3 files changed, 153 insertions, 14 deletions
diff --git a/pkg/sentry/kernel/shm/shm.go b/pkg/sentry/kernel/shm/shm.go
index 13ec7afe0..00c03585e 100644
--- a/pkg/sentry/kernel/shm/shm.go
+++ b/pkg/sentry/kernel/shm/shm.go
@@ -39,7 +39,6 @@ import (
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/log"
-	"gvisor.dev/gvisor/pkg/refs"
 	"gvisor.dev/gvisor/pkg/sentry/fs"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	ktime "gvisor.dev/gvisor/pkg/sentry/kernel/time"
@@ -252,7 +251,7 @@ func (r *Registry) newShm(ctx context.Context, pid int32, key Key, creator fs.Fi
 		creatorPID:    pid,
 		changeTime:    ktime.NowFromContext(ctx),
 	}
-	shm.EnableLeakCheck("kernel.Shm")
+	shm.EnableLeakCheck()
 
 	// Find the next available ID.
 	for id := r.lastIDUsed + 1; id != r.lastIDUsed; id++ {
@@ -337,14 +336,14 @@ func (r *Registry) remove(s *Shm) {
 //
 // +stateify savable
 type Shm struct {
-	// AtomicRefCount tracks the number of references to this segment.
+	// ShmRefs tracks the number of references to this segment.
 	//
 	// A segment holds a reference to itself until it is marked for
 	// destruction.
 	//
 	// In addition to direct users, the MemoryManager will hold references
 	// via MappingIdentity.
-	refs.AtomicRefCount
+	ShmRefs
 
 	mfp pgalloc.MemoryFileProvider
 
@@ -428,11 +427,14 @@ func (s *Shm) InodeID() uint64 {
 	return uint64(s.ID)
 }
 
-// DecRef overrides refs.RefCount.DecRef with a destructor.
+// DecRef drops a reference on s.
 //
 // Precondition: Caller must not hold s.mu.
 func (s *Shm) DecRef(ctx context.Context) {
-	s.DecRefWithDestructor(ctx, s.destroy)
+	s.ShmRefs.DecRef(func() {
+		s.mfp.MemoryFile().DecRef(s.fr)
+		s.registry.remove(s)
+	})
 }
 
 // Msync implements memmap.MappingIdentity.Msync. Msync is a no-op for shm
@@ -642,11 +644,6 @@ func (s *Shm) Set(ctx context.Context, ds *linux.ShmidDS) error {
 	return nil
 }
 
