nptl_db: Support different libpthread/ld.so load orders (bug 27744)

libthread_db is loaded once GDB encounters libpthread, and at this
point, ld.so may not have been processed by GDB yet. As a result,
_rtld_global cannot be accessed by regular means from libthread_db.
To make this work until GDB can be fixed, acess _rtld_global through
a pointer stored in libpthread.

The new test does not reproduce bug 27744 with
--disable-hardcoded-path-in-tests, but is still a valid smoke test.
With --enable-hardcoded-path-in-tests, it is necessary to avoid
add-symbol-file because this can tickle a GDB bug.

Fixes commit 1daccf403b ("nptl: Move
stack list variables into _rtld_global").

Tested-by: Emil Velikov <emil.velikov@collabora.com>

nptl/Makefile

@@ -313,7 +313,8 @@ tests = tst-attr2 tst-attr3 tst-default-attr \
 	tst-thread-affinity-sched \
 	tst-pthread-defaultattr-free \
 	tst-pthread-attr-sigmask \
-	tst-pthread-timedlock-lockloop
+	tst-pthread-timedlock-lockloop \
+	tst-pthread-gdb-attach tst-pthread-gdb-attach-static
 
 tests-container =  tst-pthread-getattr
 
@@ -359,6 +360,19 @@ CPPFLAGS-test-cond-printers.c := $(CFLAGS-printers-tests)
 CPPFLAGS-test-rwlockattr-printers.c := $(CFLAGS-printers-tests)
 CPPFLAGS-test-rwlock-printers.c := $(CFLAGS-printers-tests)
 
+# Reuse the CFLAGS setting for the GDB attaching test.  It needs
+# debugging information.
+CFLAGS-tst-pthread-gdb-attach.c := $(CFLAGS-printers-tests)
+CPPFLAGS-tst-pthread-gdb-attach.c := $(CFLAGS-printers-tests)
+ifeq ($(build-shared)$(build-hardcoded-path-in-tests),yesno)
+CPPFLAGS-tst-pthread-gdb-attach.c += -DDO_ADD_SYMBOL_FILE=1
+else
+CPPFLAGS-tst-pthread-gdb-attach.c += -DDO_ADD_SYMBOL_FILE=0
+endif
+CFLAGS-tst-pthread-gdb-attach-static.c := $(CFLAGS-printers-tests)
+CPPFLAGS-tst-pthread-gdb-attach-static.c := \
+  $(CFLAGS-printers-tests) -DDO_ADD_SYMBOL_FILE=0
+
 ifeq ($(build-shared),yes)
 tests-printers-libs := $(shared-thread-library)
 else
@@ -430,7 +444,8 @@ link-libc-static := $(common-objpfx)libc.a $(static-gnulib) \
 tests-static += tst-stackguard1-static \
 		tst-cancel24-static \
 		tst-mutex8-static tst-mutexpi8-static tst-sem11-static \
-		tst-sem12-static tst-cond11-static
+		tst-sem12-static tst-cond11-static \
+		tst-pthread-gdb-attach-static
 
 tests += tst-cancel24-static
 

nptl/pthread_create.c

@@ -51,6 +51,14 @@ static td_thr_events_t __nptl_threads_events __attribute_used__;
 /* Pointer to descriptor with the last event.  */
 static struct pthread *__nptl_last_event __attribute_used__;
 
+#ifdef SHARED
+/* This variable is used to access _rtld_global from libthread_db.  If
+   GDB loads libpthread before ld.so, it is not possible to resolve
+   _rtld_global directly during libpthread initialization.  */
+static struct rtld_global *__nptl_rtld_global __attribute_used__
+  = &_rtld_global;
+#endif
+
 /* Number of threads running.  */
 unsigned int __nptl_nthreads = 1;
 

nptl/tst-pthread-gdb-attach-static.c

@@ -0,0 +1 @@
+#include "tst-pthread-gdb-attach.c"

