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

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Date:   Tue, 19 Jan 2021 19:38:24 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        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 (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.


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