linux-kernel - Re: [PATCH 0/3][RESEND] add support for never printing hashed addresses

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Message-ID: <cd421230-f6cd-fb79-64ad-d029bcee7b4e@i-love.sakura.ne.jp>
Date:   Thu, 11 Feb 2021 02:07:21 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Timur Tabi <timur@...nel.org>, Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        akpm@...ux-foundation.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        roman.fietze@...na.com, Kees Cook <keescook@...omium.org>,
        John Ogness <john.ogness@...utronix.de>,
        akinobu.mita@...il.com, glider@...gle.com,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 0/3][RESEND] add support for never printing hashed
 addresses

On 2021/02/11 1:46, Steven Rostedt wrote:
> On Thu, 11 Feb 2021 01:39:41 +0900
> Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp> wrote:
> 
>> On 2021/02/11 1:18, Steven Rostedt wrote:
>>> The point of this exercise is to be able to debug the *same* kernel that
>>> someone is having issues with. And this is to facilitate that debugging.  
>>
>> That's too difficult to use. If a problem is not reproducible, we will have
>> no choice but always specify "never hash pointers" command line option. If a
>> problem is reproducible, we can rebuild that kernel with "never hash pointers"
>> config option turned on.
> 
> Now the question is, why do you need the unhashed pointer?

Because unhashed pointers might give some clue. We can rebuild the same kernel
using the same kernel config / compiler etc. and compare unhashed pointers with
addresses in System.map / kallsyms files without reproducing the problem.

> 
> Currently, the instruction pointer is what is fine right? You get the
> a function and its offset. If there's something that is needed, perhaps we
> should look at how to fix that, instead of just unhashing all pointers by
> default.

I'm not refusing to use kernel command line options. I'm expecting that we can
also hardcode using kernel config options. Since boot-time switching via kernel
command line options makes the kernel behave differently, less boot-time
switching is better for avoiding unexpected problems (e.g. unintended LSM was
enabled).