nptl/tst-pthread-gdb-attach.c

@@ -0,0 +1,143 @@
+/* Smoke testing GDB process attach with thread-local variable access.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* This test runs GDB against a forked copy of itself, to check
+   whether libthread_db can be loaded, and that access to thread-local
+   variables works.  */
+
+#include <errno.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/temp_file.h>
+#include <support/test-driver.h>
+#include <support/xstdio.h>
+#include <support/xthread.h>
+#include <support/xunistd.h>
+#include <unistd.h>
+
+/* Starts out as zero, changed to 1 or 2 by the debugger, depending on
+   the thread.  */
+__thread volatile int altered_by_debugger;
+
+/* Writes the GDB script to run the test to PATH.  */
+static void
+write_gdbscript (const char *path, int tested_pid)
+{
+  FILE *fp = xfopen (path, "w");
+  fprintf (fp,
+           "set trace-commands on\n"
+           "set debug libthread-db 1\n"
+#if DO_ADD_SYMBOL_FILE
+           /* Do not do this unconditionally to work around a GDB
+              assertion failure: ../../gdb/symtab.c:6404:
+              internal-error: CORE_ADDR get_msymbol_address(objfile*,
+              const minimal_symbol*): Assertion `(objf->flags &
+              OBJF_MAINLINE) == 0' failed.  */
+           "add-symbol-file %1$s/nptl/tst-pthread-gdb-attach\n"
+#endif
+           "set auto-load safe-path %1$s/nptl_db\n"
+           "set libthread-db-search-path %1$s/nptl_db\n"
+           "attach %2$d\n",
+           support_objdir_root, tested_pid);
+  fputs ("break debugger_inspection_point\n"
+         "continue\n"
+         "thread 1\n"
+         "print altered_by_debugger\n"
+         "print altered_by_debugger = 1\n"
+         "thread 2\n"
+         "print altered_by_debugger\n"
+         "print altered_by_debugger = 2\n"
+         "continue\n",
+         fp);
+  xfclose (fp);
+}
+
+/* The test sets a breakpoint on this function and alters the
+   altered_by_debugger thread-local variable.  */
+void __attribute__ ((weak))
+debugger_inspection_point (void)
+{
+}
+
+/* Thread function for the test thread in the subprocess.  */
+static void *
+subprocess_thread (void *closure)
+{
+  /* Wait until altered_by_debugger changes the value away from 0.  */
+  while (altered_by_debugger == 0)
+    {
+      usleep (100 * 1000);
+      debugger_inspection_point ();
+    }
+
+  TEST_COMPARE (altered_by_debugger, 2);
+  return NULL;
+}
+
+/* This function implements the subprocess under test.  It creates a
+   second thread, waiting for its value to change to 2, and checks
+   that the main thread also changed its value to 1.  */
+static void
+in_subprocess (void)
+{
+  pthread_t thr = xpthread_create (NULL, subprocess_thread, NULL);
+  TEST_VERIFY (xpthread_join (thr) == NULL);
+  TEST_COMPARE (altered_by_debugger, 1);
+  _exit (0);
+}
+
+static int
+do_test (void)
+{
+  pid_t tested_pid = xfork ();
+  if (tested_pid == 0)
+    in_subprocess ();
+  char *tested_pid_string = xasprintf ("%d", tested_pid);
+
+  char *gdbscript;
+  xclose (create_temp_file ("tst-pthread-gdb-attach-", &gdbscript));
+  write_gdbscript (gdbscript, tested_pid);
+
+  pid_t gdb_pid = xfork ();
+  if (gdb_pid == 0)
+    {
+      clearenv ();
+      xdup2 (STDOUT_FILENO, STDERR_FILENO);
+      execlp ("gdb", "gdb", "-nx", "-batch", "-x", gdbscript, NULL);
+      if (errno == ENOENT)
+        _exit (EXIT_UNSUPPORTED);
+      else
+        _exit (1);
+    }
+
+  int status;
+  TEST_COMPARE (xwaitpid (gdb_pid, &status, 0), gdb_pid);
+  if (WIFEXITED (status) && WEXITSTATUS (status) == EXIT_UNSUPPORTED)
+    /* gdb is not installed.  */
+    return EXIT_UNSUPPORTED;
+  TEST_COMPARE (status, 0);
+  TEST_COMPARE (xwaitpid (tested_pid, &status, 0), tested_pid);
+  TEST_COMPARE (status, 0);
+
+  free (tested_pid_string);
+  free (gdbscript);
+  return 0;
+}
+
+#include <support/test-driver.c>

nptl_db/structs.def

@@ -100,8 +100,7 @@ DB_STRUCT_FIELD (pthread, dtvp)
 #endif
 
 #if !(IS_IN (libpthread) && !defined SHARED)
-DB_STRUCT (rtld_global)
-DB_RTLD_VARIABLE (_rtld_global)
+DB_VARIABLE (__nptl_rtld_global)
 #endif
 DB_RTLD_GLOBAL_FIELD (dl_tls_dtv_slotinfo_list)
 DB_RTLD_GLOBAL_FIELD (dl_stack_user)

nptl_db/td_init.c

@@ -33,13 +33,14 @@ td_init (void)
 bool
 __td_ta_rtld_global (td_thragent_t *ta)
 {
-  if (ta->ta_addr__rtld_global == 0
-      && td_mod_lookup (ta->ph, LD_SO, SYM__rtld_global,
-                        &ta->ta_addr__rtld_global) != PS_OK)
+  if (ta->ta_addr__rtld_global == 0)
     {
-      ta->ta_addr__rtld_global = (void*)-1;
-      return false;
+      psaddr_t rtldglobalp;
+      if (DB_GET_VALUE (rtldglobalp, ta, __nptl_rtld_global, 0) == TD_OK)
+        ta->ta_addr__rtld_global = rtldglobalp;
+      else
+        ta->ta_addr__rtld_global = (void *) -1;
     }
-  else
-    return ta->ta_addr__rtld_global != (void*)-1;
+
+  return ta->ta_addr__rtld_global != (void *)-1;
 }

nptl_db/thread_dbP.h

@@ -108,6 +108,8 @@ struct td_thragent
 # undef DB_SYMBOL
 # undef DB_VARIABLE
 
+  psaddr_t ta_addr__rtld_global;
+
   /* The method of locating a thread's th_unique value.  */
   enum
     {