[gdb] Handle vfork in thread with follow-fork-mode child

[ Backport of master commit b73715df01. ]

When debugging any of the testcases added by this commit, which do a
vfork in a thread with "set follow-fork-mode child" + "set
detach-on-fork on", we run into this assertion:

...
src/gdb/nat/x86-linux-dregs.c:146: internal-error: \
  void x86_linux_update_debug_registers(lwp_info*): \
  Assertion `lwp_is_stopped (lwp)' failed.
...

The assert is caused by the following: the vfork-child exit or exec
event is handled by handle_vfork_child_exec_or_exit, which calls
target_detach to detach from the vfork parent.  During target_detach
we call linux_nat_target::detach, which:

However, during the second step we run into this code in
stop_wait_callback:

...
  /* If this is a vfork parent, bail out, it is not going to report
     any SIGSTOP until the vfork is done with.  */
  if (inf->vfork_child != NULL)
    return 0;
...

and we don't wait for the threads to be stopped, which results in this
assert in x86_linux_update_debug_registers triggering during the third
step:

...
  gdb_assert (lwp_is_stopped (lwp));
...

The fix is to reset the vfork parent's vfork_child field before
calling target_detach in handle_vfork_child_exec_or_exit.  There's
already similar code for the other paths handled by
handle_vfork_child_exec_or_exit, so this commit refactors the code a
bit so that all paths share the same code.

The new tests cover both a vfork child exiting, and a vfork child
execing, since both cases would trigger the assertion.

The new testcases also exercise following the vfork children with "set
detach-on-fork off", since it doesn't seem to be tested anywhere.

Tested on x86_64-linux, using native and native-gdbserver.

gdb/ChangeLog:
2019-04-18  Tom de Vries  <tdevries@suse.de>
	    Pedro Alves  <palves@redhat.com>

	PR gdb/24454
	* infrun.c (handle_vfork_child_exec_or_exit): Reset vfork parent's
	vfork_child field before calling target_detach.

gdb/testsuite/ChangeLog:
2019-04-18  Tom de Vries  <tdevries@suse.de>
	    Pedro Alves  <palves@redhat.com>

	PR gdb/24454
	* gdb.threads/vfork-follow-child-exec.c: New file.
	* gdb.threads/vfork-follow-child-exec.exp: New file.
	* gdb.threads/vfork-follow-child-exit.c: New file.
	* gdb.threads/vfork-follow-child-exit.exp: New file.

gdb/infrun.c

@@ -914,10 +914,14 @@ handle_vfork_child_exec_or_exit (int exec)
       int resume_parent = -1;
 
       /* This exec or exit marks the end of the shared memory region
-	 between the parent and the child.  If the user wanted to
+	 between the parent and the child.  Break the bonds.  */
-	 detach from the parent, now is the time.  */
+      inferior *vfork_parent = inf->vfork_parent;
+      inf->vfork_parent->vfork_child = NULL;
+      inf->vfork_parent = NULL;
 
-      if (inf->vfork_parent->pending_detach)
+      /* If the user wanted to detach from the parent, now is the
+	 time.  */
+      if (vfork_parent->pending_detach)
 	{
 	  struct thread_info *tp;
 	  struct program_space *pspace;
@@ -925,7 +929,7 @@ handle_vfork_child_exec_or_exit (int exec)
 
 	  /* follow-fork child, detach-on-fork on.  */
 
-	  inf->vfork_parent->pending_detach = 0;
+	  vfork_parent->pending_detach = 0;
 
 	  gdb::optional<scoped_restore_exited_inferior>
 	    maybe_restore_inferior;
@@ -940,7 +944,7 @@ handle_vfork_child_exec_or_exit (int exec)
 	    maybe_restore_thread.emplace ();
 
 	  /* We're letting loose of the parent.  */
-	  tp = any_live_thread_of_inferior (inf->vfork_parent);
+	  tp = any_live_thread_of_inferior (vfork_parent);
 	  switch_to_thread (tp);
 
