[PATCH] [gdb/symtab] Add assert in free_cached_comp_units constructor

[Tom de Vries <tdevries@suse.de>]

I wrote a patch containing:
...
      dw2_instantiate_symtab (cu->per_cu, ...);
...
and ran into a use-after-free at a following use of cu.

The problem is that dw2_instantiate_symtab contains:
...
      free_cached_comp_units freer (per_objfile);
...
and that the destructor does:
...
  ~free_cached_comp_units ()
  {
    m_per_objfile->remove_all_cus ();
  }
...
which also frees the cu we used in the cu->per_cu argument to
dw2_instantiate_symtab.

Detect this situation using an assert in the free_cached_comp_units
constructor.

Tested on aarch64-linux.
---
 gdb/dwarf2/read.c | 3 +++
 gdb/dwarf2/read.h | 6 ++++++
 2 files changed, 9 insertions(+)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index c761b732819..0dd0e944597 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -902,6 +902,9 @@ class free_cached_comp_units
   explicit free_cached_comp_units (dwarf2_per_objfile *per_objfile)
     : m_per_objfile (per_objfile)
   {
+    /* The destructor frees all cached comp units, including ones currently
+       cached, so check that there are no currently cached comp units.  */
+    gdb_assert (m_per_objfile->nr_of_cus () == 0);
   }
 
   ~free_cached_comp_units ()
diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index 15dd2abf3a1..603fe089d6a 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -969,6 +969,12 @@ struct dwarf2_per_objfile
   /* Free all cached compilation units.  */
   void remove_all_cus ();
 
+  /* Return the number of cached compilation units.  */
+  size_t nr_of_cus () const
+  {
+    return m_dwarf2_cus.size ();
+  }
+
   /* Increase the age counter on each CU compilation unit and free
      any that are too old.  */
   void age_comp_units ();

base-commit: a8740b7533b4df34d4d7bc5be9bb2108d019fce5
-- 
2.51.0