PR18006: internal error if threaded program calls clone(CLONE_VM)

On GNU/Linux, if a pthreaded program has a thread call clone(CLONE_VM)
directly, and then that clone LWP hits a debug event (breakpoint,
etc.) GDB internal errors.  Threaded programs shouldn't really be
calling clone directly, but GDB shouldn't crash either.

The crash looks like this:

 (gdb) break clone_fn
 Breakpoint 2 at 0x4007d8: file clone-thread_db.c, line 35.
 (gdb) r
 ...
 [Thread debugging using libthread_db enabled]
 ...
 src/gdb/linux-nat.c:1030: internal-error: lin_lwp_attach_lwp: Assertion `lwpid > 0' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.

The problem is that 'clone' ends up clearing the parent thread's tid
field in glibc's thread data structure.  For x86_64, the glibc code in
question is here:

  sysdeps/unix/sysv/linux/x86_64/clone.S:

   ...
          testq   $CLONE_THREAD, %rdi
          jne     1f
          testq   $CLONE_VM, %rdi
          movl    $-1, %eax            <----
          jne     2f
          movl    $SYS_ify(getpid), %eax
          syscall
  2:      movl    %eax, %fs:PID
          movl    %eax, %fs:TID        <----
  1:

When GDB refreshes the thread list out of libthread_db, it finds a
thread with LWP with pid -1 (the clone's parent), which naturally
isn't yet on the thread list.  GDB then tries to attach to that bogus
LWP id, which is caught by that assertion.

The fix is to detect the bad PID early.

Tested on x86-64 Fedora 20.  GDBserver doesn't need any fix.

gdb/ChangeLog:
2015-02-20  Pedro Alves  <palves@redhat.com>

	PR threads/18006
	* linux-thread-db.c (thread_get_info_callback): Return early if
	the thread's lwp id is -1.

gdb/testsuite/ChangeLog:
2015-02-20  Pedro Alves  <palves@redhat.com>

	PR threads/18006
	* gdb.threads/clone-thread_db.c: New file.
	* gdb.threads/clone-thread_db.exp: New file.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2015-02-20  Pedro Alves  <palves@redhat.com>
+
+	PR threads/18006
+	* linux-thread-db.c (thread_get_info_callback): Return early if
+	the thread's lwp id is -1.
+
 2015-02-20  Joel Brobecker  <brobecker@adacore.com>
 
 	GDB 7.9 released.

gdb/linux-thread-db.c

@@ -431,6 +431,14 @@ thread_get_info_callback (const td_thrhandle_t *thp, void *argp)
     error (_("thread_get_info_callback: cannot get thread info: %s"),
 	   thread_db_err_str (err));
 
+  if (ti.ti_lid == -1)
+    {
+      /* We'll get this if a threaded program has a thread call clone
+	 with CLONE_VM.  `clone' sets the pthread LID of the new LWP
+	 to -1, which ends up clearing the parent thread's LID.  */
+      return 0;
+    }
+
   /* Fill the cache.  */
   thread_ptid = ptid_build (info->pid, ti.ti_lid, 0);
   inout->thread_info = find_thread_ptid (thread_ptid);

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2015-02-20  Pedro Alves  <palves@redhat.com>
+
+	PR threads/18006
+	* gdb.threads/clone-thread_db.c: New file.
+	* gdb.threads/clone-thread_db.exp: New file.
+
 2015-02-19  Antoine Tremblay  <antoine.tremblay@ericsson.com>
 
 	PR breakpoints/16812

gdb/testsuite/gdb.threads/clone-thread_db.c

@@ -0,0 +1,73 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2015 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 that GDB doesn't lose an event for a thread it didn't know
+   about, until an event is reported for it.  */
+
+#define _GNU_SOURCE
+#include <sched.h>
+#include <assert.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+#define STACK_SIZE 0x1000
+
+int clone_pid;
+
+static int
+clone_fn (void *unused)
+{
+  return 0;
+}
+
+void *
+thread_fn (void *arg)
+{
+  unsigned char *stack;
+  int res;
+
+  stack = malloc (STACK_SIZE);
+  assert (stack != NULL);
+
+#ifdef __ia64__
+  clone_pid = __clone2 (clone_fn, stack, STACK_SIZE, CLONE_VM, NULL);
+#else
+  clone_pid = clone (clone_fn, stack + STACK_SIZE, CLONE_VM, NULL);
+#endif
+
+  assert (clone_pid > 0);
+
+  /* Wait for child.  */
+  res = waitpid (clone_pid, NULL, __WCLONE);
+  assert (res != -1);
+
+  return NULL;
+}
+
+int
+main (int argc, char **argv)
+{
+  pthread_t child;
+
+  alarm (300);
+
+  pthread_create (&child, NULL, thread_fn, NULL);
+  pthread_join (child);
+
+  return 0;
+}

gdb/testsuite/gdb.threads/clone-thread_db.exp

@@ -0,0 +1,37 @@
+# This testcase is part of GDB, the GNU debugger.
+
+# Copyright 2015 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 only works on targets with the Linux kernel.
+if ![istarget *-*-linux*] {
+    return
+}
+
+standard_testfile
+
+if [prepare_for_testing "failed to prepare" $testfile $srcfile {debug pthreads}] {
+    return -1
+}
+
+if ![runto_main] {
+    untested "could not run to main"
+    return -1
+}
+
+gdb_test "break clone_fn" "Breakpoint.*at.*file.*$srcfile.*line.*"
+gdb_test "continue" "clone_fn .* at .*" "continue to clone_fn"
+
+gdb_test "continue" "exited normally.*" "continue to end"