Make semctl IPC_INFO cmd return the index of highest used entry.

PiperOrigin-RevId: 346973338

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 {

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,

test/syscalls/linux/semaphore.cc

@@ -20,6 +20,7 @@
 #include <atomic>
 #include <cerrno>
 #include <ctime>
+#include <stack>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
@@ -774,8 +775,32 @@ TEST(SemaphoreTest, SemopGetncntOnSignal_NoRandomSave) {
 }
 
 TEST(SemaphoreTest, IpcInfo) {
+  std::stack<int> sem_ids;
+  std::stack<int> max_used_indexes;
   struct seminfo info;
-  ASSERT_THAT(semctl(0, 0, IPC_INFO, &info), SyscallSucceeds());
+  for (int i = 0; i < 3; i++) {
+    int sem_id = 0;
+    ASSERT_THAT(sem_id = semget(IPC_PRIVATE, 1, 0600 | IPC_CREAT),
+                SyscallSucceeds());
+    sem_ids.push(sem_id);
+    int max_used_index = 0;
+    EXPECT_THAT(max_used_index = semctl(0, 0, IPC_INFO, &info),
+                SyscallSucceeds());
+    if (!max_used_indexes.empty()) {
+      EXPECT_GT(max_used_index, max_used_indexes.top());
+    }
+    max_used_indexes.push(max_used_index);
+  }
+  while (!sem_ids.empty()) {
+    int sem_id = sem_ids.top();
+    sem_ids.pop();
+    ASSERT_THAT(semctl(sem_id, 0, IPC_RMID), SyscallSucceeds());
+    int max_index = max_used_indexes.top();
+    EXPECT_THAT(max_index = semctl(0, 0, IPC_INFO, &info), SyscallSucceeds());
+    EXPECT_GE(max_used_indexes.top(), max_index);
+    max_used_indexes.pop();
+  }
+  ASSERT_THAT(semctl(0, 0, IPC_INFO, &info), SyscallSucceedsWithValue(0));
 
   EXPECT_EQ(info.semmap, 1024000000);
   EXPECT_EQ(info.semmni, 32000);