[PATCH v2 1/3] tracing: fix syscall_*regfunc() vs copy_process() race

[Oleg Nesterov <oleg@redhat.com>]

syscall_regfunc() and syscall_unregfunc() should set/clear
TIF_SYSCALL_TRACEPOINT system-wide, but do_each_thread() can race
with copy_process() and miss the new child which was not added to
the process/thread lists yet.

Change copy_process() to update the child's TIF_SYSCALL_TRACEPOINT
under tasklist.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 include/trace/syscall.h |   15 +++++++++++++++
 kernel/fork.c           |    2 ++
 2 files changed, 17 insertions(+), 0 deletions(-)

diff --git a/include/trace/syscall.h b/include/trace/syscall.h
index fed853f..291c282 100644
--- a/include/trace/syscall.h
+++ b/include/trace/syscall.h
@@ -4,6 +4,7 @@
 #include <linux/tracepoint.h>
 #include <linux/unistd.h>
 #include <linux/ftrace_event.h>
+#include <linux/thread_info.h>
 
 #include <asm/ptrace.h>
 
@@ -32,4 +33,18 @@ struct syscall_metadata {
 	struct ftrace_event_call *exit_event;
 };
 
+#ifdef CONFIG_TRACEPOINTS
+static inline void syscall_tracepoint_update(struct task_struct *p)
+{
+	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
+		set_tsk_thread_flag(p, TIF_SYSCALL_TRACEPOINT);
+	else
+		clear_tsk_thread_flag(p, TIF_SYSCALL_TRACEPOINT);
+}
+#else
+static inline void syscall_tracepoint_update(struct task_struct *p)
+{
+}
+#endif
+
 #endif /* _TRACE_SYSCALL_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index a17621c..2d55a47 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1484,7 +1484,9 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 
 	total_forks++;
 	spin_unlock(&current->sighand->siglock);
+	syscall_tracepoint_update(p);
 	write_unlock_irq(&tasklist_lock);
+
 	proc_fork_connector(p);
 	cgroup_post_fork(p);
 	if (clone_flags & CLONE_THREAD)
-- 
1.5.5.1