[patch 2/2] c/r: prctl: Add ability to get clear_tid_address

[Cyrill Gorcunov <gorcunov@openvz.org>]

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Zero is written at clear_tid_address, when
the process exits. This functionality is used
by pthread_join().

We already have sys_set_tid_address() to change this
address for current task but there is no way to obtain
it from a user space.

Without ability to find this address and dump it we can't
restore pthread'ed apps which do call pthread_join() once
they have been restored.

This patch introduces PR_GET_TID_ADDRESS prctl option
which allow current process to obtain own clear_tid_address.

This feature is available iif CONFIG_CHECKPOINT_RESTORE is set.

Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
CC: Pedro Alves <palves@redhat.com>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Pavel Emelyanov <xemul@parallels.com>
CC: Tejun Heo <tj@kernel.org>
---
 include/linux/prctl.h |    2 ++
 kernel/sys.c          |   13 +++++++++++++
 2 files changed, 15 insertions(+)

Index: linux-2.6.git/include/linux/prctl.h
===================================================================
--- linux-2.6.git.orig/include/linux/prctl.h
+++ linux-2.6.git/include/linux/prctl.h
@@ -120,4 +120,6 @@
 # define PR_SET_MM_AUXV			12
 # define PR_SET_MM_EXE_FILE		13
 
+#define PR_GET_TID_ADDRESS		36
+
 #endif /* _LINUX_PRCTL_H */
Index: linux-2.6.git/kernel/sys.c
===================================================================
--- linux-2.6.git.orig/kernel/sys.c
+++ linux-2.6.git/kernel/sys.c
@@ -1897,12 +1897,22 @@ out:
 	up_read(&mm->mmap_sem);
 	return error;
 }
+
+static int prctl_get_tid_address(struct task_struct *me, int __user **tid_addr)
+{
+	return put_user(me->clear_child_tid, tid_addr);
+}
+
 #else /* CONFIG_CHECKPOINT_RESTORE */
 static int prctl_set_mm(int opt, unsigned long addr,
 			unsigned long arg4, unsigned long arg5)
 {
 	return -EINVAL;
 }
+static int prctl_get_tid_address(struct task_struct *me, int __user **tid_addr)
+{
+	return -EINVAL;
+}
 #endif
 
 SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
@@ -2057,6 +2067,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsi
 		case PR_SET_MM:
 			error = prctl_set_mm(arg2, arg3, arg4, arg5);
 			break;
+		case PR_GET_TID_ADDRESS:
+			error = prctl_get_tid_address(me, (int __user **)arg2);
+			break;
 		default:
 			error = -EINVAL;
 			break;