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* [RFC PATCH] gdb: fix eval.c assert during inferior exit event
@ 2021-05-05 15:56 Magne Hov
  2021-05-06  1:59 ` Simon Marchi
                   ` (2 more replies)
  0 siblings, 3 replies; 14+ messages in thread
From: Magne Hov @ 2021-05-05 15:56 UTC (permalink / raw)
  To: gdb-patches

Evaluating expressions from within an inferior exit event handler can
cause a crash:

    echo "int main() { return 0; }" > repro.cc
    g++ -g repro.cc -o repro
    ./gdb -q --ex "start" --ex "python gdb.events.exited.connect(lambda _: gdb.execute('set \$_a=0'))" --ex "continue" repro

    Reading symbols from repro...
    Temporary breakpoint 1 at 0x1131: file repro.cc, line 1.
    Starting program: /home/mhov/repos/binutils-gdb-master/install/bin/repro

    Temporary breakpoint 1, main () at repro.cc:1
    1	int main() { return 0; }
    Continuing.
    [Inferior 1 (process 75524) exited normally]
    ../../gdb/thread.c:72: internal-error: thread_info* inferior_thread(): Assertion `current_thread_ != nullptr' failed.
    A problem internal to GDB has been detected,
    further debugging may prove unreliable.
    Quit this debugging session? (y or n) [answered Y; input not from terminal]

    This is a bug, please report it.  For instructions, see:
    <https://www.gnu.org/software/gdb/bugs/>.

Backtrace
    0  in internal_error of ../../gdbsupport/errors.cc:51
    1  in inferior_thread of ../../gdb/thread.c:72
    2  in expression::evaluate of ../../gdb/eval.c:98
    3  in evaluate_expression of ../../gdb/eval.c:115
    4  in set_command of ../../gdb/printcmd.c:1502
    5  in do_const_cfunc of ../../gdb/cli/cli-decode.c:101
    6  in cmd_func of ../../gdb/cli/cli-decode.c:2181
    7  in execute_command of ../../gdb/top.c:670
    ...
    22 in python_inferior_exit of ../../gdb/python/py-inferior.c:182

This is the same assertion site as in
<https://sourceware.org/bugzilla/show_bug.cgi?id=26761>, but I'm not
sure if it is the same problem.

In `expression::evaluate (...)' there is a call to `inferior_thread
()' that is guarded by `target_has_execution ()':

    struct value *
    expression::evaluate (struct type *expect_type, enum noside noside)
    {
      gdb::optional<enable_thread_stack_temporaries> stack_temporaries;
      if (target_has_execution ()
          && language_defn->la_language == language_cplus
          && !thread_stack_temporaries_enabled_p (inferior_thread ()))
        stack_temporaries.emplace (inferior_thread ());

The `target_has_execution ()' guard maps onto `inf->pid' and the
`inferior_thread ()' call assumes that `current_thread_' is set to
something meaningful:

    struct thread_info*
    inferior_thread (void)
    {
      gdb_assert (current_thread_ != nullptr);
      return current_thread_;
    }

In other words, it is assumed that if `inf->pid' is set then
`current_thread_' must also be set. This does not hold at the point
where inferior exit observers are notified:
- `generic_mourn_inferior (...)'
  - `switch_to_no_thread ()'
    - `current_thread_ = nullptr;'
  - `exit_inferior (...)'
    - `gdb::observers::inferior_exit.notify (...)'
    - `inf->pid = 0'

The inferior exit notification means that a Python handler can get a
chance to run while `current_thread' has been cleared and the
`inf->pid' has not been cleared. Since the Python handler can call any
GDB command with `gdb.execute(...)' (in my case `gdb.execute("set
$_a=0")' we can end evaluating expressions and asserting in
`evaluate_subexp (...)'.

