Query on linker scripts

-----Original Message-----
From: Vaibhav Jain [mailto:vjoss197 at gmail.com]
Sent: Sunday, March 25, 2012 3:19 AM
To: Pranay Kumar Srivastava
Subject: Re: Query on linker scripts


Hi Pranay,
Thanks for replying!. I am still not clear about this as I have not
reached
the part of the tutorial which talks about pte and pgd. Could you
please explain this?point about safety of section?with a simpler
example?

I'll take example from your script.
.bss :
{
          sbss = .;
          *(COMMON)
          *(.bss)
          ebss = .;
}

What I wanted to say was instead of taking ebss within .bss section you should take it outside that section. You might need to do ABSOLUTE since . will give you relative values but you'd want absolute values since addresses that you are interested in will begin from the entry point not from a section. So you can try something like this

.bss ALIGN(4096):
{
          sbss = .;
          *(COMMON)
          *(.bss)
}
ebss = ABSOLUTE(.); /*This should be a page aligned address*/



 Also, from your reply I figured out that it is not compulsory

to define such symbols and the names can be different than sbss and
ebss. Am I right ?

The names can be anything it's your choice entirely. But being descriptive helps. You should have a close look at the redhat tutorial for linker scripts instead of following someone else's linker script since you might not require all the variables chosen or you might need some additional variables due to the design you've chosen for your kernel.

http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/simple-example.html 

Thanks
Vaibhav Jain

Please include kernelnewbies at kernelnewbies.org in cc when replying. You are likely to get more responses that way.

On Sat, Mar 24, 2012 at 11:45 AM, Pranay Kumar Srivastava
[off-list ref] wrote:
On 03/24/2012 11:52 PM, Pranay Kumar Srivastava wrote:

quoted

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quoted

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Today's Topics:

? ? 1. Query on linker scripts (Vaibhav Jain)
? ? 2. Re: Query on linker scripts (Carlo Caione)


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quoted

Message: 1
Date: Fri, 23 Mar 2012 21:43:40 -0700
From: Vaibhav Jain<redacted>
Subject: Query on linker scripts
To: kernelnewbies at kernelnewbies.org
Message-ID:

?[ref]

quoted

Content-Type: text/plain; charset="iso-8859-1"

Hi,

Recently I have started reading tutorials for writing a small kernel.

All

quoted

such tutorials mention use of linker scripts. I have
read few articles on linker scritps but I am stuck on one thing. I am
unable to understand the use of defining new symbols in linker

scripts.

quoted

Using a linker script to arrange different sections in the object

file is

quoted

understandable but defining symbols which are not referenced anywhere

in

quoted

the script
is confusing. An example is the use of symbols sbss and ebss in the

bss

quoted

section as show in the script below


ENTRY (loader)
SECTIONS
{
? ? ?. = 0x00100000;
? ? ?.text ALIGN (0x1000) :
? ? ?{
? ? ? ? ?*(.text)
? ? ?}
? ? ?.rodata ALIGN (0x1000) :
? ? ?{
? ? ? ? ?*(.rodata*)
? ? ?}
? ? ?.data ALIGN (0x1000) :
? ? ?{
? ? ? ? ?*(.data)
? ? ?}
? ? ?.bss :
? ? ?{
? ? ? ? ?sbss = .;
? ? ? ? ?*(COMMON)
? ? ? ? ?*(.bss)
? ? ? ? ?ebss = .;
? ? ?}
}

Please explain how defining such symbols is useful.

Thanks
Vaibhav Jain
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Message: 2
Date: Sat, 24 Mar 2012 16:26:38 +0100
From: Carlo Caione<redacted>
Subject: Re: Query on linker scripts
To: Vaibhav Jain<redacted>
Cc: kernelnewbies at kernelnewbies.org
Message-ID:[ref]
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

On 24/03/2012 05:43, Vaibhav Jain wrote:

quoted

Hi,

[cut]

quoted

is confusing. An example is the use of symbols sbss and ebss in the

bss

quoted

quoted

section as show in the script below
ENTRY (loader)
SECTIONS
{
? ? ? . = 0x00100000;
? ? ? .text ALIGN (0x1000) :
? ? ? {
? ? ? ? ? *(.text)
? ? ? }
? ? ? .rodata ALIGN (0x1000) :
? ? ? {
? ? ? ? ? *(.rodata*)
? ? ? }
? ? ? .data ALIGN (0x1000) :
? ? ? {
? ? ? ? ? *(.data)
? ? ? }
? ? ? .bss :
? ? ? {
? ? ? ? ? sbss = .;

The sbss will tell you the start of the section bss.

quoted

quoted

? ? ? ? ? *(COMMON)
? ? ? ? ? *(.bss)
? ? ? ? ? ebss = .;

The ebss will tell you the end of the section bss. The use of these
symbols is since you'd like to have the kernel's section be safe from
every other process.

This way you can know where your kernel code starts and ends. So you
can set up the pages( ptes and pgds as well) in a sensible manner for
your kernel.

So in the above case you'd have like two variables in your C code like
extern long sbss,ebss and then to get the location where the bss begins
you'd do &sbss while to get its ending address you'd do &ebss. So when
you subtract these two that should give you the size of your bss
section.

However you should do ebss=. after your .bss section and make it
ALIGN(0x1000) like others so you get page aligned section values cuz it
makes easier to arrange for the pte and pgd for kernel. Also you should
set the GDT values for kernel only code separately accordingly from the
values you get after an initial temporary GDT has been setup earlier by
GRUB or by you.

More appropriate would be to get the size of text and data sections as
well since you wouldn't want to accidentally bump into kernel code.

quoted

quoted

? ? ? }
}

I'm not sure if you are OT, anyway...

i.e. they are useful if you want to clear the bss section before

execution.

quoted

[snippet for ARM proc]
...
? ? ? ? ?ldr ? ? r2, =_sbss
? ? ? ? ?b ? ? ? LoopFillZerobss
/* Zero fill the bss segment. */
FillZerobss:
? ? ? ? ?movs ? ?r3, #0
? ? ? ? ?str ? ? r3, [r2], #4

LoopFillZerobss:
? ? ? ? ?ldr ? ? r3, = _ebss
? ? ? ? ?cmp ? ? r2, r3
? ? ? ? ?bcc ? ? FillZerobss
...

--
Carlo Caione



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