[PATCH] tracing: Fix missing errno when zero parser->idx in trace_pid_write

[Pu Lehui]

From: Pu Lehui <pulehui@xxxxxxxxxx>

When trace_get_user in trace_pid_write parses an only space, the
!trace_parser_loaded branch will break with no errno, causing
tr->filtered_pids to still be assigned with pid_list, which may trigger
potential problems.

This patch will also silence the fault injection syzkaller warning in
tracepoint_add_func [0]. We can reproduce the warning by following the
steps below:
1. echo 8 >> set_event_notrace_pid. Let tr->filtered_pids owns one pid
   and register sched_switch tracepoint.
2. echo ' ' >> set_event_pid, and perform fault injection during chunk
   allocation of trace_pid_list_alloc. Let pid_list with no pid and
assign to tr->filtered_pids.
3. echo ' ' >> set_event_pid. Let pid_list is NULL and assign to
   tr->filtered_pids.
4. echo 9 >> set_event_pid, will trigger the double register
   sched_switch tracepoint warning.

Link: https://lore.kernel.org/all/67cb890e.050a0220.d8275.022e.GAE@xxxxxxxxxx [0]
Fixes: b27f266f74fb ("tracing: Fix return value of trace_pid_write()")
Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
---
 kernel/trace/trace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 8d8935ed416d..feeb7eb71318 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -853,10 +853,10 @@ int trace_pid_write(struct trace_pid_list *filtered_pids,
 		ubuf += ret;
 		cnt -= ret;
 
+		ret = -EINVAL;
 		if (!trace_parser_loaded(&parser))
 			break;
 
-		ret = -EINVAL;
 		if (kstrtoul(parser.buffer, 0, &val))
 			break;
 
-- 
2.34.1