3 files changed, 60 insertions, 3 deletions
diff --git a/pkg/sentry/kernel/semaphore/semaphore.go b/pkg/sentry/kernel/semaphore/semaphore.go
index 31198d772..3dd3953b3 100644
--- a/pkg/sentry/kernel/semaphore/semaphore.go
+++ b/pkg/sentry/kernel/semaphore/semaphore.go
@@ -52,6 +52,9 @@ type Registry struct {
 	mu         sync.Mutex `state:"nosave"`
 	semaphores map[int32]*Set
 	lastIDUsed int32
+	// indexes maintains a mapping between a set's index in virtual array and
+	// its identifier.
+	indexes map[int32]int32
 }
 
 // Set represents a set of semaphores that can be operated atomically.
@@ -113,6 +116,7 @@ func NewRegistry(userNS *auth.UserNamespace) *Registry {
 	return &Registry{
 		userNS:     userNS,
 		semaphores: make(map[int32]*Set),
+		indexes:    make(map[int32]int32),
 	}
 }
 
@@ -163,6 +167,9 @@ func (r *Registry) FindOrCreate(ctx context.Context, key, nsems int32, mode linu
 	}
 
 	// Apply system limits.
+	//
+	// Map semaphores and map indexes in a registry are of the same size,
+	// check map semaphores only here for the system limit.
 	if len(r.semaphores) >= setsMax {
 		return nil, syserror.EINVAL
 	}
@@ -192,6 +199,23 @@ func (r *Registry) IPCInfo() *linux.SemInfo {
 	}
 }
 
+// HighestIndex returns the index of the highest used entry in
+// the kernel's array.
+func (r *Registry) HighestIndex() int32 {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	// By default, highest used index is 0 even though
+	// there is no semaphroe set.
+	var highestIndex int32
+	for index := range r.indexes {
+		if index > highestIndex {
+			highestIndex = index
+		}
+	}
+	return highestIndex
+}
+
 // RemoveID removes set with give 'id' from the registry and marks the set as
 // dead. All waiters will be awakened and fail.
 func (r *Registry) RemoveID(id int32, creds *auth.Credentials) error {
@@ -202,6 +226,11 @@ func (r *Registry) RemoveID(id int32, creds *auth.Credentials) error {
 	if set == nil {
 		return syserror.EINVAL
 	}
+	index, found := r.findIndexByID(id)
+	if !found {
+		// Inconsistent state.
+		panic(fmt.Sprintf("unable to find an index for ID: %d", id))
+	}
 
 	set.mu.Lock()
 	defer set.mu.Unlock()
@@ -213,6 +242,7 @@ func (r *Registry) RemoveID(id int32, creds *auth.Credentials) error {
 	}
 
 	delete(r.semaphores, set.ID)
+	delete(r.indexes, index)
 	set.destroy()
 	return nil
 }
@@ -236,6 +266,11 @@ func (r *Registry) newSet(ctx context.Context, key int32, owner, creator fs.File
 			continue
 		}
 		if r.semaphores[id] == nil {
+			index, found := r.findFirstAvailableIndex()
+			if !found {
+				panic("unable to find an available index")
+			}
+			r.indexes[index] = id
 			r.lastIDUsed = id
 			r.semaphores[id] = set
 			set.ID = id
@@ -263,6 +298,24 @@ func (r *Registry) findByKey(key int32) *Set {
 	return nil
 }
 
+func (r *Registry) findIndexByID(id int32) (int32, bool) {
+	for k, v := range r.indexes {
+		if v == id {
+			return k, true
+		}
+	}
+	return 0, false
+}
+
+func (r *Registry) findFirstAvailableIndex() (int32, bool) {
+	for index := int32(0); index < setsMax; index++ {
+		if _, present := r.indexes[index]; !present {
+			return index, true
+		}
+	}
+	return 0, false
+}
+
 func (r *Registry) totalSems() int {
 	totalSems := 0
 	for _, v := range r.semaphores {
diff --git a/pkg/sentry/kernel/semaphore/semaphore_state_autogen.go b/pkg/sentry/kernel/semaphore/semaphore_state_autogen.go
index aed6cfa29..e0578836c 100644
--- a/pkg/sentry/kernel/semaphore/semaphore_state_autogen.go
+++ b/pkg/sentry/kernel/semaphore/semaphore_state_autogen.go
@@ -15,6 +15,7 @@ func (r *Registry) StateFields() []string {
 		"userNS",
 		"semaphores",
 		"lastIDUsed",
+		"indexes",
 	}
 }
 
@@ -25,6 +26,7 @@ func (r *Registry) StateSave(stateSinkObject state.Sink) {
 	stateSinkObject.Save(0, &r.userNS)
 	stateSinkObject.Save(1, &r.semaphores)
 	stateSinkObject.Save(2, &r.lastIDUsed)
+	stateSinkObject.Save(3, &r.indexes)
 }
 
 func (r *Registry) afterLoad() {}
@@ -33,6 +35,7 @@ func (r *Registry) StateLoad(stateSourceObject state.Source) {
 	stateSourceObject.Load(0, &r.userNS)
 	stateSourceObject.Load(1, &r.semaphores)
 	stateSourceObject.Load(2, &r.lastIDUsed)
+	stateSourceObject.Load(3, &r.indexes)
 }
 
 func (s *Set) StateTypeName() string {
diff --git a/pkg/sentry/syscalls/linux/sys_sem.go b/pkg/sentry/syscalls/linux/sys_sem.go
index a1601676f..a62a6b3b5 100644
--- a/pkg/sentry/syscalls/linux/sys_sem.go
+++ b/pkg/sentry/syscalls/linux/sys_sem.go
@@ -150,9 +150,10 @@ func Semctl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscal
 		buf := args[3].Pointer()
 		r := t.IPCNamespace().SemaphoreRegistry()
 		info := r.IPCInfo()
-		_, err := info.CopyOut(t, buf)
-		// TODO(gvisor.dev/issue/137): Return the index of the highest used entry.
-		return 0, nil, err
+		if _, err := info.CopyOut(t, buf); err != nil {
+			return 0, nil, err
+		}
+		return uintptr(r.HighestIndex()), nil, nil
 
 	case linux.SEM_INFO,
 		linux.SEM_STAT,