linux-kernel - Re: [PATCH 2/3] slub: Print raw pointer addresses when debugging

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Date:   Mon, 24 May 2021 13:32:13 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org
Subject: Re: [PATCH 2/3] slub: Print raw pointer addresses when debugging

On Wed 2021-05-19 18:35:38, Stephen Boyd wrote:
> Obscuring the pointers that slub shows when debugging makes for some
> confusing slub debug messages:
> 
>  Padding overwritten. 0x0000000079f0674a-0x000000000d4dce17
> 
> Those addresses are hashed for kernel security reasons. If we're trying
> to be secure with slub_debug on the commandline we have some big
> problems given that we dump whole chunks of kernel memory to the kernel
> logs. Let's use %px here and dump buffers with the actual address for
> the buffer instead of the hashed version so that the logs are
> meaningful. This also helps if a kernel address is in some slub debug
> report so we can figure out that the object is referencing itself.

Please, do not do this!

Use "no_hash_pointers" commandling option when you want to see
raw pointers. It will make it clear when the kernel logs are save
and when not.

If "slub_debug" is useless with hashed pointers then it might enable
"no_hash_pointers". But make sure that it prints the fat warning.

This patch is the worst approach. We have to keep the number of "%px"
callers at minimum to keep it maintainable. The only safe use-case is
when the system is in panic() [*]. If the pointers might be printed
at any time then users should be warned by the fat message printed
by "no_hash_pointers".


[*] Raw pointers are currently printed also by Oops/WARN messages.
    It is from historic reasons. Anyway, they are fat warnings
    on its own. The system often need to get reported anyway.


Best Regards,
Petr