linux-kernel - Re: [PATCH v3] Add kernel config option for tweaking kernel behavior.

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Message-ID: <41a49d42-7119-62b9-085b-aa99cadc4dd1@i-love.sakura.ne.jp>
Date:   Sun, 19 Apr 2020 00:08:13 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Arnd Bergmann <arnd@...db.de>, Jiri Slaby <jslaby@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v3] Add kernel config option for tweaking kernel behavior.

On 2020/04/18 23:39, Greg Kroah-Hartman wrote:
>> Well, we could add some more text (like shown below), but this option itself
>> is neutral. This option is not limiting the target to fuzzers.
>> Below 3 patches are an example of "a set of fine-grained configs" with
>> "some umbrella uber-config" approach. Linus, are you OK with this approach?
> 
> Note, the word "tweak" is usually used where people want to get the most
> performance out of a system, while here you are using it to remove
> functionality out of the system.  You might want to pick a different
> word, naming is hard :(

Then, what about "twist" ?

>> @@ -633,6 +633,8 @@ static void k_spec(struct vc_data *vc, unsigned char value, char up_flag)
>>  	     kbd->kbdmode == VC_OFF) &&
>>  	     value != KVAL(K_SAK))
>>  		return;		/* SAK is allowed even in raw mode */
>> +	if (IS_ENABLED(CONFIG_TWEAK_DISABLE_KBD_K_SPEC_HANDLER))
>> +		return;
> 
> Are you sure this is correct?  It seems like you just cut off a number
> of other keyboard function handlers instead of just the ctrl-alt-del
> functionality.  Did you really want to do that?

Maybe someday we update this filter more fine-grained. But I think this
granularity is what syzkaller wants for now. The ctrl-alt-del ( k_spec()
=> fn_boot_it() => ctrl_alt_del()) is simply one of them. There was RCU
stall report caused by repeating SysRq-t from this path ( k_spec() =>
fn_show_state() => show_state()) due to too much printk() calls.