linux-kernel - Re: printk feature for syzbot?

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Date:   Fri, 11 May 2018 15:21:51 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        pmladek@...e.com, dvyukov@...gle.com, sergey.senozhatsky@...il.com,
        syzkaller@...glegroups.com, rostedt@...dmis.org,
        fengguang.wu@...el.com, linux-kernel@...r.kernel.org
Subject: Re: printk feature for syzbot?

On (05/11/18 11:38), Tetsuo Handa wrote:
> > 
> > So you basically want to have one more con_msg_format_flags? Do
> > you want to track a context which prints out a messages or the
> > context which "generated" the message? A CPU/task that stores
> > a logbuf entry - vprintk_emit() - is not always the same as the
> > CPU/task that prints it to consoles - console_unlock().
> > 
> 
> Well, below is the (partial) patch.

Hi,

Tetsuo, I will take a look a bit later, but at glance, there are several
ways to achieve what you are trying to do. The first one is the way you
did it - add additional buffer and make that context tracking info part of
the message body. Another one would be to extend struct printk_log and add
pid/cpu/flag there, which you then can convert into text in msg_print_text().
So far we talked about extending printk_log. Yet another one could be - add
vsprintf specifiers that would add pid/cpu/flag to the vsprintf-ed message.
You then can re-define pr_fmt, for instance, in the code you want to track
pr_fmt "%zZ" fmt, or somehow force printk to add that "%zZ" to every
message.

> By the way, when I tried to make similar change for printk_safe_log_store(),
> I noticed that printk_safe_log_store() is not safe because it is reusing
> the va_list variable after "goto again;". We need to use va_copy(), or
> we will get crash like an example shown below.

Oh, right. Can you send a patch?

	-ss