linux-kernel - Re: Absolute symbols in vmlinux

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Date:	Mon, 9 Mar 2009 22:57:19 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Sam Ravnborg <sam@...nborg.org>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>, Ingo Molnar <mingo@...e.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"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

On Mon, Mar 9, 2009 at 10:37 PM, Sam Ravnborg <sam@...nborg.org> wrote:
> 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

good to know...

anyway, more lines

  . = ALIGN(PAGE_SIZE);
  __nosave_begin = .;
  .data_nosave : AT(ADDR(.data_nosave) - LOAD_OFFSET) {
      *(.data.nosave)
  } :data.init2 /* use another section data.init2, see PERCPU_VADDR() above */
  . = ALIGN(PAGE_SIZE);
  __nosave_end = .;

  __bss_start = .;              /* BSS */
  .bss : AT(ADDR(.bss) - LOAD_OFFSET) {
        *(.bss.page_aligned)
        *(.bss)
        }
  __bss_stop = .;

  _end = . ;


there are extra ALIGN(PAGE_SIZE) between them....

YH
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