linux-kernel - Re: [PATCH] vsprintf: Do not break early boot with probing addresses

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Date:   Tue, 14 May 2019 11:07:30 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        David Laight <David.Laight@...lab.com>,
        'christophe leroy' <christophe.leroy@....fr>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "Tobin C . Harding" <me@...in.cc>, Michal Hocko <mhocko@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Russell Currey <ruscur@...sell.cc>,
        Stephen Rothwell <sfr@...abs.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH] vsprintf: Do not break early boot with probing addresses

On (05/13/19 14:42), Petr Mladek wrote:
> > The "(null)" is good enough by itself and already an established
> > practice..
> 
> (efault) made more sense with the probe_kernel_read() that
> checked wide range of addresses. Well, I still think that
> it makes sense to distinguish a pure NULL. And it still
> used also for IS_ERR_VALUE().

Wouldn't anything within first PAGE_SIZE bytes be reported as
a NULL deref?

       char *p = (char *)(PAGE_SIZE - 2);
       *p = 'a';

gives

 kernel: BUG: kernel NULL pointer dereference, address = 0000000000000ffe
 kernel: #PF: supervisor-privileged write access from kernel code
 kernel: #PF: error_code(0x0002) - not-present page


And I like Steven's "(fault)" idea.
How about this:

	if ptr < PAGE_SIZE		-> "(null)"
	if IS_ERR_VALUE(ptr)		-> "(fault)"

	-ss