[PATCH v5] gdb/gcore: interrupt all threads before generating the corefile

[Lancelot SIX <lancelot.six@amd.com>]

Hi, this V5 follows
https://sourceware.org/pipermail/gdb-patches/2022-December/194504.html.

Changes since V4:

- Update gdb/NEWS to fall into the "Changes since GDB 13" section.
- Use "build_executable" instead of "prepare_for_testing" in the test
  to avoid one useless start of GDB.
- Updated the test's description.
- Updated the copyright year to include 2023.

Best,
Lancelot.

---

In non-stop mode, if the user tries to generate a core dump (using the
gcore command) while some threads are running, a non-helpful error
message is shown.

Lets consider the following session as an example (debugging the test
program included in this patch):

    (gdb) set non-stop on
    (gdb) b 37
    (gdb) r
    Thread 1 "gcore-nonstop" hit Breakpoint 1, main () at gcore-nonstop.c:39
    (gdb) info thread
       Id   Target Id                                          Frame
     * 1    Thread 0x7ffff7d7a740 (LWP 431838) "gcore-nonstop" main () at gcore-nonstop.c:39
       2    Thread 0x7ffff7d79640 (LWP 431841) "gcore-nonstop" (running)
    (gdb) gcore
    Couldn't get registers: No such process.

The reported error ("No such process") does not help the user understand
what happens.  This is due to the fact that we cannot access the
registers of a running thread.  Even if we ignore this error, generating
a core dump while any thread might update memory would most likely
result in a core file with an inconsistent view of the process' memory.

To solve this, this patch proposes to change the gcore command so it
first stops all running threads (from the current inferior) before
generating the corefile, and then resumes them in their previous state.

To achieve this, this patch exposes the restart_threads function in infrun.h
(used to be local to infrun.c).  We also allow the first parameter
(event_thread) to be nullptr as it is possible that the gcore command is
called while all threads are running, in which case we want all threads
to be restarted at the end of the procedure.

When testing this patch against gdbserver, it appears that using
stop_all_threads / restart_threads was not compatible with all-stop
targets.  For those targets, we need to call target_stop_and_wait /
target_resume.  The gcore_command has code to handle both
configurations.  I could not use a RAII-like object to have a cleaner
way to restore the state at the end as the restore procedure could
throw.  Instead, the procedure keeps track of which method was used to
interrupt threads so the appropriate method can be used to restore their
state.

Tested on x86_64 on navite GDB and the native-extended-gdbserver board.
---
 gdb/NEWS                                 |  8 +++
 gdb/doc/gdb.texinfo                      |  5 ++
 gdb/gcore.c                              | 30 +++++++++
 gdb/infrun.c                             | 16 ++---
 gdb/infrun.h                             |  9 +++
 gdb/testsuite/gdb.base/gcore-nonstop.c   | 44 +++++++++++++
 gdb/testsuite/gdb.base/gcore-nonstop.exp | 79 ++++++++++++++++++++++++
 7 files changed, 180 insertions(+), 11 deletions(-)
 create mode 100644 gdb/testsuite/gdb.base/gcore-nonstop.c
 create mode 100644 gdb/testsuite/gdb.base/gcore-nonstop.exp

diff --git a/gdb/NEWS b/gdb/NEWS
index 445d28efed9..7f78cbc9008 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -38,6 +38,14 @@ maintenance print record-instruction [ N ]
   prints how GDB would undo the N-th previous instruction, and if N is
   positive, it prints how GDB will redo the N-th following instruction.
 
+* Changed commands
+
+gcore
+generate-core-file
+  GDB now ensures that all threads of the current inferior are stopped
+  before generating a core dump.  At the end of the command, threads are
+  restored to their previous state.
+
 * MI changes
 
 ** mi now reports 'no-history' as a stop reason when hitting the end of the
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 03033c7f9e3..c637d6eb114 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -13693,6 +13693,11 @@ Produce a core dump of the inferior process.  The optional argument
 specified, the file name defaults to @file{core.@var{pid}}, where
 @var{pid} is the inferior process ID.
 
+@value{GDBN} ensures that all threads of the current inferior are
+stopped while generating the core dump.  If any of the inferior's threads
+are running when executing this command, @value{GDBN} stops the threads
+and resumes them when the command is done.
+
 Note that this command is implemented only for some systems (as of
 this writing, @sc{gnu}/Linux, FreeBSD, Solaris, and S390).
 
diff --git a/gdb/gcore.c b/gdb/gcore.c
index 973abadb013..939737d83a1 100644
--- a/gdb/gcore.c
+++ b/gdb/gcore.c
@@ -34,6 +34,7 @@
 #include "regset.h"
 #include "gdb_bfd.h"
 #include "readline/tilde.h"
+#include "infrun.h"
 #include <algorithm>
 #include "gdbsupport/gdb_unlinker.h"
 #include "gdbsupport/byte-vector.h"
@@ -131,6 +132,28 @@ gcore_command (const char *args, int from_tty)
   if (!target_has_execution ())
     noprocess ();
 
+  scoped_restore_current_thread restore_current_thread;
+  scoped_disable_commit_resumed disable_commit_resume ("generating coredump");
+  struct inferior *inf = current_inferior ();
+  scoped_finish_thread_state finish_state (inf->process_target (),
+					   ptid_t (inf->pid));
+
+  bool all_stop_was_running = false;
+  if (exists_non_stop_target ())
+    stop_all_threads ("generating coredump", inf);
+  else
+    {
+      all_stop_was_running = any_thread_of_inferior (inf)->executing ();
+
+      if (all_stop_was_running)
+	{
+	  if (!may_stop)
+	    error (_("Cannot stop the target to generate the core dump."));
+
+	  target_stop_and_wait (ptid_t (inf->pid));
+	}
+    }
+
   if (args && *args)
     corefilename.reset (tilde_expand (args));
   else
@@ -161,6 +184,13 @@ gcore_command (const char *args, int from_tty)
     }
 
   gdb_printf ("Saved corefile %s\n", corefilename.get ());
+
+  if (exists_non_stop_target ())
+    restart_threads (nullptr, inf);
+  else if (all_stop_was_running)
+    target_resume (ptid_t (inf->pid), 0, GDB_SIGNAL_0);
+
+  disable_commit_resume.reset_and_commit ();
 }
 
 static enum bfd_architecture
diff --git a/gdb/infrun.c b/gdb/infrun.c
index edfb5ab0a91..0e6abc352aa 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -96,9 +96,6 @@ static void resume (gdb_signal sig);
 
 static void wait_for_inferior (inferior *inf);
 
-static void restart_threads (struct thread_info *event_thread,
-			     inferior *inf = nullptr);
-
 static bool start_step_over (void);
 
 static bool step_over_info_valid_p (void);
@@ -5857,18 +5854,15 @@ handle_inferior_event (struct execution_control_state *ecs)
     }
 }
 
-/* Restart threads back to what they were trying to do back when we
-   paused them (because of an in-line step-over or vfork, for example).
-   The EVENT_THREAD thread is ignored (not restarted).
-
-   If INF is non-nullptr, only resume threads from INF.  */
+/* See infrun.h.  */
 
-static void
+void
 restart_threads (struct thread_info *event_thread, inferior *inf)
 {
   INFRUN_SCOPED_DEBUG_START_END ("event_thread=%s, inf=%d",
-				 event_thread->ptid.to_string ().c_str (),
-				 inf != nullptr ? inf->num : -1);
+				 (event_thread != nullptr
+				  ? event_thread->ptid.to_string ().c_str ()
+				  : "None"), inf != nullptr ? inf->num : -1);
 
   gdb_assert (!step_over_info_valid_p ());
 
diff --git a/gdb/infrun.h b/gdb/infrun.h
index 43fd1b44f5a..f6b04934bad 100644
--- a/gdb/infrun.h
+++ b/gdb/infrun.h
@@ -175,6 +175,15 @@ extern void nullify_last_target_wait_ptid ();
    all threads of all inferiors.  */
 extern void stop_all_threads (const char *reason, inferior *inf = nullptr);
 
