linux-kernel - Re: [RFC patch] vsprintf: Allow %pe to print non PTR

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Date:   Wed, 24 Mar 2021 22:27:41 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Joe Perches <joe@...ches.com>, Arnd Bergmann <arnd@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Philipp Zabel <p.zabel@...gutronix.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Marco Felsch <m.felsch@...gutronix.de>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Liu Ying <victor.liu@....com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC patch] vsprintf: Allow %pe to print non PTR_ERR %pe uses as
 decimal

On 24/03/2021 20.24, Joe Perches wrote:
> On Wed, 2021-03-24 at 18:33 +0100, Rasmus Villemoes wrote:
>> On 24/03/2021 18.20, Joe Perches wrote:
>>
>>>
>>> Maybe it's better to output non PTR_ERR %pe uses as decimal so this
>>> sort of code would work.
>>
>> No, because that would leak the pointer value when somebody has
>> accidentally passed a real kernel pointer to %pe.
> 
> I think it's not really an issue.
> 
> _All_ code that uses %p<foo> extensions need inspection anyway.

There are now a bunch of sanity checks in place that catch e.g. an
ERR_PTR passed to an extension that would derefence the pointer;
enforcing that only ERR_PTRs are passed to %pe (or falling back to %p)
is another of those safeguards.

> It's already possible to intentionally 'leak' the ptr value
> by using %pe, -ptr so I think that's not really an issue.
> 

Huh, what? I assume -ptr is shorthand for (void*)-(unsigned long)ptr.
How would that leak the value if ptr is an ordinary kernel pointer?
That's not an ERR_PTR unless (unsigned long)ptr is < 4095 or so.

If you want to print the pointer value just do %px. No need for silly
games. What I'm talking about is preventing _un_intentionally leaking a
valid kernel pointer value. So no, a non-ERR_PTR passed to %pe is not
going to be printed as-is, not in decimal or hexadecimal or roman numerals.

>> If the code wants a cute -EFOO string explaining what's wrong, what
>> about "%pe", ERR_PTR(mux < 0 : mux : -ERANGE)? Or two separate error
>> messages
>>
>> if (mux < 0)
>>   ...
>> else if (mux >= ARRAY_SIZE())
>>   ...
> 
> Multiple tests, more unnecessary code, multiple format strings, etc...

Agreed, I'm not really advocating for the latter; the former suggestion
is IMO a pretty concise way of providing useful information in dmesg.

Rasmus