Query on linker scripts

* Query on linker scripts
@ 2012-03-24 18:45 Pranay Kumar Srivastava
       [not found] ` <CAKuUYSwPiuGTa+8r6O+-GMLu9e6dXOXNfDHu4UBx2yvrHcQyvw@mail.gmail.com>
  0 siblings, 1 reply; 9+ messages in thread
From: Pranay Kumar Srivastava @ 2012-03-24 18:45 UTC (permalink / raw)
  To: kernelnewbies

On 03/24/2012 11:52 PM, Pranay Kumar Srivastava wrote:
>
> ________________________________________
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> Sent: Saturday, March 24, 2012 9:30 PM
> To: kernelnewbies at kernelnewbies.org
> Subject: Kernelnewbies Digest, Vol 16, Issue 29
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> Today's Topics:
>
>     1. Query on linker scripts (Vaibhav Jain)
>     2. Re: Query on linker scripts (Carlo Caione)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 23 Mar 2012 21:43:40 -0700
> From: Vaibhav Jain<vjoss197@gmail.com>
> Subject: Query on linker scripts
> To: kernelnewbies at kernelnewbies.org
> Message-ID:
>          <CAKuUYSw=_zZykPWeTbJsGEYPPSroWK+whm0o5L_PnCManVcrng@mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> Hi,
>
> Recently I have started reading tutorials for writing a small kernel. All
> 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.
> Using a linker script to arrange different sections in the object file is
> understandable but defining symbols which are not referenced anywhere in
> the script
> is confusing. An example is the use of symbols sbss and ebss in the bss
> 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<carlo.caione@gmail.com>
> Subject: Re: Query on linker scripts
> To: Vaibhav Jain<vjoss197@gmail.com>
> Cc: kernelnewbies at kernelnewbies.org
> Message-ID:<4F6DE7AE.9070808@gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> On 24/03/2012 05:43, Vaibhav Jain wrote:
>> Hi,
> [cut]
>> is confusing. An example is the use of symbols sbss and ebss in the bss
>> 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.
>>           *(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.

>>       }
>> }
>
> I'm not sure if you are OT, anyway...
>
> i.e. they are useful if you want to clear the bss section before execution.
>
> [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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