From: Arnd Bergmann <arnd@arndb.de>
To: monstr@monstr.eu
Subject: Re: [PATCH 20/30] microblaze_mmu_v1: uaccess MMU update
Date: Wed, 29 Apr 2009 17:55:37 +0200
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Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
       john.williams@petalogix.com
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On Wednesday 29 April 2009, Michal Simek wrote:
> For powerpc I found these tree references. Where is the __get_user_bad definition?

> arch/powerpc/include/asm/uaccess.h:216:extern long __get_user_bad(void);
> arch/powerpc/include/asm/uaccess.h:263: ? ? ? ? (x) = __get_user_bad(); ? ? ? ? ? ? ? ? ? ? ? ? \
> arch/powerpc/include/asm/uaccess.h:269: default: (x) = __get_user_bad(); ? ? ? ? ? ? ? ? ? ? ? ?\
> 

As I tried to explain, the whole point is that there is no definition
at all, just a declaration.
Normally, the compiler will discard the 'default:' case of get_user,
so the function is not called. However, if you pass a variable of
a length other than 1, 2, or 4, the compiler will generate a function
call to __get_user_bad in the object file. When you try to link
the kernel afterwards, you will get an error message like

Unknown symbol __get_user_bad" for drivers/foo/bar.o

which tells you that some function in drivers/foo/bar.c calls
get_user with an invalid argument.

	Arnd <><
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