 	  /* We're about to detach from the parent, which implicitly
@@ -963,7 +967,7 @@ handle_vfork_child_exec_or_exit (int exec)
 	  if (print_inferior_events)
 	    {
 	      const char *pidstr
-		= target_pid_to_str (ptid_t (inf->vfork_parent->pid));
+		= target_pid_to_str (ptid_t (vfork_parent->pid));
 
 	      target_terminal::ours_for_output ();
 
@@ -981,7 +985,7 @@ handle_vfork_child_exec_or_exit (int exec)
 		}
 	    }
 
-	  target_detach (inf->vfork_parent, 0);
+	  target_detach (vfork_parent, 0);
 
 	  /* Put it back.  */
 	  inf->pspace = pspace;
@@ -996,10 +1000,7 @@ handle_vfork_child_exec_or_exit (int exec)
 	  inf->removable = 1;
 	  set_current_program_space (inf->pspace);
 
-	  resume_parent = inf->vfork_parent->pid;
+	  resume_parent = vfork_parent->pid;
-
-	  /* Break the bonds.  */
-	  inf->vfork_parent->vfork_child = NULL;
 	}
       else
 	{
@@ -1029,17 +1030,13 @@ handle_vfork_child_exec_or_exit (int exec)
 	  set_current_program_space (pspace);
 	  inf->removable = 1;
 	  inf->symfile_flags = SYMFILE_NO_READ;
-	  clone_program_space (pspace, inf->vfork_parent->pspace);
+	  clone_program_space (pspace, vfork_parent->pspace);
 	  inf->pspace = pspace;
 	  inf->aspace = pspace->aspace;
 
-	  resume_parent = inf->vfork_parent->pid;
+	  resume_parent = vfork_parent->pid;
-	  /* Break the bonds.  */
-	  inf->vfork_parent->vfork_child = NULL;
 	}
 
-      inf->vfork_parent = NULL;
-
       gdb_assert (current_program_space == inf->pspace);
 
       if (non_stop && resume_parent != -1)

gdb/testsuite/ChangeLog

@@ -1,3 +1,12 @@
+2019-04-18  Tom de Vries  <tdevries@suse.de>
+	    Pedro Alves  <palves@redhat.com>
+
+	PR gdb/24454
+	* gdb.threads/vfork-follow-child-exec.c: New file.
+	* gdb.threads/vfork-follow-child-exec.exp: New file.
+	* gdb.threads/vfork-follow-child-exit.c: New file.
+	* gdb.threads/vfork-follow-child-exit.exp: New file.
+
 2019-07-08  Alan Hayward  <alan.hayward@arm.com>
 
 	* gdb.base/break-idempotent.exp: Test both PIE and non PIE.

gdb/testsuite/gdb.threads/vfork-follow-child-exec.c

@@ -0,0 +1,66 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2019 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 <stdio.h>
+#include <unistd.h>
+#include <pthread.h>
+#include <string.h>
+#include <stdlib.h>
+
+static char *program_name;
+
+static void *
+f (void *arg)
+{
+  int res = vfork ();
+
+  if (res == -1)
+    {
+      perror ("vfork");
+      return NULL;
+    }
+  else if (res == 0)
+    {
+      /* Child.  */
+      execl (program_name, program_name, "1", NULL);
+      perror ("exec");
+      abort ();
+    }
+  else
+    {
+      /* Parent.  */
+      return NULL;
+    }
+}
+
+int
+main (int argc, char **argv)
+{
+  pthread_t tid;
+
+  if (argc > 1)
+    {
+      /* Getting here via execl.  */
+      return 0;
+    }
+
+  program_name = argv[0];
+
+  pthread_create (&tid, NULL, f, NULL);
+  pthread_join (tid, NULL);
+  return 0;
+}

