linux-kernel - Re: UML fails to locate address space

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Message-ID: <Pine.LNX.4.61.0805201117540.12360@chaos.analogic.com>
Date:	Tue, 20 May 2008 11:22:08 -0400
From:	"linux-os (Dick Johnson)" <linux-os@...logic.com>
To:	"Jeff Dike" <jdike@...toit.com>
Cc:	"Tom Spink" <tspink@...il.com>,
	"LKML" <linux-kernel@...r.kernel.org>
Subject: Re: UML fails to locate address space

On Tue, 20 May 2008, Jeff Dike wrote:

> On Tue, May 20, 2008 at 12:08:24PM +0100, Tom Spink wrote:
>> I've just recently pulled the latest GIT and compiled UML, however,
>> when I run it a message appears saying "Locating the top of the
>> address space... Address 0x0 no good?" and the program exits.
>
> Can you strace it and send me the output?
>
>> After some digging, it appears that page_ok is returning false when
>> checking 'bottom', in os_get_task_size
>> (arch/um/os-Linux/sys-i386/task_size.c:96).  After running through
>> GDB, it seems that in line 31 of that file is where the segfault
>> occurs:
>>
>> n = *address;
>>
>> i.e. when trying to read from the address space (at address zero).
>
> The segfaults are on purpose - it will trap SIGSEGV and longjmp out of
> the handler and mark the affected address as not-usable.
>
>    	       	  Jeff

Does this mean that I cannot mmap() address zero in the kernel
anymore? I should be able (in the kernel) read anything, and
then mmap() it to something usable in user-space. If address
zero is now artifically trapped there are problems being created
for no good reason at all.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.22.1 on an i686 machine (5588.29 BogoMips).
My book : http://www.AbominableFirebug.com/
_

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