-func (s *Shm) destroy(context.Context) {
-	s.mfp.MemoryFile().DecRef(s.fr)
-	s.registry.remove(s)
-}
-
 // MarkDestroyed marks a segment for destruction. The segment is actually
 // destroyed once it has no references. MarkDestroyed may be called multiple
 // times, and is safe to call after a segment has already been destroyed. See
diff --git a/pkg/sentry/kernel/shm/shm_refs.go b/pkg/sentry/kernel/shm/shm_refs.go
new file mode 100644
index 000000000..51e07d0b3
--- /dev/null
+++ b/pkg/sentry/kernel/shm/shm_refs.go
@@ -0,0 +1,118 @@
+package shm
+
+import (
+	"fmt"
+	"runtime"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/log"
+	refs_vfs1 "gvisor.dev/gvisor/pkg/refs"
+)
+
+// ownerType is used to customize logging. Note that we use a pointer to T so
+// that we do not copy the entire object when passed as a format parameter.
+var ShmownerType *Shm
+
+// Refs implements refs.RefCounter. It keeps a reference count using atomic
+// operations and calls the destructor when the count reaches zero.
+//
+// Note that the number of references is actually refCount + 1 so that a default
+// zero-value Refs object contains one reference.
+//
+// TODO(gvisor.dev/issue/1486): Store stack traces when leak check is enabled in
+// a map with 16-bit hashes, and store the hash in the top 16 bits of refCount.
+// This will allow us to add stack trace information to the leak messages
+// without growing the size of Refs.
+//
+// +stateify savable
+type ShmRefs struct {
+	// refCount is composed of two fields:
+	//
+	//	[32-bit speculative references]:[32-bit real references]
+	//
+	// Speculative references are used for TryIncRef, to avoid a CompareAndSwap
+	// loop. See IncRef, DecRef and TryIncRef for details of how these fields are
+	// used.
+	refCount int64
+}
+
+func (r *ShmRefs) finalize() {
+	var note string
+	switch refs_vfs1.GetLeakMode() {
+	case refs_vfs1.NoLeakChecking:
+		return
+	case refs_vfs1.UninitializedLeakChecking:
+		note = "(Leak checker uninitialized): "
+	}
+	if n := r.ReadRefs(); n != 0 {
+		log.Warningf("%sRefs %p owned by %T garbage collected with ref count of %d (want 0)", note, r, ShmownerType, n)
+	}
+}
+
+// EnableLeakCheck checks for reference leaks when Refs gets garbage collected.
+func (r *ShmRefs) EnableLeakCheck() {
+	if refs_vfs1.GetLeakMode() != refs_vfs1.NoLeakChecking {
+		runtime.SetFinalizer(r, (*ShmRefs).finalize)
+	}
+}
+
+// ReadRefs returns the current number of references. The returned count is
+// inherently racy and is unsafe to use without external synchronization.
+func (r *ShmRefs) ReadRefs() int64 {
+
+	return atomic.LoadInt64(&r.refCount) + 1
+}
+
+// IncRef implements refs.RefCounter.IncRef.
+//
+//go:nosplit
+func (r *ShmRefs) IncRef() {
+	if v := atomic.AddInt64(&r.refCount, 1); v <= 0 {
+		panic(fmt.Sprintf("Incrementing non-positive ref count %p owned by %T", r, ShmownerType))
+	}
+}
+
+// TryIncRef implements refs.RefCounter.TryIncRef.
+//
+// To do this safely without a loop, a speculative reference is first acquired
+// on the object. This allows multiple concurrent TryIncRef calls to distinguish
+// other TryIncRef calls from genuine references held.
+//
+//go:nosplit
+func (r *ShmRefs) TryIncRef() bool {
+	const speculativeRef = 1 << 32
+	v := atomic.AddInt64(&r.refCount, speculativeRef)
+	if int32(v) < 0 {
+
+		atomic.AddInt64(&r.refCount, -speculativeRef)
+		return false
+	}
+
+	atomic.AddInt64(&r.refCount, -speculativeRef+1)
+	return true
+}
+
+// DecRef implements refs.RefCounter.DecRef.
+//
+// Note that speculative references are counted here. Since they were added
+// prior to real references reaching zero, they will successfully convert to
+// real references. In other words, we see speculative references only in the
+// following case:
+//
+//	A: TryIncRef [speculative increase => sees non-negative references]
+//	B: DecRef [real decrease]
+//	A: TryIncRef [transform speculative to real]
+//
+//go:nosplit
+func (r *ShmRefs) DecRef(destroy func()) {
+	switch v := atomic.AddInt64(&r.refCount, -1); {
+	case v < -1:
+		panic(fmt.Sprintf("Decrementing non-positive ref count %p, owned by %T", r, ShmownerType))
+
+	case v == -1:
+
+		if destroy != nil {
+			destroy()
+		}
+	}
+}
diff --git a/pkg/sentry/kernel/shm/shm_state_autogen.go b/pkg/sentry/kernel/shm/shm_state_autogen.go
index c98632087..fa80353b6 100644
--- a/pkg/sentry/kernel/shm/shm_state_autogen.go
+++ b/pkg/sentry/kernel/shm/shm_state_autogen.go
@@ -47,7 +47,7 @@ func (x *Shm) StateTypeName() string {
 
 func (x *Shm) StateFields() []string {
 	return []string{
-		"AtomicRefCount",
+		"ShmRefs",
 		"mfp",
 		"registry",
 		"ID",
@@ -71,7 +71,7 @@ func (x *Shm) beforeSave() {}
 
 func (x *Shm) StateSave(m state.Sink) {
 	x.beforeSave()
-	m.Save(0, &x.AtomicRefCount)
+	m.Save(0, &x.ShmRefs)
 	m.Save(1, &x.mfp)
 	m.Save(2, &x.registry)
 	m.Save(3, &x.ID)
@@ -93,7 +93,7 @@ func (x *Shm) StateSave(m state.Sink) {
 func (x *Shm) afterLoad() {}
 
 func (x *Shm) StateLoad(m state.Source) {
-	m.Load(0, &x.AtomicRefCount)
+	m.Load(0, &x.ShmRefs)
 	m.Load(1, &x.mfp)
 	m.Load(2, &x.registry)
 	m.Load(3, &x.ID)
@@ -112,7 +112,31 @@ func (x *Shm) StateLoad(m state.Source) {
 	m.Load(16, &x.pendingDestruction)
 }
 
+func (x *ShmRefs) StateTypeName() string {
+	return "pkg/sentry/kernel/shm.ShmRefs"
+}
+
+func (x *ShmRefs) StateFields() []string {
+	return []string{
+		"refCount",
+	}
+}
+
+func (x *ShmRefs) beforeSave() {}
+
+func (x *ShmRefs) StateSave(m state.Sink) {
+	x.beforeSave()
+	m.Save(0, &x.refCount)
+}
+
+func (x *ShmRefs) afterLoad() {}
+
+func (x *ShmRefs) StateLoad(m state.Source) {
+	m.Load(0, &x.refCount)
+}
+
 func init() {
 	state.Register((*Registry)(nil))
 	state.Register((*Shm)(nil))
+	state.Register((*ShmRefs)(nil))
 }