This patch adds a test in `evaluate_subexp (...)' to check the global
`inferior_ptid' which is reset at the same time as `current_thread_'.
Checking `inferior_ptid' at the same time as `target_has_execution ()'
seems to be a common pattern:

    $ git grep -n -e inferior_ptid --and -e target_has_execution
    gdb/breakpoint.c:2998:    && (inferior_ptid == null_ptid || !target_has_execution ()))
    gdb/breakpoint.c:3054:    && (inferior_ptid == null_ptid || !target_has_execution ()))
    gdb/breakpoint.c:4587:  if (inferior_ptid == null_ptid || !target_has_execution ())
    gdb/infcmd.c:360:  if (inferior_ptid != null_ptid && target_has_execution ())
    gdb/infcmd.c:2380:  /* FIXME:  This should not really be inferior_ptid (or target_has_execution).
    gdb/infrun.c:3438:  if (!target_has_execution () || inferior_ptid == null_ptid)
    gdb/remote.c:11961:  if (!target_has_execution () || inferior_ptid == null_ptid)
    gdb/solib.c:725:  if (target_has_execution () && inferior_ptid != null_ptid)

The testsuite has been run on 5.4.0-59-generic x86_64 GNU/Linux:
- Ubuntu 20.04.1 LTS
- gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
- DejaGnu version 1.6.2
  - Expect version 5.45.4
  - Tcl version 8.6
- Native configuration: x86_64-pc-linux-gnu
- Target: unix

Diffing results before and after the patch shows a few XFAIL in
gdb.threads/attach-many-short-lived-threads.exp, and one unexpected
failure in gdb.base/run-attach-while-running.exp which seems to be
flaky on master as well.

gdb/ChangeLog:

2021-05-05  Magne Hov  <mhov@undo.io>

    * eval.c (expression::evaluate): Check inferior_ptid.

gdb/testsuite/ChangeLog:

2021-05-05  Magne Hov  <mhov@undo.io>

    * gdb.python/py-events.exp: Extend inferior exit tests.
    * gdb.python/py-events.py: Print inferior exit PID.
    * gdb.python/py-events.cc: New test.
---
 gdb/eval.c                             |  2 +-
 gdb/testsuite/gdb.python/py-events.cc  | 21 ++++++++++++++
 gdb/testsuite/gdb.python/py-events.exp | 39 ++++++++++++++++++++++++++
 gdb/testsuite/gdb.python/py-events.py  |  1 +
 4 files changed, 62 insertions(+), 1 deletion(-)
 create mode 100644 gdb/testsuite/gdb.python/py-events.cc

diff --git a/gdb/eval.c b/gdb/eval.c
index 1b3c974009a..34c6c6160d4 100644
--- a/gdb/eval.c
+++ b/gdb/eval.c
@@ -93,7 +93,7 @@ struct value *
 expression::evaluate (struct type *expect_type, enum noside noside)
 {
   gdb::optional<enable_thread_stack_temporaries> stack_temporaries;
-  if (target_has_execution ()
+  if (target_has_execution ()  && inferior_ptid != null_ptid
       && language_defn->la_language == language_cplus
       && !thread_stack_temporaries_enabled_p (inferior_thread ()))
     stack_temporaries.emplace (inferior_thread ());
diff --git a/gdb/testsuite/gdb.python/py-events.cc b/gdb/testsuite/gdb.python/py-events.cc
new file mode 100644
index 00000000000..b865a5a9652
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-events.cc
@@ -0,0 +1,21 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2021 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 main(void)
+{
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.python/py-events.exp b/gdb/testsuite/gdb.python/py-events.exp
index e89cd8b021b..bec3c9f2cab 100644
--- a/gdb/testsuite/gdb.python/py-events.exp
+++ b/gdb/testsuite/gdb.python/py-events.exp
@@ -197,18 +197,33 @@ gdb_test_multiple "continue" $test {
 gdb_test_no_output "delete $second_breakpoint"
 
