Correct FDSize in /proc/[pid]/status.

In Linux, FDSize is fs/proc/array.c:task_state() => struct fdtable::max_fds,
which is set to the underlying array's length in fs/file.c:alloc_fdtable().

Follow-up changes:

- Remove FDTable.GetRefs() and FDTable.GetRefsVFS2(), which are unused.

- Reset FDTable.used to 0 during restore, since the subsequent calls to
  FDTable.setAll() increment it again, causing its value to be doubled. (After
  this CL, FDTable.used is only used to avoid reallocation in FDTable.GetFDs(),
  so this fix is not very visible.)

PiperOrigin-RevId: 331588190

4 files changed, 10 insertions, 32 deletions

diff --git a/pkg/sentry/fs/proc/task.go b/pkg/sentry/fs/proc/task.go
index 9cf7f2a62..103bfc600 100644
--- a/pkg/sentry/fs/proc/task.go
+++ b/pkg/sentry/fs/proc/task.go
@@ -604,7 +604,7 @@ func (s *statusData) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) (
 	var vss, rss, data uint64
 	s.t.WithMuLocked(func(t *kernel.Task) {
 		if fdTable := t.FDTable(); fdTable != nil {
-			fds = fdTable.Size()
+			fds = fdTable.CurrentMaxFDs()
 		}
 		if mm := t.MemoryManager(); mm != nil {
 			vss = mm.VirtualMemorySize()
diff --git a/pkg/sentry/fsimpl/proc/task_files.go b/pkg/sentry/fsimpl/proc/task_files.go
index 356036b9b..ce87b0d47 100644
--- a/pkg/sentry/fsimpl/proc/task_files.go
+++ b/pkg/sentry/fsimpl/proc/task_files.go
@@ -543,7 +543,7 @@ func (s *statusData) Generate(ctx context.Context, buf *bytes.Buffer) error {
 	var vss, rss, data uint64
 	s.task.WithMuLocked(func(t *kernel.Task) {
 		if fdTable := t.FDTable(); fdTable != nil {
-			fds = fdTable.Size()
+			fds = fdTable.CurrentMaxFDs()
 		}
 		if mm := t.MemoryManager(); mm != nil {
 			vss = mm.VirtualMemorySize()
diff --git a/pkg/sentry/kernel/fd_table.go b/pkg/sentry/kernel/fd_table.go
index 89223fa36..e9c382f82 100644
--- a/pkg/sentry/kernel/fd_table.go
+++ b/pkg/sentry/kernel/fd_table.go
@@ -111,6 +111,7 @@ func (f *FDTable) saveDescriptorTable() map[int32]descriptor {
 func (f *FDTable) loadDescriptorTable(m map[int32]descriptor) {
 	ctx := context.Background()
 	f.init() // Initialize table.
+	f.used = 0
 	for fd, d := range m {
 		f.setAll(ctx, fd, d.file, d.fileVFS2, d.flags)
 
@@ -186,12 +187,6 @@ func (f *FDTable) DecRef(ctx context.Context) {
 	})
 }
 
-// Size returns the number of file descriptor slots currently allocated.
-func (f *FDTable) Size() int {
-	size := atomic.LoadInt32(&f.used)
-	return int(size)
-}
-
 // forEach iterates over all non-nil files in sorted order.
 //
 // It is the caller's responsibility to acquire an appropriate lock.
@@ -550,30 +545,6 @@ func (f *FDTable) GetFDs(ctx context.Context) []int32 {
 	return fds
 }
 
-// GetRefs returns a stable slice of references to all files and bumps the
-// reference count on each. The caller must use DecRef on each reference when
-// they're done using the slice.
-func (f *FDTable) GetRefs(ctx context.Context) []*fs.File {
-	files := make([]*fs.File, 0, f.Size())
-	f.forEach(ctx, func(_ int32, file *fs.File, _ *vfs.FileDescription, _ FDFlags) {
-		file.IncRef() // Acquire a reference for caller.
-		files = append(files, file)
-	})
-	return files
-}
-
-// GetRefsVFS2 returns a stable slice of references to all files and bumps the
-// reference count on each. The caller must use DecRef on each reference when
-// they're done using the slice.
-func (f *FDTable) GetRefsVFS2(ctx context.Context) []*vfs.FileDescription {
-	files := make([]*vfs.FileDescription, 0, f.Size())
-	f.forEach(ctx, func(_ int32, _ *fs.File, file *vfs.FileDescription, _ FDFlags) {
-		file.IncRef() // Acquire a reference for caller.
-		files = append(files, file)
-	})
-	return files
-}
-
 // Fork returns an independent FDTable.
 func (f *FDTable) Fork(ctx context.Context) *FDTable {
 	clone := f.k.NewFDTable()
diff --git a/pkg/sentry/kernel/fd_table_unsafe.go b/pkg/sentry/kernel/fd_table_unsafe.go
index 555b14f8e..1c977b60f 100644
--- a/pkg/sentry/kernel/fd_table_unsafe.go
+++ b/pkg/sentry/kernel/fd_table_unsafe.go
@@ -79,6 +79,13 @@ func (f *FDTable) getAll(fd int32) (*fs.File, *vfs.FileDescription, FDFlags, boo
 	return d.file, d.fileVFS2, d.flags, true
 }
 
+// CurrentMaxFDs returns the number of file descriptors that may be stored in f
+// without reallocation.
+func (f *FDTable) CurrentMaxFDs() int {
+	slice := *(*[]unsafe.Pointer)(atomic.LoadPointer(&f.slice))
+	return len(slice)
+}
+
 // set sets an entry.
 //
 // This handles accounting changes, as well as acquiring and releasing the