Re: [patch] Fix internal error on breaking at a multi-locations caller

Jan Kratochvil
2009-05-01T09:20:43Z

> On Fri, 24 Apr 2009 03:00:50 +0200, Tom Tromey wrote:
> > Based on the documentation of `break', and also my mental model of
> > debugging with gdb, I think that the best behavior here would be to
> > simply set a single breakpoint here -- the one corresponding to the
> > instance that is currently being executed.

info '(gdb)Set Breaks'
`break'
When called without any arguments, `break' sets a breakpoint at
the next instruction to be executed in the selected stack frame
[...]

Do you refer here to the "selected stack frame" part of the doc?

On Tue, 28 Apr 2009 22:32:35 +0200, Joel Brobecker wrote:
> That's a good point. I actually had a different interpretation
> of the "break" command without arguments, but the documentation
> is very specific about it. I agree with you.

Attaching here such a patch variant; it at least prints a warning in such case.

Regression tested on x86_64-unknown-linux-gnu.

Thanks,
Jan

gdb/
2009-05-01 Jan Kratochvil <jan.kratochvil@redhat.com>

Fix internal error on breaking at a multi-locations caller source line.
* breakpoint.c (expand_line_sal_maybe): Remove the variable `found'.
(expand_line_sal_maybe <original_pc && expanded.nelts >= 2>): New
initialized variable `best'. Trim `expanded.sals' for the caller lines.

gdb/testsuite/
2009-05-01 Jan Kratochvil <jan.kratochvil@redhat.com>

* gdb.cp/expand-sals.exp, gdb.cp/expand-sals.cc: New.

--- gdb/breakpoint.c 29 Apr 2009 19:31:58 -0000 1.392
+++ gdb/breakpoint.c 1 May 2009 08:54:09 -0000
@@ -5336,7 +5336,6 @@ expand_line_sal_maybe (struct symtab_and
struct symtabs_and_lines expanded;
CORE_ADDR original_pc = sal.pc;
char *original_function = NULL;
- int found;
int i;

/* If we have explicit pc, don't expand.
@@ -5412,14 +5411,56 @@ expand_line_sal_maybe (struct symtab_and

if (original_pc)
{
- found = 0;
+ /* There are multiple PCs for this line of code with multiple instances
+ (locations). If the instruction is in the middle of an instruction
+ block for source line GDB cannot safely find the same instruction in
+ the other compiled instances of the same source line because the other
+ instances may have been compiled completely differently.
+
+ The testcase gdb.cp/expand-sals.exp shows that breaking at the return
+ address in a caller of the current frame works for the current
+ instance but the breakpoint cannot catch the point (instruction) where
+ the callee returns in the other compiled instances of this source line.
+
+ The current implementation will place the breakpoint at the expected
+ returning address of the current instance of the caller. But the
+ other instances get no breakpoint at all.
+
+ One possibility would be to put the breakpoint at first instruction of
+ the same source line - therefore before the call would be made.
+ Another possibility would be to place the breakpoint in the other
+ instances at the start of the next source line.
+
+ A possible heuristics would compare the instructions length of each of
+ the instances of the current source line and if it matches it would
+ place the breakpoint at the same offset. Unfortunately a mistaken
+ guess would possibly crash the inferior. */
+
+ CORE_ADDR best = -1;
+
+ /* Find the nearest preceding PC and set it to ORIGINAL_PC. */
for (i = 0; i < expanded.nelts; ++i)
- if (expanded.sals[i].pc == original_pc)
- {
- found = 1;
- break;
- }
- gdb_assert (found);
+ if (expanded.sals[i].pc <= original_pc
+ && (best == -1 || expanded.sals[best].pc < expanded.sals[i].pc))
+ best = i;
+
+ if (best == -1)
+ error (_("Cannot find the best address for %s out of the %d locations"),
+ paddr (original_pc), expanded.nelts);
+
+ /* ORIGINAL_PC is set even for regular `break LINENO' commands which
+ should cover all the locations. Catch specifically the
+ `up'-into-caller case where SAL.PC does not match the first
+ instruction of the line but still SAL.EXPLICIT_PC is not set. */
+
+ if (expanded.sals[best].pc != original_pc)
+ {
+ warning (_("Breakpoint has been set only in the current location "
+ "out of %d existing ones for this line."), expanded.nelts);
+ expanded.sals[best].pc = original_pc;
+ expanded.sals[0] = expanded.sals[best];
+ expanded.nelts = 1;
+ }
}

return expanded;
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ gdb/testsuite/gdb.cp/expand-sals.cc 1 May 2009 08:54:10 -0000
@@ -0,0 +1,53 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+ Copyright (C) 2009 Free Software Foundation, Inc.
+
+ This file is part of GDB.
+
+ 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
+func ()
+{
+ return 42; /* func-line */
+}
+
+volatile int global_x;
+
+class A
+{
+public:
+ A ()
+ {
+ global_x = func (); /* caller-line */
+ }
+};
+
+/* class B is here just to make the `func' calling line above having multiple
+ instances - multiple locations. Template cannot be used as its instances
+ would have different function names which get discarded by GDB
+ expand_line_sal_maybe. */
+
+class B : public A
+{
+};
+
+int
+main (void)
+{
+ A a;
+ B b;
+
+ return 0; /* exit-line */
+}
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ gdb/testsuite/gdb.cp/expand-sals.exp 1 May 2009 08:54:10 -0000
@@ -0,0 +1,86 @@
+# 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/>.
+
+if { [skip_cplus_tests] } { continue }
+
+set srcfile expand-sals.cc
+if { [prepare_for_testing expand-sals.exp expand-sals $srcfile {debug c++}] } {
+ return -1
+}
+if ![runto_main] {
+ return -1
+}
+
+gdb_breakpoint [gdb_get_line_number "exit-line"]
+
+gdb_breakpoint [gdb_get_line_number "func-line"]
+gdb_continue_to_breakpoint "func" ".*func-line.*"
+
+gdb_test "up" "caller-line.*"
+
+# PC should not be at the boundary of source lines to make the original bug
+# exploitable.
+
+set test "p/x \$pc"
+set pc {}
+gdb_test_multiple $test $test {
+ -re "\\$\[0-9\]+ = (0x\[0-9a-f\]+)\r\n$gdb_prompt $" {
+ set pc $expect_out(1,string)
+ pass $test
+ }
+}
+
+set test "info line"
+set end {}
+gdb_test_multiple $test $test {
+ -re "Line \[0-9\]+ of .* starts at address 0x\[0-9a-f\]+.* and ends at (0x\[0-9a-f\]+).*\\.\r\n$gdb_prompt $" {
+ set end $expect_out(1,string)
+ pass $test
+ }
+}
+
+set test "caller line has trailing code"
+if {$pc != $end} {
+ pass $test
+} else {
+ fail $test
+}
+
+# Original problem was an internal error here.
+set test "break"
+gdb_test_multiple $test $test {
+ -re "Breakpoint \[0-9\]+ at .*, line \[0-9\]+\\. \\(\[2-9\] locations\\)\r\n$gdb_prompt $" {
+ fail $test
+ }
+ -re "warning: Breakpoint has been set only in the current location out of 2 existing ones for this line\\.\r\nBreakpoint \[0-9\]+ at .*, line \[0-9\]+\\.\r\n$gdb_prompt $" {
+ pass $test
+ }
+ -re "Breakpoint \[0-9\]+ at .*, line \[0-9\]+\\.\r\n$gdb_prompt $" {
+ fail $test
+ }
+}
+
+gdb_continue_to_breakpoint "caller" ".*caller-line.*"
+
+# Test GDB caught this return call and not the next one through B::B()
+gdb_test "bt" \
+ "#0 \[^\r\n\]* A \[^\r\n\]*\r\n#1 \[^\r\n\]* main \[^\r\n\]*" \
+ "bt from A"
+
+gdb_continue_to_breakpoint "next caller func" ".*func-line.*"
+
+# Verify GDB really could not catch any other breakpoint location.
+
+gdb_continue_to_breakpoint "uncaught return" ".*exit-line.*"