linux-kernel - Re: Issue with 'lib/vsprintf.c: don't try to fix pointer wrap-around'

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Message-Id: <20150116122404.e82eb86b5c46afdd0d00df1d@linux-foundation.org>
Date:	Fri, 16 Jan 2015 12:24:04 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	linux-kernel@...r.kernel.org
Subject: Re: Issue with 'lib/vsprintf.c: don't try to fix pointer
 wrap-around'

On Fri, 16 Jan 2015 11:23:57 -0500 Johannes Weiner <hannes@...xchg.org> wrote:

> Hi Rasmus,
> 
> I have trouble booting my test machine with this patch in -mm:
> 
> commit bb2e066c6943e62e9650bb129f416dacf138f8b1
> Author: Rasmus Villemoes <linux@...musvillemoes.dk>
> Date:   Wed Jan 14 01:00:44 2015 +0000
> 
>     lib/vsprintf.c: don't try to fix pointer wrap-around
>     
>     Actual kernel buffers can't wrap into the user address space.  If someone
>     manages to pass a buf/size combination that wraps, it is most likely due
>     to a bug in the caller.  Instead of trying to fix it by using a smaller
>     part of the buffer, bail out.
>     
>     Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
>     Cc: Jiri Kosina <jkosina@...e.cz>
>     Cc: Randy Dunlap <rdunlap@...radead.org>
>     Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> 
> After I get "Loading bzImage-new... ok" from the bootloader, the
> serial console remains quiet.
> 
> A WARN_ON_ONCE() inside vsnprintf() looks like it would deadlock
> instantly when triggering this overflow from printk(), no?

Dammit, I was starting at that printk, ended up deciding it was OK,
didn't think about deadlocks.  logbuf_lock and recursion_bug, for a
start...

I'll drop the patch.  I assume all this means that your machine is
trying to trigger that warning condition?  I wonder what the call
site is.

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