[PATCH 2/3] ptrace: shift user_*_step() from ptrace_resume() to ptrace_stop() path

Ignoring the buggy PTRACE_KILL, ptrace_resume() calls user_*_step()
when the tracee sleeps in ptrace_stop(). Now that ptrace_resume()
sets PT_SINGLE* flags, we can reassign user_*_step() from the tracer
to the tracee.

Introduce ptrace_finish_stop(), it is called by ptrace_stop() after
schedule(). Move user_*_step() call sites from ptrace_resume() to
ptrace_finish_stop().

This way:

	- we can remove user_disable_single_step() from detach paths.
	
	  This is the main motivation, we can implement asynchronous
	  detach.

	- this makes the detach-on-exit more correct, we do not leak
	  TIF_SINGLESTEP if the tracer dies.

	- user_enable_*_step(tsk) can be implemented more efficiently
	  if tsk == current, we can avoid access_process_vm().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

 include/linux/ptrace.h |    1 +
 kernel/signal.c        |    1 +
 kernel/ptrace.c        |   16 ++++++++++++----
 3 files changed, 14 insertions(+), 4 deletions(-)

--- ptrace/include/linux/ptrace.h~2_ptrace_finish_resume	2011-07-03 21:55:17.000000000 +0200
+++ ptrace/include/linux/ptrace.h	2011-07-03 21:55:37.000000000 +0200
@@ -112,6 +112,7 @@
 #include <linux/compiler.h>		/* For unlikely.  */
 #include <linux/sched.h>		/* For struct task_struct.  */
 
+extern void ptrace_finish_stop(void);
 
 extern long arch_ptrace(struct task_struct *child, long request,
 			unsigned long addr, unsigned long data);
--- ptrace/kernel/signal.c~2_ptrace_finish_resume	2011-07-03 21:55:17.000000000 +0200
+++ ptrace/kernel/signal.c	2011-07-03 21:55:37.000000000 +0200
@@ -1879,6 +1879,7 @@ static void ptrace_stop(int exit_code, i
 	 */
 	try_to_freeze();
 
+	ptrace_finish_stop();
 	/*
 	 * We are back.  Now reacquire the siglock before touching
 	 * last_siginfo, so that we are sure to have synchronized with
--- ptrace/kernel/ptrace.c~2_ptrace_finish_resume	2011-07-03 21:55:17.000000000 +0200
+++ ptrace/kernel/ptrace.c	2011-07-03 21:55:37.000000000 +0200
@@ -581,6 +581,18 @@ static int ptrace_setsiginfo(struct task
 #define is_sysemu_singlestep(request)	0
 #endif
 
+void ptrace_finish_stop(void)
+{
+	struct task_struct *task = current;
+
+	if (task->ptrace & PT_BLOCKSTEP)
+		user_enable_block_step(task);
+	else if (task->ptrace & PT_SINGLESTEP)
+		user_enable_single_step(task);
+	else
+		user_disable_single_step(task);
+}
+
 static int ptrace_resume(struct task_struct *child, long request,
 			 unsigned long data)
 {
@@ -604,14 +616,10 @@ static int ptrace_resume(struct task_str
 		if (unlikely(!arch_has_block_step()))
 			return -EIO;
 		child->ptrace |= PT_BLOCKSTEP;
-		user_enable_block_step(child);
 	} else if (is_singlestep(request) || is_sysemu_singlestep(request)) {
 		if (unlikely(!arch_has_single_step()))
 			return -EIO;
 		child->ptrace |= PT_SINGLESTEP;
-		user_enable_single_step(child);
-	} else {
-		user_disable_single_step(child);
 	}
 
 	child->exit_code = data;