linux-kernel - [for-next][PATCH 04/21] ftrace: fpid_next() should increase position index

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Date:   Thu, 30 Jan 2020 09:47:47 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org
Cc:     Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vasily Averin <vvs@...tuozzo.com>
Subject: [for-next][PATCH 04/21] ftrace: fpid_next() should increase position index

From: Vasily Averin <vvs@...tuozzo.com>

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@...tuozzo.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.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 fdb1a9532420..0e9612c30995 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -7026,9 +7026,10 @@ 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);
 }
 
-- 
2.24.1