Commit e3492872 authored Jan 24, 2020 by Vasily Averin Committed by Greg Kroah-Hartman Feb 24, 2020

ftrace: fpid_next() should increase position index

[ 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>

parent 8a7bfa3d

kernel/trace/ftrace.c

+3 −2

Original line number	Diff line number	Diff line
		@@ -6537,9 +6537,10 @@ 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);
		}