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Date:	Wed, 24 Dec 2014 11:22:38 +0200
From:	Oded Gabbay <oded.gabbay@....com>
To:	Rusty Russell <rusty@...tcorp.com.au>,
	Andi Kleen <ak@...ux.intel.com>,
	Alex Deucher <alexdeucher@...il.com>
CC:	Dana Elifaz <Dana.Elifaz@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Maling list - DRI developers 
	<dri-devel@...ts.freedesktop.org>,
	"Alexander Deucher" <Alexander.Deucher@....com>,
	LKP ML <lkp@...org>
Subject: Re: [LKP] [PATCH] drm/radeon: Try to init amdkfd only if 64 bit kernel



On 12/24/2014 01:01 AM, Rusty Russell wrote:
> Oded Gabbay <oded.gabbay@....com> writes:
>> I didn't say it doesn't always work.
>> The actual thing that doesn't work is the define symbol_get and only in a
>> specific case of 32bit kernel AND CONFIG_MODULES is unset AND
>> CONFIG_RANDOMIZE_BASE is set.
>> The define in that case is:
>> #define symbol_get(x) ({ extern typeof(x) x __attribute__((weak)); &(x); })
>>
>> Why it doesn't work (doesn't return NULL when symbol doesn't exists) ?
>
> Hmm, I'd guess CONFIG_RANDOMIZE_BASE is relocating NULL symbols...
>
> No, I can't reproduce this.  Please send your .config privately.
>
> Here's my test case:
>
> diff --git a/init/main.c b/init/main.c
> index 61b993767db5..a3ee1ec97ec3 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -683,6 +683,12 @@ asmlinkage __visible void __init start_kernel(void)
>
>   	ftrace_init();
>
> +	{
> +		extern void nonexistent_fn(void);
> +		printk("symbol_get(nonexistent_fn) = %p\n",
> +		       symbol_get(nonexistent_fn));
> +	}
> +
>   	/* Do the rest non-__init'ed, we're now alive */
>   	rest_init();
>   }
>
> Thanks,
> Rusty.
>
Hi Rusty,

Attached is the bad config file. (config-bad)
I have narrowed the changes you need to do to the config file in order to 
reproduce this bug.
The base assumption is a 32-bit kernel and without modules support. Rest of the 
config file is pretty standard, IMO.
Then, its not enough to enable CONFIG_RANDOMIZE_BASE like I wrote in my original 
post. You need also to unset CONFIG_HIBERNATION.

If you do only one of the changes above, it isn't sufficient to trigger the problem.

I also attached a config-good file, which is almost the same as config-bad, 
except the changes I mentioned above that reproduces the problem.

To quickly reproduce it, I run the following command:
qemu-system-i386 -enable-kvm -kernel arch/x86/boot/bzImage -initrd 
~/tmp/test32.cpio.gz

The kernel is 3.19-rc1 from Linus' tree. No additional patches on top of that.

I appreciate your help.

	Oded


View attachment "config-bad" of type "text/plain" (84361 bytes)

View attachment "config-good" of type "text/plain" (84426 bytes)

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