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

Pierre-Marie de Rodat
2016-06-27T09:11:17Z

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Hi Pedro,

On 06/24/2016 06:41 PM, Pedro Alves wrote:
> I think this comment should be adjusted.

Done.

> But I think this would be even better:
>
> # ... and when it did, as a result, the following breakpoint creation
> # (not initiated by the Python API) would dereference the
> # already-freed Python breakpoint wrapper, resulting in undefined
> # behavior, sometimes observed as a gdb crash, and other times causing
> # the next stop to invoke the Python wrapper "stop" method for the
> # object that is not supposed to exist.

Yours is definitely better: done!

> Three things here:
>
> - Please make pass/fail messages here the same.
>
> - With gdb_test_multiple, you also need to match $gdb_prompt,
> otherwise you confuse the next test.
>
> - No need for leading ".*" in regexes, it's implicit.

Done.

> While at it, how about renaming the new files to avoid
> the meaningless "2"?
>
> Maybe py-breakpoint-create-fail.[py|exp|c] ?

Done as well.

Thank you for your help! I’m not familiar with the testsuite so I had a
hard time analyzing how other testcases do and investigating why my
testcase did not work… Anyway, thank you for the rationale! Updated
patch is attached.

--
Pierre-Marie de Rodat

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From 8e8bf8cfd8c07b65cdde21066cf4a25b63b3fec5 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-breakpoint-create-fail.c,
gdb.python/py-breakpoint-create-fail.exp,
gdb.python/py-breakpoint-create-fail.py: New testcase.
---
gdb/python/py-breakpoint.c | 1 +
.../gdb.python/py-breakpoint-create-fail.c | 28 +++++++++++
.../gdb.python/py-breakpoint-create-fail.exp | 58 ++++++++++++++++++++++
.../gdb.python/py-breakpoint-create-fail.py | 31 ++++++++++++
4 files changed, 118 insertions(+)
create mode 100644 gdb/testsuite/gdb.python/py-breakpoint-create-fail.c
create mode 100644 gdb/testsuite/gdb.python/py-breakpoint-create-fail.exp
create mode 100644 gdb/testsuite/gdb.python/py-breakpoint-create-fail.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-breakpoint-create-fail.c b/gdb/testsuite/gdb.python/py-breakpoint-create-fail.c
new file mode 100644
index 0000000..c346bdd
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-breakpoint-create-fail.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-breakpoint-create-fail.exp b/gdb/testsuite/gdb.python/py-breakpoint-create-fail.exp
new file mode 100644
index 0000000..68cdccd
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-breakpoint-create-fail.exp
@@ -0,0 +1,58 @@
+# 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 proper handling for
+# breakpoint creation failure.
+
+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) would dereference the already-freed Python
+# breakpoint wrapper, resulting in undefined behavior, sometimes observed as a
+# gdb crash, and other times causing the next stop to invoke the Python wrapper
+# "stop" method for the object that is not supposed to exist.
+gdb_test "break foo"
+
+# ... eventually, triggering this breakpoint will invoke the Python wrapper
+# "stop" method for an object that is not supposed to exist.
+set test "continuing to foo"
+gdb_test_multiple "continue" "$test" {
+ -re "MyBP\.stop was invoked\!.*$gdb_prompt $" {
+ fail "$test"
+ }
+ -re "Continuing.*Breakpoint 2, foo.*$gdb_prompt $" {
+ pass "$test"
+ }
+}
diff --git a/gdb/testsuite/gdb.python/py-breakpoint-create-fail.py b/gdb/testsuite/gdb.python/py-breakpoint-create-fail.py
new file mode 100644
index 0000000..845eb0f
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-breakpoint-create-fail.py
@@ -0,0 +1,31 @@
+# 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/>.
+
+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