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Date:	Mon, 05 Jan 2015 14:58:54 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Oded Gabbay <oded.gabbay@....com>, Andi Kleen <ak@...ux.intel.com>,
	Alex Deucher <alexdeucher@...il.com>,
	Kees Cook <keescook@...omium.org>,
	"H. Peter Anvin" <hpa@...or.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

Oded Gabbay <oded.gabbay@....com> writes:
> 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.

Indeed, thanks; your config breaks as reported.  With CONFIG_HIBERNATION
the kernel offset is 0, so we don't see this.

Kees, as far as I can tell you need another 0-terminated vmlinux.relocs
section for weak symbols.  These should not be relocated if already 0.

Put this somewhere to test.  It fails for x86_64, too:

diff --git a/init/main.c b/init/main.c
index 61b993767db5..c9e0195c792a 100644
--- a/init/main.c
+++ b/init/main.c
@@ -683,6 +683,12 @@ asmlinkage __visible void __init start_kernel(void)
 
 	ftrace_init();
 
+	{
+		extern void __attribute__((weak)) nonexistent_fn(void);
+		printk("nonexistent_fn = %p\n", nonexistent_fn);
+		BUG_ON(nonexistent_fn != NULL);
+	}
+
 	/* Do the rest non-__init'ed, we're now alive */
 	rest_init();
 }

Cheers,
Rusty.
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