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Re: [PATCH] Fix use of a dangling pointer for Python breakpoint objects
- From: Pierre-Marie de Rodat <derodat at adacore dot com>
- To: Pedro Alves <palves at redhat dot com>, gdb-patches at sourceware dot org
- Date: Fri, 24 Jun 2016 11:21:10 +0200
- Subject: Re: [PATCH] Fix use of a dangling pointer for Python breakpoint objects
- Authentication-results: sourceware.org; auth=none
- References: <20160621104021 dot 15093-1-derodat at adacore dot com> <02138a22-5087-44ad-6023-7c87251f3d19 at redhat dot com>
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