[patch] Do not allow unwinder to terminate inferior in an inferior function call if a C++ exception handler cannot be found.

Phil Muldoon
2009-04-24T08:32:53Z

[-- Attachment #1: Type: text/plain, Size: 2068 bytes --]

This patch addresses the bug detailed in gnats pr 2495 and after the
conversion to bugzilla: pr 9600

Due to the method that GDB uses to construct a frame for an inferior
function call, the unwinder can terminate the inferior if it raises an
exception and relies on an out-of-frame handler to normally handle that
exception. For most systems GCC uses the exception handling strategy
documented here: http://www.codesourcery.com/public/cxx-abi/ . If the
function that raises the exception relies on an out-of-frame handler,
the single dummy frame constructed by GDB for the inferior function call
does not allow the unwinder to walk the stack looking for an appropriate
handler. Briefly, if this happens the unwinder terminates the inferior
via a call to the default exception handler on the (wrong) assumption
that there is no user-created handler for the exception.

This patch addresses that by gating access to std::terminate in an
inferior function call. If it enters that function, it is popped back to
the calling state and "rescues" the inferior. This patch also adds the
functionality to turn this gating behaviour on or off. The default is on.

This patch applies cleanly to CVS GDB head. Tested on x8664 with no
regressions.

Regards

Phil

gdb/ChangeLog
2009-04-23 Phil Muldoon <pmuldoon@redhat.com>

* infcall.c (show_unwind_on_terminating_exception_p): New
function.
(call_function_by_hand): Create breakpoint and clean-up call for
std::terminate.breakpoint. Add unwind_on_terminating_exception_p
gate. Pop frame on breakpoint hit.
(_initialize_infcall): Add add_setshow_boolean_cmd for
unwind-on-terminating-exception.

gdb/testsuite/ChangeLog
2009-04-23 Phil Muldoon <pmuldoon@redhat.com>

* gdb.cp/gdb2495.cc: New file.
* gdb.cp/gdb2495.exp: New file.

gdb/doc/ChangeLog
2009-04-23 Phil Muldoon <pmuldoon@redhat.com>

* doc/gdb.texinfo (Calling): Document
set-unwind-on-terminating-exception usage.

[-- Attachment #2: infcall_exceptions.patch --]
[-- Type: text/x-patch, Size: 16128 bytes --]

Index: infcall.c
===================================================================
RCS file: /cvs/src/src/gdb/infcall.c,v
retrieving revision 1.111
diff -u -r1.111 infcall.c
--- infcall.c 22 Mar 2009 17:59:59 -0000 1.111
+++ infcall.c 24 Apr 2009 08:28:47 -0000
@@ -98,6 +98,29 @@
value);
}

+/* This boolean tells what gdb should do if a std::terminate call is
+ made while in a function called from gdb (call dummy).
+ As the confines of a single dummy stack prohibit out-of-frame
+ handlers from handling a raised exception, and as out-of-frame
+ handlers are common in C++, this can lead to no handler being found
+ by the unwinder, and a std::terminate call. This is a false positive.
+ If set, gdb unwinds the stack and restores the context to what it
+ was before the call.
+
+ The default is to unwind the frame if a std::terminate call is
+ made. */
+
+static int unwind_on_terminating_exception_p = 1;
+static void
+show_unwind_on_terminating_exception_p (struct ui_file *file, int from_tty,
+ struct cmd_list_element *c,
+ const char *value)
+
+{
+ fprintf_filtered (file, _("\
+Unwind stack if a C++ exception is unhandled while in a call dummy is %s.\n"),
+ value);
+}

/* Perform the standard coercions that are specified
for arguments to be passed to C or Ada functions.
@@ -415,6 +438,8 @@
struct cleanup *args_cleanup;
struct frame_info *frame;
struct gdbarch *gdbarch;
+ struct breakpoint *terminate_bp = 0;
+ struct minimal_symbol *tm;
ptid_t call_thread_ptid;
struct gdb_exception e;
const char *name;
@@ -714,6 +739,29 @@
bpt = set_momentary_breakpoint (sal, dummy_id, bp_call_dummy);
bpt->disposition = disp_del;
}
+
+ /* Create a breakpoint in std::terminate.
+ If a C++ exception is raised in the dummy-frame, and the
+ exception handler is (normally, and expected to be) out-of-frame,
+ the default C++ handler will (wrongly) be called in an inferior
+ function call. This is wrong, as an exception can be normally
+ and legally handled out-of-frame. The confines of the dummy frame
+ prevent the unwinder from finding the correct handler (or any
+ handler, unless it is in-frame). The default handler calls
+ std::terminate. This will kill the inferior. Assert that
+ terminate should never be called in an inferior function
+ call. Place a momentary breakpoint in the std::terminate function
+ and if triggered in the call, rewind. */
+ if (unwind_on_terminating_exception_p)
+ {
+ tm = lookup_minimal_symbol ("std::terminate()", NULL, NULL);
+ if (tm != NULL)
+ {
+ terminate_bp = set_momentary_breakpoint_at_pc
+ (SYMBOL_VALUE_ADDRESS (tm), bp_breakpoint);
+ make_cleanup_delete_breakpoint (terminate_bp);
+ }
+ }

