don't take an unnecessary reference in proc.fdSymlink.Valid()

Reported-by: syzbot+8622a8a08287adc17bc9@syzkaller.appspotmail.com
PiperOrigin-RevId: 510454946

pkg/sentry/fsimpl/proc/task_fds.go

@@ -43,12 +43,13 @@ func getTaskFD(t *kernel.Task, fd int32) (*vfs.FileDescription, kernel.FDFlags)
 }
 
 func taskFDExists(ctx context.Context, fs *filesystem, t *kernel.Task, fd int32) bool {
-	file, _ := getTaskFD(t, fd)
-	if file == nil {
-		return false
-	}
-	fs.SafeDecRefFD(ctx, file)
-	return true
+	var exists bool
+	t.WithMuLocked(func(task *kernel.Task) {
+		if fdt := t.FDTable(); fdt != nil {
+			exists = fdt.Exists(fd)
+		}
+	})
+	return exists
 }
 
 // +stateify savable

pkg/sentry/kernel/fd_table.go

@@ -440,6 +440,17 @@ func (f *FDTable) GetFDs(ctx context.Context) []int32 {
 	return fds
 }
 
+// Exists returns whether fd is defined in the table. It is inherently racy.
+//
+//go:nosplit
+func (f *FDTable) Exists(fd int32) bool {
+	if fd < 0 {
+		return false
+	}
+	file, _, _ := f.get(fd)
+	return file != nil
+}
+
 // Fork returns an independent FDTable, cloning all FDs up to maxFds (non-inclusive).
 func (f *FDTable) Fork(ctx context.Context, maxFds int32) *FDTable {
 	clone := f.k.NewFDTable()