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Re: [PATCH] Fix use of a dangling pointer for Python breakpoint objects


Thank you for your review, Pedro!

On 06/23/2016 06:14 PM, Pedro Alves wrote:
So the problem is that the next time gdbpy_breakpoint_created
is called, for some other breakpoint, we'll dereference the dangling
pointer then, correct?

Exactly. This happens when a second breakpoint is created but not using the Python API, otherwise bppy_pending_object is set to a new Python wrapper before it is reused.

"start" doesn't work with "target remote" testing.  Try:

$ make check \
   RUNTESTFLAGS="--target_board=native-gdbserver" \
   TESTS="gdb.python/py-breakpoint2.exp"

Can we instead run to main first, and then source the python
script?

Ah, I did not know that, thanks.

Wouldn't it better to create a breakpoint after the one
that failed, explicitly?  Either in python, or perhaps
simpler, a regular command line breakpoint directly in
the .exp file.

Good idea! Iâve reworked the testcase as you said. The bug does not manifest with a crash anymore, though: itâs just that a Python method is called whereas it should not. But it may be a more reliable testcase.

--
Pierre-Marie de Rodat
>From 4b0b57cd285297f533ef9fc27f28e5da5f8ccd0a Mon Sep 17 00:00:00 2001
From: Pierre-Marie de Rodat <derodat@adacore.com>
Date: Tue, 21 Jun 2016 12:32:56 +0200
Subject: [PATCH] Fix use of a dangling pointer for Python breakpoint objects

When a Python script tries to create a breakpoint but fails to do so,
gdb.Breakpoint.__init__ raises an exception and the breakpoint does not
exist anymore in the Python interpreter. However, GDB still keeps a
reference to the Python object to be used for a later hook, which is
wrong.

This commit adds the necessary cleanup code so that there is no stale
reference to this Python object. It also adds a new testcase to
reproduce the bug and check the fix.

2016-06-24  Pierre-Marie de Rodat  <derodat@adacore.com>

gdb/
	* python/py-breakpoint.c (bppy_init): Clear bppy_pending_object
	when there is an error during the breakpoint creation.

gdb/testsuite

	* gdb.python/py-breakpoint2.c, gdb.python/py-breakpoint2.exp,
	gdb.python/py-breakpoint2.py: New testcase.
---
 gdb/python/py-breakpoint.c                  |  1 +
 gdb/testsuite/gdb.python/py-breakpoint2.c   | 28 +++++++++++++++
 gdb/testsuite/gdb.python/py-breakpoint2.exp | 54 +++++++++++++++++++++++++++++
 gdb/testsuite/gdb.python/py-breakpoint2.py  | 34 ++++++++++++++++++
 4 files changed, 117 insertions(+)
 create mode 100644 gdb/testsuite/gdb.python/py-breakpoint2.c
 create mode 100644 gdb/testsuite/gdb.python/py-breakpoint2.exp
 create mode 100644 gdb/testsuite/gdb.python/py-breakpoint2.py

diff --git a/gdb/python/py-breakpoint.c b/gdb/python/py-breakpoint.c
index ed9cae6..5918bcc 100644
--- a/gdb/python/py-breakpoint.c
+++ b/gdb/python/py-breakpoint.c
@@ -705,6 +705,7 @@ bppy_init (PyObject *self, PyObject *args, PyObject *kwargs)
     }
   CATCH (except, RETURN_MASK_ALL)
     {
+      bppy_pending_object = NULL;
       PyErr_Format (except.reason == RETURN_QUIT
 		    ? PyExc_KeyboardInterrupt : PyExc_RuntimeError,
 		    "%s", except.message);
diff --git a/gdb/testsuite/gdb.python/py-breakpoint2.c b/gdb/testsuite/gdb.python/py-breakpoint2.c
new file mode 100644
index 0000000..c346bdd
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-breakpoint2.c
@@ -0,0 +1,28 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2016 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see  <http://www.gnu.org/licenses/>.  */
+
+int
+foo (int a)
+{
+  return a * 2;
+}
+
+int
+main (void)
+{
+  return foo (2);
+}
diff --git a/gdb/testsuite/gdb.python/py-breakpoint2.exp b/gdb/testsuite/gdb.python/py-breakpoint2.exp
new file mode 100644
index 0000000..e835fd8
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-breakpoint2.exp
@@ -0,0 +1,54 @@
+# Copyright (C) 2016 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This file is part of the GDB testsuite.  It tests the mechanism
+# exposing breakpoints to Python.
+
+load_lib gdb-python.exp
+
+standard_testfile
+
+if { [prepare_for_testing ${testfile}.exp ${testfile} ${srcfile}] } {
+    return -1
+}
+
+# Skip all tests if Python scripting is not enabled.
+if { [skip_python_tests] } { continue }
+
+clean_restart "${testfile}"
+if ![runto_main] {
+    perror "could not run to main"
+    continue
+}
+
+# The following will create a breakpoint Python wrapper whose construction will
+# abort: the requested symbol is not defined.  GDB should not keep a reference
+# to the wrapper; however it used to...
+gdb_test "source py-breakpoint2.py"
+
+# ... and when it did, as a result, the following breakpoint creation (not
+# initiated by the Python API) will re-use the previous Python wrapper...
+gdb_test "break foo"
+
+# ... eventually, triggering this breakpoint will invoke the Python wrapper
+# "stop" method for an object that is not supposed to exist.
+gdb_test_multiple "continue" "continuing to foo" {
+    -re ".*MyBP\.stop was invoked\!.*" {
+        fail "wrong breakpoint Python wrapper involved"
+    }
+    -re "Continuing.*Breakpoint 2, foo.*" {
+        pass "ok"
+    }
+}
diff --git a/gdb/testsuite/gdb.python/py-breakpoint2.py b/gdb/testsuite/gdb.python/py-breakpoint2.py
new file mode 100644
index 0000000..6cd2ff2
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-breakpoint2.py
@@ -0,0 +1,34 @@
+# Copyright (C) 2016 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This file is part of the GDB testsuite.  It tests the mechanism
+# exposing breakpoints to Python.
+
+import gdb
+
+
+class MyBP(gdb.Breakpoint):
+    def stop(self):
+        print('MyBP.stop was invoked!')
+        # Don't make this breakpoint stop
+        return False
+
+
+try:
+    bp = MyBP('does_not_exist', gdb.BP_WATCHPOINT)
+except RuntimeError:
+    pass
+else:
+    assert False
-- 
2.8.3


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