2 files changed, 58 insertions, 0 deletions
diff --git a/pkg/sentry/kernel/fd_table.go b/pkg/sentry/kernel/fd_table.go
index cc3f43a45..11f613a11 100644
--- a/pkg/sentry/kernel/fd_table.go
+++ b/pkg/sentry/kernel/fd_table.go
@@ -81,6 +81,9 @@ type FDTable struct {
 	// mu protects below.
 	mu sync.Mutex `state:"nosave"`
 
+	// next is start position to find fd.
+	next int32
+
 	// used contains the number of non-nil entries. It must be accessed
 	// atomically. It may be read atomically without holding mu (but not
 	// written).
@@ -226,6 +229,11 @@ func (f *FDTable) NewFDs(ctx context.Context, fd int32, files []*fs.File, flags
 	f.mu.Lock()
 	defer f.mu.Unlock()
 
+	// From f.next to find available fd.
+	if fd < f.next {
+		fd = f.next
+	}
+
 	// Install all entries.
 	for i := fd; i < end && len(fds) < len(files); i++ {
 		if d, _, _ := f.get(i); d == nil {
@@ -242,6 +250,11 @@ func (f *FDTable) NewFDs(ctx context.Context, fd int32, files []*fs.File, flags
 		return nil, syscall.EMFILE
 	}
 
+	if fd == f.next {
+		// Update next search start position.
+		f.next = fds[len(fds)-1] + 1
+	}
+
 	return fds, nil
 }
 
@@ -361,6 +374,11 @@ func (f *FDTable) Remove(fd int32) *fs.File {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 
+	// Update current available position.
+	if fd < f.next {
+		f.next = fd
+	}
+
 	orig, _, _ := f.get(fd)
 	if orig != nil {
 		orig.IncRef()             // Reference for caller.
@@ -377,6 +395,10 @@ func (f *FDTable) RemoveIf(cond func(*fs.File, FDFlags) bool) {
 	f.forEach(func(fd int32, file *fs.File, flags FDFlags) {
 		if cond(file, flags) {
 			f.set(fd, nil, FDFlags{}) // Clear from table.
+			// Update current available position.
+			if fd < f.next {
+				f.next = fd
+			}
 		}
 	})
 }
diff --git a/pkg/sentry/kernel/fd_table_test.go b/pkg/sentry/kernel/fd_table_test.go
index 2413788e7..2bcb6216a 100644
--- a/pkg/sentry/kernel/fd_table_test.go
+++ b/pkg/sentry/kernel/fd_table_test.go
@@ -70,6 +70,42 @@ func TestFDTableMany(t *testing.T) {
 		if err := fdTable.NewFDAt(ctx, 1, file, FDFlags{}); err != nil {
 			t.Fatalf("fdTable.NewFDAt(1, r, FDFlags{}): got %v, wanted nil", err)
 		}
+
+		i := int32(2)
+		fdTable.Remove(i)
+		if fds, err := fdTable.NewFDs(ctx, 0, []*fs.File{file}, FDFlags{}); err != nil || fds[0] != i {
+			t.Fatalf("Allocated %v FDs but wanted to allocate %v: %v", i, maxFD, err)
+		}
+	})
+}
+
+func TestFDTableOverLimit(t *testing.T) {
+	runTest(t, func(ctx context.Context, fdTable *FDTable, file *fs.File, _ *limits.LimitSet) {
+		if _, err := fdTable.NewFDs(ctx, maxFD, []*fs.File{file}, FDFlags{}); err == nil {
+			t.Fatalf("fdTable.NewFDs(maxFD, f): got nil, wanted error")
+		}
+
+		if _, err := fdTable.NewFDs(ctx, maxFD-2, []*fs.File{file, file, file}, FDFlags{}); err == nil {
+			t.Fatalf("fdTable.NewFDs(maxFD-2, {f,f,f}): got nil, wanted error")
+		}
+
+		if fds, err := fdTable.NewFDs(ctx, maxFD-3, []*fs.File{file, file, file}, FDFlags{}); err != nil {
+			t.Fatalf("fdTable.NewFDs(maxFD-3, {f,f,f}): got %v, wanted nil", err)
+		} else {
+			for _, fd := range fds {
+				fdTable.Remove(fd)
+			}
+		}
+
+		if fds, err := fdTable.NewFDs(ctx, maxFD-1, []*fs.File{file}, FDFlags{}); err != nil || fds[0] != maxFD-1 {
+			t.Fatalf("fdTable.NewFDAt(1, r, FDFlags{}): got %v, wanted nil", err)
+		}
+
+		if fds, err := fdTable.NewFDs(ctx, 0, []*fs.File{file}, FDFlags{}); err != nil {
+			t.Fatalf("Adding an FD to a resized map: got %v, want nil", err)
+		} else if len(fds) != 1 || fds[0] != 0 {
+			t.Fatalf("Added an FD to a resized map: got %v, want {1}", fds)
+		}
 	})
 }