/* Everything's ready, push all the info needed to restore the
caller (and identify the dummy-frame) onto the dummy-frame
@@ -824,6 +872,16 @@
name);
}

+ if (! target_has_execution)
+ {
+ /* If we try to restore the inferior status (via the cleanup),
+ we'll crash as the inferior is no longer running. */
+ discard_cleanups (inf_status_cleanup);
+ discard_inferior_status (inf_status);
+ error (_("\
+The program being debugged exited while in a function called from GDB."));
+ }
+
if (stopped_by_random_signal || !stop_stack_dummy)
{
const char *name = get_function_name (funaddr,
@@ -880,6 +938,39 @@

if (!stop_stack_dummy)
{
+
+ /* Check if unwind on terminating exception behaviour is on. */
+ if (unwind_on_terminating_exception_p)
+ {
+ /* Check that the breakpoint is our special std::terminate
+ breakpoint. If it is, we do not want to kill the inferior
+ in an inferior function call. Rewind, and warn the
+ user. */
+
+ if ((terminate_bp != NULL) &&
+ (inferior_thread()->stop_bpstat->breakpoint_at->address
+ == terminate_bp->loc->address))
+
+
+ {
+
+ /* We must get back to the frame we were before the
+ dummy call. */
+ dummy_frame_pop (dummy_id);
+
+ /* We also need to restore inferior status to that before the
+ dummy call. */
+ restore_inferior_status (inf_status);
+
+ error (_("\
+The program being debugged entered a std::terminate call which would\n\
+have terminated the program being debugged. GDB has restored the\n\
+context to what it was before the call\n\
+To change this behaviour use \"set unwind-on-terminating-exception off\"\n\
+Evaluation of the expression containing the function (%s) will be abandoned."),
+ name);
+ }
+ }
/* We hit a breakpoint inside the FUNCTION.
Keep the dummy frame, the user may want to examine its state.
Discard inferior status, we're not at the same point
@@ -988,4 +1079,19 @@
NULL,
show_unwind_on_signal_p,
&setlist, &showlist);
+
+ add_setshow_boolean_cmd ("unwind-on-terminating-exception", no_class,
+ &unwind_on_terminating_exception_p, _("\
+Set unwinding of stack if a std::terminate() call originates from\n\
+the default C++ exception handler."), _("\
+Show unwinding of stack if a std::terminate() call originates from\n\
+the default C++ exception handler."), _("\
+The unwind on terminating exception flag lets the user determine\n\
+what gdb should do if a std::terminate() call is made from the\n\
+default exception handler.\n\
+The default is to unwind the frame."),
+ NULL,
+ show_unwind_on_terminating_exception_p,
+ &setlist, &showlist);
+
}
Index: doc/gdb.texinfo
===================================================================
RCS file: /cvs/src/src/gdb/doc/gdb.texinfo,v
retrieving revision 1.584
diff -u -r1.584 gdb.texinfo
--- doc/gdb.texinfo 22 Apr 2009 17:24:59 -0000 1.584
+++ doc/gdb.texinfo 24 Apr 2009 08:28:53 -0000
@@ -12660,6 +12660,12 @@
the function, or if you passed it incorrect arguments). What happens
in that case is controlled by the @code{set unwindonsignal} command.

