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>

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 71a40e5c3a9f0d253406bead7b4848b1b173cd52..2ae98f8bce81ba76df227acf5d9ebfcd4bf0ff2e 100644 (file)
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5455,9 +5455,10 @@ 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);
 }