linux-kernel - Re: [PATCH] reduce export symbol CRC table size on 64-bit archs

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Date:	Mon, 06 Jul 2009 08:42:51 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Rusty Russell" <rusty@...tcorp.com.au>,
	"Michal Marek" <mmarek@...e.cz>
Cc:	"Ingo Molnar" <mingo@...e.hu>, <tony.luck@...el.com>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	<linux-arch@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-modules@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH] reduce export symbol CRC table size on 64-bit
	 archs

>>> Michal Marek <mmarek@...e.cz> 03.07.09 22:16 >>>
>Jan Beulich napsal(a):
>> Since these CRCs are really only 32-bit quantities, there's no need to
>> store them in 64-bit slots. Since, however, gcc doesn't allow
>> respective initializations, asm() constructs get used to create the CRC
>> tables (and its for that reason that the patch only makes x86-64 and
>> ia64 utilize that functionality, as I can't verify this doesn't break
>> in some subtle way elsewhere).
>> 
>...
>>  struct modversion_info
>>  {
>> -	unsigned long crc;
>> +	ksym_crc_t crc;
>>  	char name[MODULE_NAME_LEN];
>>  };
>
>This change breaks module-init-tools:
>Before:
>$ /sbin/modprobe --dump-modversions _build/drivers/usb/core/usbcore.ko
>| head
>0xb49b735a      module_layout
>0xdb7e6a70      bus_register
>...
>After:
>$ /sbin/modprobe --dump-modversions
>_build-crc-int/drivers/usb/core/usbcore.ko  | head
>0x75646f6d91ea7b5c      le_layout
>0x5f7375623e215f43      register
>...
>It also breaks the newly added depmod -E option (check symbol versions),
>which also reads the struct modversion_info array (*). Is it possible
>name the section differently (__versions2?) on those architectures where
>the size changes, so that it is possible to fix m-i-t in a
>backwards-compatible manner?

First of all I'd view it as a design bug if user mode code assumptions prevent
changes to the kernel.

But taking this as an uncorrectable fact, I'd think that renaming the section
would certainly be an option (though I'm unsure whether that would have
other consequences - Rusty?), however I could also imagine other means to
communicate to user land the width of a CRC value (e.g. adding an absolute
symbol during the .ko linking stage).

Jan

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