+/* Restart threads back to what they were trying to do back when we
+   paused them (because of an in-line step-over or vfork, for example).
+   The EVENT_THREAD thread, if non-nullptr, is ignored (not restarted).
+
+   If INF is non-nullptr, only resume threads from INF.  */
+
+extern void restart_threads (struct thread_info *event_thread,
+			     inferior *inf = nullptr);
+
 extern void prepare_for_detach (void);
 
 extern void fetch_inferior_event ();
diff --git a/gdb/testsuite/gdb.base/gcore-nonstop.c b/gdb/testsuite/gdb.base/gcore-nonstop.c
new file mode 100644
index 00000000000..a96c011ad14
--- /dev/null
+++ b/gdb/testsuite/gdb.base/gcore-nonstop.c
@@ -0,0 +1,44 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2022-2023 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>
+
+static pthread_barrier_t barrier;
+
+static void *
+worker_func (void *ignored)
+{
+  pthread_barrier_wait (&barrier);
+  return NULL;
+}
+
+int
+main (void)
+{
+  pthread_t worker_thread;
+  pthread_barrier_init (&barrier, NULL, 2);
+
+  pthread_create (&worker_thread, NULL, worker_func, NULL);
+
+  /* Break here.  */
+
+  pthread_barrier_wait (&barrier);
+  pthread_join (worker_thread, NULL);
+  pthread_barrier_destroy (&barrier);
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/gcore-nonstop.exp b/gdb/testsuite/gdb.base/gcore-nonstop.exp
new file mode 100644
index 00000000000..172dd760e73
--- /dev/null
+++ b/gdb/testsuite/gdb.base/gcore-nonstop.exp
@@ -0,0 +1,79 @@
+# Copyright 2022-2023 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/>.
+
+# Check that in non-stop mode, if some threads are running when the user
+# launches the "gcore" command, GDB suspends all threads, generates the core
+# file and resumes threads which where running before the "gcore" command
+# got issued.  If running threads were not stopped, GDB would report errors
+# when trying to capture the thread's state.
+
+standard_testfile
+
+if { [build_executable "failed to build" \
+	${testfile} ${srcfile} {pthreads debug}] } {
+    return
+}
+
+save_vars { GDBFLAGS } {
+    append GDBFLAGS " -ex \"set non-stop on\""
+    clean_restart ${binfile}
+}
+
+set lineno [gdb_get_line_number "Break here"]
+if { ![runto $lineno] } {
+    return
+}
+
+# We should be stopped in thread 1 while thread 2 is running.
+gdb_test_sequence "info threads" "info threads" {
+    {Id\s+Target Id\s+Frame}
+    {\*\s+1[^\n]*\n}
+    {\s+2\s+[^\n]*\(running\)[^\n]*\n}
+}
+
+set th1_pc ""
+gdb_test_multiple "p/x \$pc" "fetch thread 1 PC" {
+  -wrap -re "$::decimal = ($::hex)" {
+    set th1_pc $expect_out(1,string)
+    pass $gdb_test_name
+  }
+}
+
+set corefile [standard_output_file "corefile"]
+if {![gdb_gcore_cmd $corefile "generate corefile"]} {
+  # gdb_gcore_cmd issues a "UNSUPPORTED".
+  return
+}
+
+# After the core file is generated, thread 2 should be back running
+# and thread 1 should still be selected.
+gdb_test_sequence "info threads" "correct thread selection after gcore" {
+    {Id\s+Target Id\s+Frame}
+    {\*\s+1[^\n]*\n}
+    {\s+2\s+[^\n]*\(running\)[^\n]*\n}
+}
+
+# Thread 1 is at the same PC it was before calling the gcore command.
+gdb_test "p/x \$pc" "\\\$$::decimal = $th1_pc" "thread 1 unchanged"
+
+clean_restart $binfile
+gdb_test "core-file $corefile" "Core was generated by.*" "load corefile"
+
+# The core file has the 2 threads.
+gdb_test_sequence "info threads" "threads in corefile" {
+    {Id\s+Target Id\s+Frame}
+    {\s+1\s+Thread[^\n]*\n}
+    {\s+2\s+Thread[^\n]*\n}
+}

base-commit: 5867ab870b8aa36ae490ec6e4e8e4c55be11ccf1
-- 
2.34.1