ptrace: PTRACE_DETACH should do flush_ptrace_hw_breakpoint(child)

Change ptrace_detach() to call flush_ptrace_hw_breakpoint(child).  This
frees the slots for non-ptrace PERF_TYPE_BREAKPOINT users, and this
ensures that the tracee won't be killed by SIGTRAP triggered by the
active breakpoints.

Test-case:

	unsigned long encode_dr7(int drnum, int enable, unsigned int type, unsigned int len)
	{
		unsigned long dr7;

		dr7 = ((len | type) & 0xf)
			<< (DR_CONTROL_SHIFT + drnum * DR_CONTROL_SIZE);
		if (enable)
			dr7 |= (DR_GLOBAL_ENABLE << (drnum * DR_ENABLE_SIZE));

		return dr7;
	}

	int write_dr(int pid, int dr, unsigned long val)
	{
		return ptrace(PTRACE_POKEUSER, pid,
				offsetof (struct user, u_debugreg[dr]),
				val);
	}

	void func(void)
	{
	}

	int main(void)
	{
		int pid, stat;
		unsigned long dr7;

		pid = fork();
		if (!pid) {
			assert(ptrace(PTRACE_TRACEME, 0,0,0) == 0);
			kill(getpid(), SIGHUP);

			func();
			return 0x13;
		}

		assert(pid == waitpid(-1, &stat, 0));
		assert(WSTOPSIG(stat) == SIGHUP);

		assert(write_dr(pid, 0, (long)func) == 0);
		dr7 = encode_dr7(0, 1, DR_RW_EXECUTE, DR_LEN_1);
		assert(write_dr(pid, 7, dr7) == 0);

		assert(ptrace(PTRACE_DETACH, pid, 0,0) == 0);
		assert(pid == waitpid(-1, &stat, 0));
		assert(stat == 0x1300);

		return 0;
	}

Before this patch the child is killed after PTRACE_DETACH.

Signed-off-by: Oleg Nesterov <[email protected]>
Acked-by: Frederic Weisbecker <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jan Kratochvil <[email protected]>
Cc: Michael Neuling <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Paul Mundt <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Prasad <[email protected]>
Cc: Russell King <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

kernel/ptrace.c

@@ -469,6 +469,7 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
 	/* Architecture-specific hardware disable .. */
 	ptrace_disable(child);
 	clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
+	flush_ptrace_hw_breakpoint(child);
 
 	write_lock_irq(&tasklist_lock);
 	/*