Warn users about mismatched PID namespaces

Linux supports multiple "PID namespaces".  Processes in different PID
namespaces have different views of the system process list.  Sometimes,
a single process can appear in more than one PID namespace, but with a
different PID in each.  When GDB and its target are in different PID
namespaces, various features can break due to the mismatch between
what the target believes its PID to be and what GDB believes its PID
to be.  The most visible broken functionality is thread enumeration
silently failing.

This patch explicitly warns users against trying to debug across PID
namespaces.

The patch introduced no new failures in my test suite run on an x86_64
installation of Ubuntu 14.10.  It doesn't include a test: writing an
automated test that exercises this code would be very involved because
CLONE_NEWNS requires CAP_SYS_ADMIN; the easier way to reproduce the
problem is to start a new lxc container.

gdb/
2014-11-11  Daniel Colascione  <dancol@dancol.org>

	Warn about cross-PID-namespace debugging.
	* nat/linux-procfs.h (linux_proc_pid_get_ns): New prototype.
	* nat/linux-procfs.c (linux_proc_pid_get_ns): New function.
	* linux-thread-db.c (check_pid_namespace_match): New function.
	(thread_db_inferior_created): Call it.

gdb/ChangeLog

@@ -1,3 +1,11 @@
+2014-11-11  Daniel Colascione  <dancol@dancol.org>
+
+	Warn about cross-PID-namespace debugging.
+	* nat/linux-procfs.h (linux_proc_pid_get_ns): New prototype.
+	* nat/linux-procfs.c (linux_proc_pid_get_ns): New function.
+	* linux-thread-db.c (check_pid_namespace_match): New function.
+	(thread_db_inferior_created): Call it.
+
 2014-11-10  Doug Evans  <xdje42@gmail.com>
 
 	* symmisc.c (print_objfile_statistics): Remove trailing whitespace.

gdb/linux-thread-db.c

@@ -1217,12 +1217,41 @@ thread_db_new_objfile (struct objfile *objfile)
     check_for_thread_db ();
 }
 
+static void
+check_pid_namespace_match (void)
+{
+  /* Check is only relevant for local targets targets.  */
+  if (target_can_run (&current_target))
+    {
+      /* If the child is in a different PID namespace, its idea of its
+	 PID will differ from our idea of its PID.  When we scan the
+	 child's thread list, we'll mistakenly think it has no threads
+	 since the thread PID fields won't match the PID we give to
+	 libthread_db.  */
+      char *our_pid_ns = linux_proc_pid_get_ns (getpid (), "pid");
+      char *inferior_pid_ns = linux_proc_pid_get_ns (
+	ptid_get_pid (inferior_ptid), "pid");
+
+      if (our_pid_ns != NULL && inferior_pid_ns != NULL
+	  && strcmp (our_pid_ns, inferior_pid_ns) != 0)
+	{
+	  warning (_ ("Target and debugger are in different PID "
+		      "namespaces; thread lists and other data are "
+		      "likely unreliable"));
+	}
+
+      xfree (our_pid_ns);
+      xfree (inferior_pid_ns);
+    }
+}
+
 /* This function is called via the inferior_created observer.
    This handles the case of debugging statically linked executables.  */
 
 static void
 thread_db_inferior_created (struct target_ops *target, int from_tty)
 {
+  check_pid_namespace_match ();
   check_for_thread_db ();
 }
 

gdb/nat/linux-procfs.c

@@ -113,3 +113,22 @@ linux_proc_pid_is_zombie (pid_t pid)
 {
   return linux_proc_pid_has_state (pid, "Z (zombie)");
 }
+
+/* See linux-procfs.h declaration.  */
+
+char *
+linux_proc_pid_get_ns (pid_t pid, const char *ns)
+{
+  char buf[100];
+  char nsval[64];
+  int ret;
+  xsnprintf (buf, sizeof (buf), "/proc/%d/ns/%s", (int) pid, ns);
+  ret = readlink (buf, nsval, sizeof (nsval));
+  if (0 < ret && ret < sizeof (nsval))
+    {
+      nsval[ret] = '\0';
+      return xstrdup (nsval);
+    }
+
+  return NULL;
+}

gdb/nat/linux-procfs.h

@@ -40,4 +40,10 @@ extern int linux_proc_pid_is_stopped (pid_t pid);
 
 extern int linux_proc_pid_is_zombie (pid_t pid);
 
+/* Return an opaque string identifying PID's NS namespace or NULL if
+ * the information is unavailable.  The returned string must be
+ * released with xfree.  */
+
+extern char *linux_proc_pid_get_ns (pid_t pid, const char *ns);
+
 #endif /* COMMON_LINUX_PROCFS_H */