 #test exited event.
+proc get_process_id {test} {
+    global gdb_prompt
+    global process_id
+    gdb_test_multiple "info proc" $test {
+	-re "process (\\d+).*$gdb_prompt $" {
+	    set process_id $expect_out(1,string)
+	    pass $test
+	}
+    }
+}
+
+get_process_id "get inferior 1 process id"
 gdb_test "continue" ".*event type: continue.*
 .*clear_objfiles\[\r\n\]*progspace: .*py-events.*
 .*event type: exit.*
 .*exit code: 12.*
 .*exit inf: 1.*
+.*exit pid: $process_id.*
 dir ok: True.*" "Inferior 1 terminated."
 
 gdb_test "inferior 2" ".*Switching to inferior 2.*"
+get_process_id "get inferior 2 process id"
 gdb_test "continue" ".*event type: continue.*
 .*event type: exit.*
 .*exit code: 12.*
 .*exit inf: 2.*
+.*exit pid: $process_id.*
 dir ok: True.*" "Inferior 2 terminated."
 
 
@@ -235,3 +250,27 @@ gdb_test "python print(count)" 2 "check for before_prompt event"
 
 gdb_test_no_output "xxz" "run a canned sequence"
 gdb_test "python print(count)" 4 "check for before_prompt event again"
+
+# Test evaluating expressions from within an inferior exit event handler. This used to cause a crash
+# when C++ expression were evaluated.
+standard_testfile .cc
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile]} {
+    return -1
+}
+
+gdb_test_multiline "add exited listener" \
+    "python" "" \
+    "def increment_foo(_):" "" \
+    "  gdb.execute('set \$_foo=\$_foo+1')" "" \
+    "gdb.events.exited.connect(increment_foo)" "" \
+    "end" ""
+gdb_test "set \$_foo=0" "" "initialize foo variable"
+gdb_test "print \$_foo" "= 0" "check foo initialized"
+gdb_test "run" "exited normally.*"
+gdb_test "print \$_foo" "= 1" "check foo after run"
+gdb_test "start" "Temporary breakpoint .* main .*" "stop on a C++ frame"
+gdb_test "continue" "exited normally.*"
+gdb_test "print \$_foo" "= 2" "check foo after start continue"
+
+>>>>>>> 2d04478f2ca... gdb: check inferior_ptid before calling inferior_thread in eval.c
diff --git a/gdb/testsuite/gdb.python/py-events.py b/gdb/testsuite/gdb.python/py-events.py
index bd1dd749452..53c7f9f20a4 100644
--- a/gdb/testsuite/gdb.python/py-events.py
+++ b/gdb/testsuite/gdb.python/py-events.py
@@ -44,6 +44,7 @@ def exit_handler (event):
     print ("event type: exit")
     print ("exit code: %d" % (event.exit_code))
     print ("exit inf: %d" % (event.inferior.num))
+    print ("exit pid: %d" % (event.inferior.pid))
     print ("dir ok: %s" % str('exit_code' in dir(event)))
 
 def continue_handler (event):
-- 
2.25.1


^ permalink raw reply	[flat|nested] 14+ messages in thread
end of thread, other threads:[~2021-06-03 21:07 UTC | newest]

Thread overview: 14+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-05-05 15:56 [RFC PATCH] gdb: fix eval.c assert during inferior exit event Magne Hov
2021-05-06  1:59 ` Simon Marchi
2021-05-10 16:57   ` Magne Hov
2021-05-10 17:33     ` Simon Marchi
2021-05-10 17:21 ` [PATCH v2] " Magne Hov
2021-05-10 17:44   ` Simon Marchi
2021-05-14 16:24     ` Magne Hov
2021-05-11  7:16   ` Aktemur, Tankut Baris
2021-05-14 16:22     ` Magne Hov
2021-05-24 19:23   ` Joel Brobecker
2021-05-26 13:02 ` [PATCH v3] " Magne Hov
2021-05-29  2:25   ` Simon Marchi
2021-06-02 11:34     ` Magne Hov
2021-06-03 21:07   ` Magne Hov

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