+Similarly, with a C++ program it is possible for the function you
+call via the @code{print} or @code{call} command to generate an
+exception that is not handled due to the constraints of the dummy
+frame. What happens in that case is controlled by the
+@code{set unwind-on-terminating-exception} command.
+
@table @code
@item set unwindonsignal
@kindex set unwindonsignal
@@ -12676,6 +12682,23 @@
@kindex show unwindonsignal
Show the current setting of stack unwinding in the functions called by
@value{GDBN}.
+
+@item set unwind-on-terminating-exception
+@kindex set unwind-on-terminating-exception
+@cindex unwind stack in called functions
+@cindex call dummy stack unwinding on unhandled exception.
+Set unwinding of the stack if a C++ exception is raised but unhandled
+while in a function that @value{GDBN} called in the program being
+debugged. If set to on (the default), @value{GDBN} unwinds the stack
+it created for the call and restores the context to what it was before
+the call. If set to off, @value{GDBN} the exception is delivered to
+the default C++ exception handler.
+
+@item show unwind-on-terminating-exception
+@kindex show unwind-on-terminating-exception
+Show the current setting of stack unwinding in the functions called by
+@value{GDBN}.
+
@end table

@cindex weak alias functions
Index: testsuite/gdb.cp/Makefile.in
===================================================================
RCS file: /cvs/src/src/gdb/testsuite/gdb.cp/Makefile.in,v
retrieving revision 1.5
diff -u -r1.5 Makefile.in
--- testsuite/gdb.cp/Makefile.in 3 Feb 2009 01:09:01 -0000 1.5
+++ testsuite/gdb.cp/Makefile.in 24 Apr 2009 08:28:53 -0000
@@ -4,7 +4,7 @@
EXECUTABLES = ambiguous annota2 anon-union cplusfuncs cttiadd \
derivation inherit local member-ptr method misc \
overload ovldbreak ref-typ ref-typ2 templates userdef virtfunc namespace \
- ref-types ref-params method2 pr9594
+ ref-types ref-params method2 pr9594 gdb2495

