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ftrace: fpid_next() should increase position index

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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>
This commit is contained in:
Vasily Averin 2020-01-24 10:02:56 +03:00 committed by Steven Rostedt (VMware)
parent 64ae572bc7
commit e4075e8bdf
1 changed files with 3 additions and 2 deletions
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5
kernel/trace/ftrace.c
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@ -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);
}