[PATCH] tracing: Allow raw_syscall tracepoints to work from boot

[Michael Ellerman <mpe@ellerman.id.au>]

In commit 5f893b2639b2 "tracing: Move enabling tracepoints to just after
rcu_init()", tracing was enabled earlier in boot.

This broke tracing of the raw_syscall tracepoints from boot using the
trace_event kernel parameter.

When the registration function for the raw_syscall tracepoints runs, it
iterates over all tasks setting TIF_SYSCALL_TRACEPOINT. However now that
this happens earlier in boot, the loop has no effect because there are
no tasks in existence other than init_task, which is skipped by the
for_each_process_thread() macro.

We can fix it by explicitly setting TIF_SYSCALL_TRACEPOINT for the
init_task. That way when pid 1 is cloned from init_task it will inherit
TIF_SYSCALL_TRACEPOINT.

Fixes: 5f893b2639b2 ("tracing: Move enabling tracepoints to just after rcu_init()")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 kernel/tracepoint.c | 6 ++++++
 1 file changed, 6 insertions(+)


It feels a bit naughty to be whacking init_task like this, but it also
seems like the right fix?

Should we also clear it in the unregfunc? I can't see how that would
ever be needed in practice?


diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 3490407dc7b7..43a03e22c16c 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -496,6 +496,12 @@ void syscall_regfunc(void)
 
 	if (!sys_tracepoint_refcount) {
 		read_lock(&tasklist_lock);
+		/*
+		 * If we run very early in boot then we need to hit init_task
+		 * directly so that pid 1 will inherit the TIF flag.
+		 */
+		set_tsk_thread_flag(&init_task, TIF_SYSCALL_TRACEPOINT);
+
 		for_each_process_thread(p, t) {
 			set_tsk_thread_flag(t, TIF_SYSCALL_TRACEPOINT);
 		}
-- 
2.1.0