Re: [PATCH][gdb] Fix hang after ext sigkill

[Tom de Vries <tdevries@suse.de>]

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On 25-03-2020 15:44, Simon Marchi wrote:
> On 2020-03-25 6:29 a.m., Tom de Vries wrote:
>> Here's the updated patch.
> 
> Thanks.  Some comments about the test:
> 
> - Please add a comment at the top to describe briefly what this is testing.
> - Please replace the infinite loops with bounded ones (e.g. for (i = 0; i < 300; i++)),
>   so that the test program eventually exits if something goes wrong and it is allowed to run
>   freely.

Done.

Thanks,
- Tom



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[gdb] Fix hang after ext sigkill

Consider the test-case from this patch, compiled with pthread support:
...
$ gcc src/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.c -lpthread
...

After running, the program sleeps:
...
$ gdb a.out
Reading symbols from a.out...
(gdb) r
Starting program: /data/gdb_versions/devel/a.out
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[New Thread 0x7ffff77fe700 (LWP 22604)]
...

Until we interrupt it with a control-C:
...
^C
Thread 1 "a.out" received signal SIGINT, Interrupt.
0x00007ffff78c50f0 in nanosleep () from /lib64/libc.so.6
(gdb)
...

If we then kill the inferior using an external SIGKILL:
...
(gdb) shell killall -s SIGKILL a.out
...
and subsequently continue:
...
(gdb) c
Continuing.
Couldn't get registers: No such process.
Couldn't get registers: No such process.
(gdb) Couldn't get registers: No such process.
(gdb) Couldn't get registers: No such process.
(gdb) Couldn't get registers: No such process.
<repeat>
...
gdb hangs repeating the same warning.  Typing control-C no longer helps,
and we have to kill gdb.

This is a regression since commit 873657b9e8 "Preserve selected thread in
all-stop w/ background execution".  The commit adds a
scoped_restore_current_thread typed variable restore_thread to
fetch_inferior_event, and the hang is caused by the constructor throwing an
exception.

Fix this by catching the exception in the constructor.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-02-24  Tom de Vries  <tdevries@suse.de>

	PR gdb/25471
	* thread.c
	(scoped_restore_current_thread::scoped_restore_current_thread): Catch
	exception in get_frame_id.

gdb/testsuite/ChangeLog:

2020-02-24  Tom de Vries  <tdevries@suse.de>

	PR gdb/25471
	* gdb.threads/hang-after-ext-sigkill.c: New test.
	* gdb.threads/hang-after-ext-sigkill.exp: New file.
	* lib/gdb.exp (runto): Handle "Temporary breakpoint" string.

---
 gdb/testsuite/gdb.threads/hang-after-ext-sigkill.c | 43 +++++++++++
 .../gdb.threads/hang-after-ext-sigkill.exp         | 88 ++++++++++++++++++++++
 gdb/testsuite/lib/gdb.exp                          |  2 +-
 gdb/thread.c                                       | 12 ++-
 4 files changed, 142 insertions(+), 3 deletions(-)

diff --git a/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.c b/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.c
new file mode 100644
index 0000000000..b93d6c644a
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.c
@@ -0,0 +1,43 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 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 <pthread.h>
+#include <unistd.h>
+
+static void *
+fun (void *dummy)
+{
+  int i;
+
+  for (i = 0; i < 300; i++)
+    sleep (1);
+
+  return NULL;
+}
+
+int
+main (void)
+{
+  int i;
+  pthread_t thread;
+  pthread_create (&thread, NULL, fun, NULL);
+
+  for (i = 0; i < 300; i++)
+    sleep (1);
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.exp b/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.exp
new file mode 100644
index 0000000000..89b38b1f6c
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/hang-after-ext-sigkill.exp
@@ -0,0 +1,88 @@
+# Copyright (C) 2020 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 test-case tests that continuing an inferior that has been killed
+# using an external sigkill does not make gdb hang.
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile \
+	 {pthreads}] == -1} {
+    return -1
+}
+
+set res [runto main no-message temporary]
+if { $res != 1 } {
+    return -1
+}
+
+set pid -1
+gdb_test_multiple "info inferior 1" "get inferior pid" {
+    -re -wrap "process (\[0-9\]*).*" {
+       set pid $expect_out(1,string)
+       pass $gdb_test_name
+    }
+}
+if { $pid == -1 } {
+    return -1
+}
+
+gdb_test_multiple "continue" "" {
+    -re "Continuing" {
+	pass $gdb_test_name
+    }
+}
+
+send_gdb "\003"
+
+gdb_test_multiple "" "get sigint" {
+    -re -wrap "received signal SIGINT, Interrupt\..*" {
+       pass $gdb_test_name
+   }
+}
+
+gdb_test_no_output "shell kill -s SIGKILL $pid" "shell kill -s SIGKILL pid"
+
+set no_such_process_msg "Couldn't get registers: No such process\."
+set killed_msg "Program terminated with signal SIGKILL, Killed\."
+set no_longer_exists_msg "The program no longer exists\."
+set not_being_run_msg "The program is not being run\."
+
+gdb_test_multiple "continue" "prompt after first continue" {
+    -re "Continuing\.\r\n\r\n$killed_msg\r\n$no_longer_exists_msg\r\n$gdb_prompt $" {
+	pass $gdb_test_name
+	# Regular output, bug condition was not triggered, we're done.
+	return -1
+    }
+    -re "Continuing\.\r\n$no_such_process_msg\r\n$no_such_process_msg\r\n$gdb_prompt " {
+	pass $gdb_test_name
+	# Two times $no_such_process_msg.  The bug condition was triggered, go
+	# check for it.
+    }
+    -re "Continuing\.\r\n$no_such_process_msg\r\n$gdb_prompt $" {
+	pass $gdb_test_name
+	# One time $no_such_process_msg.  We're stuck here.  The bug condition
+	# was not triggered, but we're not getting correct gdb behaviour either:
+	# every subsequent continue produces one no_such_process_msg.  Give up.
+	return -1
+    }
+}
+
+gdb_test_multiple "" "messages" {
+    -re ".*$killed_msg.*$no_longer_exists_msg\r\n" {
+	pass $gdb_test_name
+	gdb_test "continue" $not_being_run_msg "second continue"
+    }
+}
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index e17ac0ef75..4cf2beca00 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -570,7 +570,7 @@ proc runto { function args } {
 	    }
 	    return 1
 	}
-	-re "Breakpoint \[0-9\]*, \[0-9xa-f\]* in .*$gdb_prompt $" { 
+	-re "\[Bb\]reakpoint \[0-9\]*, \[0-9xa-f\]* in .*$gdb_prompt $" {
 	    if { $print_pass } {
 		pass $test_name
 	    }
diff --git a/gdb/thread.c b/gdb/thread.c
index c6e3d356a5..d287bce45f 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -1488,8 +1488,16 @@ scoped_restore_current_thread::scoped_restore_current_thread ()
       else
 	frame = NULL;
 
-      m_selected_frame_id = get_frame_id (frame);
-      m_selected_frame_level = frame_relative_level (frame);
+      try
+	{
+	  m_selected_frame_id = get_frame_id (frame);
+	  m_selected_frame_level = frame_relative_level (frame);
+	}
+      catch (const gdb_exception &ex)
+	{
+	  m_selected_frame_id = null_frame_id;
+	  m_selected_frame_level = -1;
+	}
 
       tp->incref ();
       m_thread = tp;