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Subject[PATCH] ptrace: add ability to get clear_tid_address
Zero is written at clear_tid_address, when the process exits.
This functionality is used by pthread_join().

sys_set_tid_address() changes this address for current task.

Before this patch clear_tid_address could not be got from user space.
I want to dump a full state of a task, so I need this address.
Also I think it may be useful for debugging a multithreading program.

I am not sure that ptrace is suitable place. It may be added in prctl,
but I think it's a bit useless and strange. I can't image a real
situation (avoid checkpointing) when a thread will want to get own
clear_tid_address from itself, this address is used by parent ussually.

Signed-off-by: Andrew Vagin <>
include/linux/ptrace.h | 3 +++
kernel/ptrace.c | 3 +++
2 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index c2f1f6a..79b84a3 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -51,6 +51,9 @@
#define PTRACE_INTERRUPT 0x4207
#define PTRACE_LISTEN 0x4208

+/* Get clear_child_tid address */
+#define PTRACE_GET_TID_ADDRESS 0x4209
/* flags in @data for PTRACE_SEIZE */
#define PTRACE_SEIZE_DEVEL 0x80000000 /* temp flag for development */

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 00ab2ca..ed7fbe7 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -845,6 +845,9 @@ int ptrace_request(struct task_struct *child, long request,
+ return put_user(child->clear_child_tid, (int __user **) data);

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