all info install-info dvi install uninstall installcheck check:
@echo "Nothing to be done for $@..."
Index: testsuite/gdb.cp/gdb2495.cc
===================================================================
RCS file: testsuite/gdb.cp/gdb2495.cc
diff -N testsuite/gdb.cp/gdb2495.cc
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ testsuite/gdb.cp/gdb2495.cc 24 Apr 2009 08:28:53 -0000
@@ -0,0 +1,90 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+ Copyright 2009 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/>.
+ */
+
+#include <iostream>
+#include <signal.h>
+
+using namespace std;
+
+class SimpleException
+{
+
+public:
+
+ void raise_signal (int dummy)
+ {
+ if (dummy > 0)
+ raise(SIGABRT);
+ }
+
+ int no_throw_function ()
+ {
+ return 1;
+ }
+
+ void throw_function ()
+ {
+ throw 1;
+ }
+
+ int throw_function_with_handler ()
+ {
+ try
+ {
+ throw 1;
+ }
+ catch (...)
+ {
+ cout << "Handled" << endl;
+ }
+
+ return 2;
+ }
+
+ void call_throw_function_no_handler ()
+ {
+ throw_function ();
+ }
+
+ void call_throw_function_handler ()
+ {
+ throw_function_with_handler ();
+ }
+};
+SimpleException exceptions;
+
+int
+main()
+{
+ /* Have to call these functions so GCC does not optimize them
+ away. */
+ exceptions.raise_signal (-1);
+ exceptions.no_throw_function ();
+ exceptions.throw_function_with_handler ();
+ exceptions.call_throw_function_handler ();
+ try
+ {
+ exceptions.throw_function ();
+ exceptions.call_throw_function_no_handler ();
+ }
+ catch (...)
+ {
+ }
+ return 0;
+}
+
Index: testsuite/gdb.cp/gdb2495.exp
===================================================================
RCS file: testsuite/gdb.cp/gdb2495.exp
diff -N testsuite/gdb.cp/gdb2495.exp
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ testsuite/gdb.cp/gdb2495.exp 24 Apr 2009 08:28:54 -0000
@@ -0,0 +1,160 @@
+# Copyright 2009 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/>.
+
+
+# In gdb inferior function calls, if a C++ exception is raised in the
+# dummy-frame, and the exception handler is (normally, and expected to
+# be) out-of-frame, the default C++ handler will (wrongly) be called
+# in an inferior function call.
+# This is incorrect as an exception can normally and legally be handled
+# out-of-frame. The confines of the dummy frame prevent the unwinder
+# from finding the correct handler (or any handler, unless it is
+# in-frame). The default handler calls std::terminate. This will kill
+# the inferior. Assert that terminate should never be called in an
+# inferior function call. These tests test the functionality around
+# unwinding that sequence and also tests the flag behaviour gating this
+# functionality.
+
+# This test is largley based off gdb.base/callfuncs.exp.
+
+if $tracelevel then {
+ strace $tracelevel
+}
+
+if { [skip_cplus_tests] } { continue }
+
+set prms_id 2495
+set bug_id 0
+
+set testfile "gdb2495"
+set srcfile ${testfile}.cc
+set binfile $objdir/$subdir/$testfile
+
+# Create and source the file that provides information about the compiler
+# used to compile the test case.
+if [get_compiler_info ${binfile} "c++"] {
+ return -1
+}
+
+if { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable {debug c++}] != "" } {
+ untested gdb2495.exp
+ return -1
+}
+
+# Some targets can't do function calls, so don't even bother with this
+# test.
+if [target_info exists gdb,cannot_call_functions] {
+ setup_xfail "*-*-*" 2416
+ fail "This target can not call functions"
+ continue
+}
+
+gdb_exit
+gdb_start
+gdb_reinitialize_dir $srcdir/$subdir
+gdb_load ${binfile}
+
+if ![runto_main] then {
+ perror "couldn't run to main"
+ continue
+}
+
+# See http://sources.redhat.com/gdb/bugs/2495
+
+# Test normal baseline behaviour. Call a function that
+# does not raise an exception.
+gdb_test "p exceptions.no_throw_function()" " = 1"
+# And one that does but handles it in-frame.
+gdb_test "p exceptions.throw_function_with_handler()" " = 2"
+# Both should return normally.
+
+# Test basic unwind. Call a function that raises an exception but
+# does not handle it. It should be rewound.
+gdb_test "p exceptions.throw_function()" \
+ "The program being debugged entered a std::terminate call .*" \
+ "Call a function that raises an exception without a handler."
+
+# Make sure that after rewinding we are back at the call parent.
+gdb_test "bt" \
+ "#0 main.*" \
+ "bt after returning from a popped frame"
+
+# Make sure the only breakpoint is the one set via the runto_main
+# call and that the std::terminate breakpoint has evaporated and
+# cleaned-up.
+gdb_test "info breakpoints" \
+ "gdb.cp/gdb2495\.cc.*"
+
+# Turn off this new behaviour.
+send_gdb "set unwind-on-terminating-exception off\n"
+gdb_expect {
+ -re "$gdb_prompt $" {pass "set unwind-on-terminating-exception"}
+ timeout {fail "(timeout) set unwind-on-terminating-exception"}
+}
+
+# Check that it is turned off.
+gdb_test "show unwind-on-terminating-exception" \
+ "exception is unhandled while in a call dummy is off.*" \
+ "Turn off unwind on terminating exception flag"
+
+# Check that the old behaviour is restored.
+gdb_test "p exceptions.throw_function()" \
+ "The program being debugged stopped while in a function called .*" \
+ "Call a function that raises an exception with unwinding off.."
+
+
+# Restart the inferior back at main.
+if ![runto_main] then {
+ perror "couldn't run to main"
+ continue
+}
+
+
+# Check to see if the new behaviour alters the unwind signal
+# behaviour; it should not. Test both on and off states.
+
+# Turn on unwind on signal behaviour.
+send_gdb "set unwindonsignal on\n"
+gdb_expect {
+ -re "$gdb_prompt $" {pass "set unwindonsignal on"}
+ timeout {fail "(timeout) set unwindonsignal on"}
+}
+
+# Check that it is turned on.
+gdb_test "show unwindonsignal" \
+ "signal is received while in a call dummy is on.*" \
+ "Turn on unwind on signal"
+
+# Check to see if new behaviour interferes with
+# normal signal handling in inferior function calls.
+gdb_test "p exceptions.raise_signal(1)" \
+ "To change this behavior use \"set unwindonsignal off\".*"
+
+# And reverse - turn off again.
+send_gdb "set unwindonsignal off\n"
+gdb_expect {
+ -re "$gdb_prompt $" {pass "set unwindonsignal off"}
+ timeout {fail "(timeout) set unwindonsignal off"}
+}
+
+# Check that it is actually turned off.
+gdb_test "show unwindonsignal" \
+ "signal is received while in a call dummy is off.*" \
+ "Turn off unwind on signal"
+
+# Check to see if new behaviour interferes with
+# normal signal handling in inferior function calls.
+gdb_test "p exceptions.raise_signal(1)" \
+ "To change this behavior use \"set unwindonsignal on\".*"