module_has_debuginfo: avoid heuristic for DWARF

The heuristic approach for detecting whether DWARF debuginfo is loaded
for kernel modules proved to be even less reliable than previously
expected. For instance, on some kernel versions, over half of kernel
modules are wrongly detected as "missing" debuginfo.

To avoid this causing problems in corelens, let's add the less elegant,
but obvious solution. Keep track of which modules we've loaded
debuginfo, and consult this set first. Fall back to heuristic detection
in case we did not record that we had loaded debuginfo. This is
unlikely, given that the common use cases of drgn-tools (corelens and
the CLI) both rely on the APIs here to load debuginfo.

Orabug: 37894875

Signed-off-by: Stephen Brennan <[email protected]>

Author: brenns10

drgn_tools/module.py

@@ -470,6 +470,13 @@ def module_has_debuginfo(module: Object) -> bool:
     """
     if module.prog_.cache.get("using_ctf"):
         return True
+    # The common case for DWARF is that we record which modules' debuginfo we
+    # load as we do it. Then we can easily check the name in the cache.
+    name = module.name.string_().decode().replace("-", "_")
+    if name in module.prog_.cache.get("drgn-tools-loaded-mods", set()):
+        return True
+    # Otherwise, fallback to a heuristic. TODO: drgn 0.0.31 use the module API
+    # for this.
     addrs = _first_kallsyms_symbols(module, 5)
     for addr in addrs:
         try:
@@ -883,6 +890,7 @@ def load_debuginfo(
                 info = fetch_debuginfo(release, [name]).get(name)
             if info:
                 self.obj.prog_.load_debug_info([info])
+                self.obj.prog_.cache.setdefault("drgn-tools-loaded-mods", set()).add(name)
             else:
                 raise FileNotFoundError("Could not find debuginfo for module")
 
@@ -1107,6 +1115,7 @@ def load_module_debuginfo(
         )
 
     prog.load_debug_info(to_load)
+    prog.cache.setdefault("drgn-tools-loaded-mods", set()).update(found_set)
 
 
 def ensure_debuginfo(prog: Program, modules: List[str]) -> Optional[str]: