[PATCH 2/8] gdb/linux-nat: remove check based on current_inferior in linux_handle_extended_wait

Simon Marchi via Gdb-patches
2022-01-17T16:32:31Z

The check removed by this patch, using current_inferior, looks wrong.
When debugging multiple inferiors with the Linux native target and
linux_handle_extended_wait is called, there's no guarantee about which
is the current inferior. The vfork-done event we receive could be for
any inferior. If the vfork-done event is for a non-current inferior, we
end up wrongfully ignoring it. As a result, the core never processes a
TARGET_WAITKIND_VFORK_DONE event, program_space::breakpoints_not_allowed
is never cleared, and breakpoints are never reinserted. However,
because the Linux native target decided to ignore the event, it resumed
the thread - while breakpoints out. And that's bad.

The proposed fix is to remove this check. Always report vfork-done
events and let infrun's logic decide if it should be ignored. We don't
save much cycles by filtering the event here.

Add a test that sets replicates the situation described above. See
comments in the test for more details.

Change-Id: Ibe33c1716c3602e847be6c2093120696f2286fbf
---
gdb/linux-nat.c | 17 +--
.../gdb.threads/vfork-multi-inferior-sleep.c | 25 ++++
.../gdb.threads/vfork-multi-inferior.c | 55 +++++++++
.../gdb.threads/vfork-multi-inferior.exp | 115 ++++++++++++++++++
4 files changed, 199 insertions(+), 13 deletions(-)
create mode 100644 gdb/testsuite/gdb.threads/vfork-multi-inferior-sleep.c
create mode 100644 gdb/testsuite/gdb.threads/vfork-multi-inferior.c
create mode 100644 gdb/testsuite/gdb.threads/vfork-multi-inferior.exp

diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index b926fb5eba9e..7de4da03a34f 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -2096,20 +2096,11 @@ linux_handle_extended_wait (struct lwp_info *lp, int status)

if (event == PTRACE_EVENT_VFORK_DONE)
{
- if (current_inferior ()->waiting_for_vfork_done)
- {
- linux_nat_debug_printf
- ("Got expected PTRACE_EVENT_VFORK_DONE from LWP %ld: stopping",
- lp->ptid.lwp ());
-
- ourstatus->set_vfork_done ();
- return 0;
- }
-
linux_nat_debug_printf
- ("Got PTRACE_EVENT_VFORK_DONE from LWP %ld: ignoring", lp->ptid.lwp ());
-
- return 1;
+ ("Got expected PTRACE_EVENT_VFORK_DONE from LWP %ld",
+ lp->ptid.lwp ());
+ ourstatus->set_vfork_done ();
+ return 0;
}

internal_error (__FILE__, __LINE__,
diff --git a/gdb/testsuite/gdb.threads/vfork-multi-inferior-sleep.c b/gdb/testsuite/gdb.threads/vfork-multi-inferior-sleep.c
new file mode 100644
index 000000000000..a26ae07d53b7
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/vfork-multi-inferior-sleep.c
@@ -0,0 +1,25 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+ Copyright 2022 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 <unistd.h>
+
+int
+main (void)
+{
+ sleep (30);
+ return 0;
+}
diff --git a/gdb/testsuite/gdb.threads/vfork-multi-inferior.c b/gdb/testsuite/gdb.threads/vfork-multi-inferior.c
new file mode 100644
index 000000000000..1b7b00d3c755
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/vfork-multi-inferior.c
@@ -0,0 +1,55 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+ Copyright 2022 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 <unistd.h>
+#include <sys/wait.h>
+#include <assert.h>
+
+static void
+should_break_here (void)
+{
+}
+
+int
+main (void)
+{
+ int i;
+
+ for (i = 0; i < NR_LOOPS; i++)
+ {
+ int pid = vfork ();
+
+ if (pid != 0)
+ {
+ /* Parent */
+ int stat;
+ int ret = waitpid (pid, &stat, 0);
+ assert (ret == pid);
+ assert (WIFEXITED (stat));
+ assert (WEXITSTATUS (stat) == 12);
+
+ should_break_here ();
+ }
+ else
+ {
+ /* Child */
+ _exit(12);
+ }
+ }
+
+ return 0;
+}
diff --git a/gdb/testsuite/gdb.threads/vfork-multi-inferior.exp b/gdb/testsuite/gdb.threads/vfork-multi-inferior.exp
new file mode 100644
index 000000000000..22a1476eaacc
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/vfork-multi-inferior.exp
@@ -0,0 +1,115 @@
+# Copyright 2020-2022 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/>.
+
+# Test handling a vfork while another inferior is running. The bug that
+# prompted writing this test case was in the Linux native target. The target
+# assumed that the vfork-done event it received was for the current inferior
+# (an invalid assumption, the current inferior is the one randomly selected by
+# do_target_wait (at the time of writing). This caused the target to drop the
+# vfork-done event, because it was seen as unneeded and to restart the thread
+# as if nothing happened. This however resulted in the thread running with
+# breakpoints not inserted.
+#
+# To catch the bug, this test verifies that we can hit a breakpoint after a
+# vfork call, while a second inferior runs in the background.
+
+if [use_gdb_stub] {
+ unsupported "test uses multiple inferiors"
+ return
+}
+
+standard_testfile .c -sleep.c
+
+set srcfile_sleep $srcfile2
+set binfile_sleep ${binfile}-sleep
+
+# The reproducibility of the bug depends on which inferior randomly selects in
+# do_target_wait when consuming the vfork-done event. Since GDB doesn't call
+# srand(), we are likely to always see the same sequence of inferior selected by
+# do_target_wait, which can hide the bug if you are not "lucky". To work
+# around that, call vfork and hit the breakpoint in a loop, it makes it
+# somewhat likely that the wrong inferior will be selected eventually.
+set nr_loops 20
+
+# Compile the main program that calls vfork and hits a breakpoint.
+set opts [list debug additional_flags=-DNR_LOOPS=$nr_loops]
+if { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable \
+ $opts] != "" } {
+ untested "failed to compile"
+ return -1
+}
+
+# Compile the secondary program, which just sleeps.
+if { [gdb_compile "${srcdir}/${subdir}/${srcfile_sleep}" "${binfile_sleep}" executable \
+ {debug}] != "" } {
+ untested "failed to compile"
+ return -1
+}
+
+# We exercise two methods of getting a second inferior to execute while the
+# first one vforks. METHOD can be:
+#
+# - non-stop: start GDB with non-stop on and run the second inferior in
+# background.
+# - schedule-multiple: set "schedule-multiple on", this will make "continue"
+# resume both inferiors.
+proc do_test {method} {
+ save_vars { ::GDBFLAGS } {
+ if { $method == "non-stop" } {
+ append ::GDBFLAGS " -ex \"set non-stop on\""
+ }
+ clean_restart
+ }
+
+ # Start the second inferior in background.
+ gdb_test "add-inferior" "Added inferior 2.*"
+ gdb_test "inferior 2" "Switching to inferior 2 .*"
+ gdb_file_cmd ${::binfile_sleep}
+ if { $method == "non-stop" } {
+ gdb_test "run &" "Starting program: .*" "run inferior 2"
+ } else {
+ gdb_test "start" "Temporary breakpoint $::decimal, main .*" \
+ "start inferior 2"
+ }
+
+ # Start the first inferior.
+ gdb_test "inferior 1" "Switching to inferior 1 .*"
+ gdb_file_cmd ${::binfile}
+ gdb_test "break should_break_here" "Breakpoint $::decimal at .*"
+ gdb_test "start" "Thread 1.1 .* hit Temporary breakpoint.*" \
+ "start inferior 1"
+
+ # Only enable schedule-multiple this late, because of:
+ # https://sourceware.org/bugzilla/show_bug.cgi?id=28777
+ if { $method == "schedule-multiple" } {
+ gdb_test_no_output "set schedule-multiple on"
+ }
+
+
+ # Continue over vfork and until the breakpoint. The number of loops here
+ # matches the number of loops in the program. So if a breakpoint is missed
+ # at some point, a "continue" will wrongfully continue until the end of the
+ # program, which will fail the test.
+ for {set i 0} {$i < $::nr_loops} {incr i} {
+ with_test_prefix "i=$i" {
+ gdb_test "continue" \
+ "Thread 1.1 .* hit Breakpoint $::decimal, should_break_here.*"
+ }
+ }
+}
+
+foreach_with_prefix method {schedule-multiple non-stop} {
+ do_test $method
+}
--
2.34.1