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[RFA] nameless LOAD_DLL_DEBUG_EVENT causes ntdll.dll to be missing


We observed on Windows 2012 that we were unable to unwind past
exception handlers. For instance, with any Ada program raising
an exception that does not get handled:

    % gnatmake -g a -bargs -shared
    % gdb a
    (gdb) start
    (gdb) catch exception unhandled
    Catchpoint 2: unhandled Ada exceptions
    (gdb) c
    Catchpoint 2, unhandled CONSTRAINT_ERROR at <__gnat_unhandled_exception> (
        e=0x645ff820 <constraint_error>) at s-excdeb.adb:53
    53      s-excdeb.adb: No such file or directory.

At this point, we can already see that something went wrong, since
the frame selected by the debugger corresponds to a runtime function
rather than the function in the user code that caused the exception
to be raised (in our case procedure A).

This is further confirmed by the fact that we are unable to unwind
all the way to procedure A:

    (gdb) bt
    #0  <__gnat_unhandled_exception> (e=0x645ff820 <constraint_error>)
        at s-excdeb.adb:53
    #1  0x000000006444e9a3 in <__gnat_notify_unhandled_exception> (excep=0x284d20)
        at a-exextr.adb:144
    #2  0x00000000645f106a in __gnat_personality_imp ()
       from C:\[...]\libgnat-7.3.dll
    #3  0x000000006144d1b7 in _GCC_specific_handler (ms_exc=0x242fab0,
        this_frame=0x242fe60, ms_orig_context=0x242f5c0, ms_disp=0x242ef70,
        gcc_per=0x645f0960 <__gnat_personality_imp>)
        at ../../../src/libgcc/unwind-seh.c:289
    #4  0x00000000645f1211 in __gnat_personality_seh0 ()
       from C:\[...]\libgnat-7.3.dll
    #5  0x000007fad3879f4d in ?? ()
    Backtrace stopped: previous frame inner to this frame (corrupt stack?)

It turns out that the unwinder has been doing its job flawlessly
up until frame #5. The address in frame #5 is correct, but GDB
is not able to associate it with any symbol or unwind record.

And this is because this address is inside ntdll.dll, and when
we received the LOAD_DLL_DEBUG_EVENT for that DLL, the system
was not able to tell us the name of the library, thus causing us
to silently ignoring the event. Because GDB does not know about
ntdll.dll, it is unable to access the unwind information from it.
And because the function at that address does not use a frame
pointer, the unwinding becomes impossible.

This patch helps recovering ntdll.dll at the end of the "run/attach"
phase, simply by trying to locate that specific DLL again.

In terms of our medium to long term planning, it seems to me that
we should be able to simplify the code by ignoring LOAD_DLL_DEBUG_EVENT
during the startup phase, and modify windows_ensure_ntdll_loaded
to then detect and report all shared libraries after we've finished
inferior creation.  But for a change just before 7.7 branch creation,
I thought it was safest to just handle ntdll.dll specifically. This
is less intrusive, and ntdll is the only DLL affected by the problem
I know so far.


	* windows-nat.c (windows_ensure_ntdll_loaded): New function.
	(do_initial_windows_stuff): Use windows_ensure_ntdll_loaded.

Tested on all versions of Windows at AdaCore (XP all the way to 2012),
both x86 and x86_64, but using the AdaCore gdb-testsuite. No regression.

OK to commit?


 gdb/windows-nat.c | 60 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)

diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c
index e32c701..be5a132 100644
--- a/gdb/windows-nat.c
+++ b/gdb/windows-nat.c
@@ -1703,6 +1703,64 @@ windows_wait (struct target_ops *ops,
+/* On certain versions of Windows, the information about ntdll.dll
+   is not available yet at the time we get the LOAD_DLL_DEBUG_EVENT,
+   thus preventing us from reporting this DLL as an SO. This has been
+   witnessed on Windows 8.1, for instance.  A possible explanation
+   is that ntdll.dll might be mapped before the SO info gets created
+   by the Windows system -- ntdll.dll is the first DLL to be reported
+   via LOAD_DLL_DEBUG_EVENT and other DLLs do not seem to suffer from
+   that problem.
+   If we indeed are missing ntdll.dll, this function tries to recover
+   from this issue, after the fact.  Do nothing if we encounter any
+   issue trying to locate that DLL.  */
+static void
+windows_ensure_ntdll_loaded (void)
+  struct so_list *so;
+  HMODULE dummy_hmodule;
+  DWORD cb_needed;
+  HMODULE *hmodules;
+  int i;
+  for (so =; so != NULL; so = so->next)
+    if (FILENAME_CMP (lbasename (so->so_name), "ntdll.dll") == 0)
+      return;  /* ntdll.dll already loaded, nothing to do.  */
+  if (EnumProcessModules (current_process_handle, &dummy_hmodule,
+			  sizeof (HMODULE), &cb_needed) == 0)
+    return;
+  if (cb_needed < 1)
+    return;
+  hmodules = (HMODULE *) alloca (cb_needed);
+  if (EnumProcessModules (current_process_handle, hmodules,
+			  cb_needed, &cb_needed) == 0)
+    return;
+  for (i = 0; i < (int) (cb_needed / sizeof (HMODULE)); i++)
+    {
+      MODULEINFO mi;
+      char dll_name[__PMAX];
+      if (GetModuleInformation (current_process_handle, hmodules[i],
+				&mi, sizeof (mi)) == 0)
+	continue;
+      if (GetModuleFileNameEx (current_process_handle, hmodules[i],
+			       dll_name, sizeof (dll_name)) == 0)
+	continue;
+      if (FILENAME_CMP (lbasename (dll_name), "ntdll.dll") == 0)
+	{
+	  solib_end->next = windows_make_so (dll_name, mi.lpBaseOfDll);
+	  solib_end = solib_end->next;
+	  return;
+	}
+    }
 static void
 do_initial_windows_stuff (struct target_ops *ops, DWORD pid, int attaching)
@@ -1756,6 +1814,8 @@ do_initial_windows_stuff (struct target_ops *ops, DWORD pid, int attaching)
+  windows_ensure_ntdll_loaded ();
   windows_initialization_done = 1;
   inf->control.stop_soon = NO_STOP_QUIETLY;
   stop_after_trap = 0;

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