gdb/testsuite/gdb.threads/vfork-follow-child-exec.exp

@@ -0,0 +1,64 @@
+# Copyright (C) 2019 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 following a vfork child that execs, when the vfork parent is a
+# threaded program, and it's a non-main thread that vforks.
+
+standard_testfile
+
+if {[build_executable "failed to prepare" $testfile $srcfile {debug pthreads}]} {
+    return -1
+}
+
+# DETACH indicates whether "set detach-on-fork" is enabled.  It is
+# either "on" or "off".
+
+proc test_vfork {detach} {
+    global binfile
+
+    clean_restart $binfile
+
+    if ![runto_main] then {
+	fail "can't run to main"
+	return 0
+    }
+
+    delete_breakpoints
+
+    gdb_test_no_output "set follow-fork-mode child"
+    gdb_test_no_output "set detach-on-fork $detach"
+
+    if {$detach == "off"} {
+	gdb_test "continue" \
+	    [multi_line \
+		 "Attaching after .* vfork to child .*" \
+		 ".*New inferior 2 .*" \
+		 ".* is executing new program: .*" \
+		 ".*Inferior 2 .* exited normally.*"]
+    } else {
+	gdb_test "continue" \
+	    [multi_line \
+		 "Attaching after .* vfork to child .*" \
+		 ".*New inferior 2 .*" \
+		 ".*Detaching vfork parent process .* after child exec.*" \
+		 ".*Inferior 1 .* detached.*" \
+		 ".*is executing new program: .*" \
+		 ".*Inferior 2 .*exited normally.*"]
+    }
+}
+
+foreach_with_prefix detach-on-fork {"off" "on"} {
+    test_vfork ${detach-on-fork}
+}

gdb/testsuite/gdb.threads/vfork-follow-child-exit.c

@@ -0,0 +1,52 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2019 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 <stdio.h>
+#include <unistd.h>
+#include <pthread.h>
+
+static void *
+f (void *arg)
+{
+  int res = vfork ();
+
+  if (res == -1)
+    {
+      perror ("vfork");
+      return NULL;
+    }
+  else if (res == 0)
+    {
+      /* Child.  */
+      _exit (0);
+    }
+  else
+    {
+      /* Parent.  */
+      return NULL;
+    }
+}
+
+int
+main (void)
+{
+  pthread_t tid;
+
+  pthread_create (&tid, NULL, f, NULL);
+  pthread_join (tid, NULL);
+  return 0;
+}

gdb/testsuite/gdb.threads/vfork-follow-child-exit.exp

@@ -0,0 +1,60 @@
+# Copyright (C) 2019 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 following a vfork child that exits, when the vfork parent is a
+# threaded program, and it's a non-main thread that vforks.
+
+standard_testfile
+
+if {[build_executable "failed to prepare" $testfile $srcfile {debug pthreads}]} {
+    return -1
+}
+
+# DETACH indicates whether "set detach-on-fork" is enabled.  It is
+# either "on" or "off".
+
+proc test_vfork {detach} {
+    global binfile
+
+    clean_restart $binfile
+
+    if ![runto_main] then {
+	fail "can't run to main"
+	return 0
+    }
+
+    gdb_test_no_output "set follow-fork-mode child"
+    gdb_test_no_output "set detach-on-fork $detach"
+
+    if {$detach == "off"} {
+	gdb_test "continue" \
+	    [multi_line \
+		 "Attaching after .* vfork to child .*" \
+		 ".*New inferior 2 .*" \
+		 ".*Inferior 2 .*exited normally.*"]
+    } else {
+	gdb_test "continue" \
+	    [multi_line \
+		 "Attaching after .* vfork to child .*" \
+		 ".*New inferior 2 .*" \
+		 ".*Detaching vfork parent process .* after child exit.*" \
+		 ".*Inferior 1 .* detached.*" \
+		 ".*Inferior 2 .*exited normally.*"]
+    }
+}
+
+foreach_with_prefix detach-on-fork {"off" "on"} {
+    test_vfork ${detach-on-fork}
+}