Add prctl to set sibling thread names

Taking a very raw attempt at this, I scratched out the following simple
implementation. I'd appreciate any review or suggestions for
improvements. I'm not at all certain the passing of the thread pid_t
through the unsigned long is valid, for instance, or if
same_thread_group() is the right check to make sure we only change
siblings and not tid from other processes. So any advice on better
approaches would be great.

thanks
-john

========================

Setting a thread's comm to be something unique is a very useful ability
and is helpful for debugging complicated threaded applications. However
currently the only way to set a thread name is for the thread to name
itself via the PR_SET_NAME prctl.

However, there may be situations where it would be advantageous for a
thread dispatcher to be naming the threads its managing, rather then
having the threads self-describe themselves. This sort of behavior is
available on other systems via the pthread_setname_np() interface.

This patch allows a thread to name its sibling threads via a new
PR_SET_THREAD_NAME prctrl.

Signed-off-by: John Stultz <johnstul@us.ibm.com>

diff --git a/include/linux/prctl.h b/include/linux/prctl.h
index 9311505..2726527 100644
--- a/include/linux/prctl.h
+++ b/include/linux/prctl.h
@@ -52,6 +52,7 @@
 
 #define PR_SET_NAME    15		/* Set process name */
 #define PR_GET_NAME    16		/* Get process name */
+#define PR_SET_THREAD_NAME    17	/* Set sibling thread name */
 
 /* Get/set process endian */
 #define PR_GET_ENDIAN	19
diff --git a/kernel/sys.c b/kernel/sys.c
index 255475d..d25851a 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1506,6 +1506,27 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 					 sizeof(comm)))
 				return -EFAULT;
 			return 0;
+		case PR_SET_THREAD_NAME:
+		{
+			struct task_struct *tsk;
+			pid_t tid;
+
+			comm[sizeof(me->comm)-1] = 0;
+			tid = (pid_t)arg2;
+			if (strncpy_from_user(comm, (char __user *)arg3,
+					      sizeof(me->comm) - 1) < 0)
+				return -EFAULT;
+
+			tsk = get_pid_task(find_get_pid(tid), PIDTYPE_PID);
+			if (!tsk)
+				return -EINVAL;
+
+			if (!same_thread_group(me, tsk))
+				return -EINVAL;
+
+			set_task_comm(tsk, comm);
+			return 0;
+		}
 		case PR_GET_ENDIAN:
 			error = GET_ENDIAN(me, arg2);
 			break;


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