linux-kernel - Re: [PATCH] kconfig: Add kernel config option for fuzz testing.

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Date:   Tue, 17 Dec 2019 09:38:41 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kconfig: Add kernel config option for fuzz testing.

On Tue, Dec 17, 2019 at 9:24 AM Sergey Senozhatsky
<sergey.senozhatsky.work@...il.com> wrote:
>
> On (19/12/17 08:54), Dmitry Vyukov wrote:
> > On Tue, Dec 17, 2019 at 6:12 AM Sergey Senozhatsky
> > <sergey.senozhatsky.work@...il.com> wrote:
> > >
> > > On (19/12/16 18:59), Tetsuo Handa wrote:
> > >
> > > Can you fuzz test with `ignore_loglevel'?
> >
> > We can set ignore_loglevel in syzbot configs, but won't it then print
> > everything including verbose debug output?
>
> What would be the most active source of debug output?
>
> dev_dbg(), which is dev_printk(KERN_DEBUG), can be compiled out, if I'm
> not mistaken. It probably depends on CONFIG_DEBUG or something similar,
> unlike dev_printk(), which depends on CONFIG_PRINTK. It seems that we have
> significantly more dev_dbg() users, than direct dev_printk(KERN_DEBUG).
>
> Does fuzz tester hit pr_debug() often? File systems? Some of them
> have ways to compile out debugging output as well. E.g. jbd_debug,
> _debug, ext4_debug, and so on.

As far as I remember it produces an infinite amount of output on debug
level. Even for normal level it produces much more than one would
normally expect as it does random things all over the kernel with
maximum frequency.