linux: Improve compatibility with ancient kernels

Author: gcmoreira

volatility3/framework/symbols/linux/extensions/__init__.py

@@ -316,16 +316,26 @@ def is_valid(self) -> bool:
         if self.pid < 0:
             return False
 
-        if not (self.signal and self.signal.is_readable()):
+        if self.has_member("signal") and not (
+            self.signal and self.signal.is_readable()
+        ):
             return False
 
-        if not (self.nsproxy and self.nsproxy.is_readable()):
+        if self.has_member("nsproxy") and not (
+            self.nsproxy and self.nsproxy.is_readable()
+        ):
             return False
 
-        if not (self.real_parent and self.real_parent.is_readable()):
+        if self.has_member("real_parent") and not (
+            self.real_parent and self.real_parent.is_readable()
+        ):
             return False
 
-        if self.active_mm and not self.active_mm.is_readable():
+        if (
+            self.has_member("active_mm")
+            and self.active_mm
+            and not self.active_mm.is_readable()
+        ):
             return False
 
         if self.mm: