }
+
+// Remove removes the specified socket at name from the abstract socket
+// namespace, if it has not yet been replaced.
+func (a *AbstractSocketNamespace) Remove(name string, socket refs_vfs2.RefCounter) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+
+	ep, ok := a.endpoints[name]
+	if !ok {
+		// We never delete a map entry apart from a socket's destructor (although the
+		// map entry may be overwritten). Therefore, a socket should exist, even if it
+		// may not be the one we expect.
+		panic(fmt.Sprintf("expected socket to exist at '%s' in abstract socket namespace", name))
+	}
+
+	// A Bind() operation may race with callers of Remove(), e.g. in the
+	// following case:
+	//   socket1 reaches zero references and begins destruction
+	//   a.Bind("foo", ep, socket2) replaces socket1 with socket2
+	//   socket1's destructor calls a.Remove("foo", socket1)
+	//
+	// Therefore, we need to check that the socket at name is what we expect
+	// before modifying the map.
+	if ep.socket == socket {
+		delete(a.endpoints, name)
+	}
+}
diff --git a/pkg/sentry/socket/unix/BUILD b/pkg/sentry/socket/unix/BUILD
index 061a689a9..cb953e4dc 100644
--- a/pkg/sentry/socket/unix/BUILD
+++ b/pkg/sentry/socket/unix/BUILD
@@ -1,12 +1,25 @@
 load("//tools:defs.bzl", "go_library")
+load("//tools/go_generics:defs.bzl", "go_template_instance")
 
 package(licenses = ["notice"])
 
+go_template_instance(
+    name = "socket_refs",
+    out = "socket_refs.go",
+    package = "unix",
+    prefix = "socketOpsCommon",
+    template = "//pkg/refs_vfs2:refs_template",
+    types = {
+        "T": "socketOpsCommon",
+    },
+)
+
 go_library(
     name = "unix",
     srcs = [
         "device.go",
         "io.go",
+        "socket_refs.go",
         "unix.go",
         "unix_vfs2.go",
     ],
@@ -15,6 +28,7 @@ go_library(
         "//pkg/abi/linux",
         "//pkg/context",
         "//pkg/fspath",
+        "//pkg/log",
         "//pkg/refs",
         "//pkg/safemem",
         "//pkg/sentry/arch",
diff --git a/pkg/sentry/socket/unix/unix.go b/pkg/sentry/socket/unix/unix.go
index 2b8454edb..b7e8e4325 100644
--- a/pkg/sentry/socket/unix/unix.go
+++ b/pkg/sentry/socket/unix/unix.go
@@ -24,7 +24,6 @@ import (
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/fspath"
-	"gvisor.dev/gvisor/pkg/refs"
 	"gvisor.dev/gvisor/pkg/sentry/arch"
 	"gvisor.dev/gvisor/pkg/sentry/fs"
 	"gvisor.dev/gvisor/pkg/sentry/fs/fsutil"
@@ -80,7 +79,7 @@ func NewWithDirent(ctx context.Context, d *fs.Dirent, ep transport.Endpoint, sty
 			stype: stype,
 		},
 	}
-	s.EnableLeakCheck("unix.SocketOperations")
+	s.EnableLeakCheck()
 
 	return fs.NewFile(ctx, d, flags, &s)
 }
@@ -89,17 +88,26 @@ func NewWithDirent(ctx context.Context, d *fs.Dirent, ep transport.Endpoint, sty
 //
 // +stateify savable
 type socketOpsCommon struct {
-	refs.AtomicRefCount
+	socketOpsCommonRefs
 	socket.SendReceiveTimeout
 
 	ep    transport.Endpoint
 	stype linux.SockType
+
+	// abstractName and abstractNamespace indicate the name and namespace of the
+	// socket if it is bound to an abstract socket namespace. Once the socket is
+	// bound, they cannot be modified.
+	abstractName      string
+	abstractNamespace *kernel.AbstractSocketNamespace
 }
 
 // DecRef implements RefCounter.DecRef.
 func (s *socketOpsCommon) DecRef(ctx context.Context) {
-	s.DecRefWithDestructor(ctx, func(context.Context) {
+	s.socketOpsCommonRefs.DecRef(func() {
 		s.ep.Close(ctx)
+		if s.abstractNamespace != nil {
+			s.abstractNamespace.Remove(s.abstractName, s)
+		}
 	})
 }
 
@@ -284,10 +292,14 @@ func (s *SocketOperations) Bind(t *kernel.Task, sockaddr []byte) *syserr.Error {
 			if t.IsNetworkNamespaced() {
 				return syserr.ErrInvalidEndpointState
 			}
-			if err := t.AbstractSockets().Bind(t, p[1:], bep, s); err != nil {
+			asn := t.AbstractSockets()
+			name := p[1:]
+			if err := asn.Bind(t, name, bep, s); err != nil {
 				// syserr.ErrPortInUse corresponds to EADDRINUSE.
 				return syserr.ErrPortInUse
 			}
+			s.abstractName = name
+			s.abstractNamespace = asn
 		} else {
 			// The parent and name.
 			var d *fs.Dirent
diff --git a/pkg/sentry/socket/unix/unix_vfs2.go b/pkg/sentry/socket/unix/unix_vfs2.go
index dfa25241a..d066ef8ab 100644
--- a/pkg/sentry/socket/unix/unix_vfs2.go
+++ b/pkg/sentry/socket/unix/unix_vfs2.go
@@ -183,10 +183,14 @@ func (s *SocketVFS2) Bind(t *kernel.Task, sockaddr []byte) *syserr.Error {
 			if t.IsNetworkNamespaced() {
 				return syserr.ErrInvalidEndpointState
 			}
-			if err := t.AbstractSockets().Bind(t, p[1:], bep, s); err != nil {
+			asn := t.AbstractSockets()
+			name := p[1:]
+			if err := asn.Bind(t, name, bep, s); err != nil {
 				// syserr.ErrPortInUse corresponds to EADDRINUSE.
 				return syserr.ErrPortInUse
 			}
+			s.abstractName = name
+			s.abstractNamespace = asn
 		} else {
 			path := fspath.Parse(p)
 			root := t.FSContext().RootDirectoryVFS2()