linux-kernel - Re: Absolute symbols in vmlinux

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Date:	Mon, 09 Mar 2009 22:49:21 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Sam Ravnborg <sam@...nborg.org>
Cc:	Yinghai Lu <yinghai@...nel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Absolute symbols in vmlinux_64.lds.S

Sam Ravnborg <sam@...nborg.org> writes:

> On Mon, Mar 09, 2009 at 06:23:55PM -0700, Yinghai Lu wrote:
>> Jeremy Fitzhardinge wrote:
>> > Why does vmlinux_64.lds.S use absolute symbols for things like
>> > __bss_start/stop:
>> > 
>> >  __bss_start = .;        /* BSS */
>> >  .bss : AT(ADDR(.bss) - LOAD_OFFSET) {
>> >     *(.bss.page_aligned)
>> >     *(.bss)
>> >     }
>> >  __bss_stop = .;
>> > 
>> > 
>> > vmlinux_32.lds.S puts __bss_start/stop into the .bss section itself.  Is
>> > there some particular reason they need to be absolute symbols
>> > (relocation?).
>> > 
>> 
>> they are the same.
>
> Thats depends on the value of '.' where you assign __bss_start.
> We have had several bugs where the symbol assinged outside the
> section was less than expected because the linker aling the
> start of the section equal to the lrgest alignment requirement
> of a member in the section.
>
> So in this case if '.' equals to 0xabcd and the lagest
> alignment requirement inside the block is 0x1000 and we have
> __bss_start1 = .;
> .bss : {
> 	__bss_start2 = .;
> 	*(.bss.page_aligned)
> }
>
> Then you would see that:
> __bss_start1 equals 0xabcd
> __bss_start2 equals 0xb000
>
> Which may result in unexpected behaviour.
>
> The case I have in mind prevented the kernel from booting!
> So unless there are specific reasons (which should be documented)
> then always move the assignmnets inside the {} block.

I have no complaint with that.  I believe the symbols are absolute
simply because they were originally coded that way and the relocatable
kernel work on x86_64 didn't need them to change.

Eric
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