linux-kernel - Re: [PATCH 0/2] introduce DUMP_PREFIX

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Date:   Tue, 26 Jan 2021 17:47:33 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Timur Tabi <timur@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
        Petr Mladek <pmladek@...e.com>, roman.fietze@...na.com,
        keescook@...omium.org, Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>, linux-mm@...ck.org,
        Akinobu Mita <akinobu.mita@...il.com>
Subject: Re: [PATCH 0/2] introduce DUMP_PREFIX_UNHASHED for hex dumps

On 1/19/21 11:38 AM, Sergey Senozhatsky wrote:
> On (21/01/19 01:47), Matthew Wilcox wrote:
> [..]
>> 
>> > So maybe DUMP_PREFIX_UNHASHED can do the unhashed dump only when
>> > CONFIG_DEBUG_KERNEL=y and fallback to DUMP_PREFIX_ADDRESS otherwise?
>> 
>> Distros enable CONFIG_DEBUG_KERNEL.
> 
> Oh, I see.
> 
>> If you want to add CONFIG_DEBUG_LEAK_ADDRESSES, then that's great,
>> and you won't even have to change users, you can just change how %p
>> behaves.
> 
> I like the name. config dependent behaviour of %p wouldn't be new,
> well, to some extent, e.g. XFS does something similar (see below).
> I don't think Linus will be sold on this, however.

Given Linus' current stance later in this thread, could we revive the idea of a
boot time option, or at least a CONFIG (I assume a runtime toggle would be too
much, even if limited to !kernel_lockdown :) , that would disable all hashing?
It would be really useful for a development/active debugging, as evidenced
below. Thanks.

> fs/xfs/xfs_linux.h:
> 
> /*
>  * Starting in Linux 4.15, the %p (raw pointer value) printk modifier
>  * prints a hashed version of the pointer to avoid leaking kernel
>  * pointers into dmesg.  If we're trying to debug the kernel we want the
>  * raw values, so override this behavior as best we can.
>  */
> #ifdef DEBUG
> # define PTR_FMT "%px"
> #else
> # define PTR_FMT "%p"
> #endif
> 
> And then they just use it as
> 
> 	xfs_alert(mp, "%s: bad inode magic number, dip = "ptr_fmt",
> 		  dino bp = "ptr_fmt", ino = %ld",
> 		  __func__, dip, bp, in_f->ilf_ino);
> 
> 	-ss
>