releases/5.5.6/ftrace-fpid_next-should-increase-position-index.patch

From a030596c7a0f673d89f3947d56bded3aeabe1ec7 Mon Sep 17 00:00:00 2001
From: Sasha Levin <sashal@kernel.org>
Date: Fri, 24 Jan 2020 10:02:56 +0300
Subject: ftrace: fpid_next() should increase position index

From: Vasily Averin <vvs@virtuozzo.com>

[ Upstream commit e4075e8bdffd93a9b6d6e1d52fabedceeca5a91b ]

if seq_file .next fuction does not change position index,
read after some lseek can generate unexpected output.

Without patch:
 # dd bs=4 skip=1 if=/sys/kernel/tracing/set_ftrace_pid
 dd: /sys/kernel/tracing/set_ftrace_pid: cannot skip to specified offset
 id
 no pid
 2+1 records in
 2+1 records out
 10 bytes copied, 0.000213285 s, 46.9 kB/s

Notice the "id" followed by "no pid".

With the patch:
 # dd bs=4 skip=1 if=/sys/kernel/tracing/set_ftrace_pid
 dd: /sys/kernel/tracing/set_ftrace_pid: cannot skip to specified offset
 id
 0+1 records in
 0+1 records out
 3 bytes copied, 0.000202112 s, 14.8 kB/s

Notice that it only prints "id" and not the "no pid" afterward.

Link: http://lkml.kernel.org/r/4f87c6ad-f114-30bb-8506-c32274ce2992@virtuozzo.com

https://bugzilla.kernel.org/show_bug.cgi?id=206283
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/trace/ftrace.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 3581bd96d6eb3..ddb47a0af854b 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -7038,9 +7038,10 @@ static void *fpid_next(struct seq_file *m, void *v, loff_t *pos)
 	struct trace_array *tr = m->private;
 	struct trace_pid_list *pid_list = rcu_dereference_sched(tr->function_pids);
 
-	if (v == FTRACE_NO_PIDS)
+	if (v == FTRACE_NO_PIDS) {
+		(*pos)++;
 		return NULL;
-
+	}
 	return trace_pid_next(pid_list, v, pos);
 }
 
-- 
2.20.1