Re: Query on linker scripts

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Hi,
 
Just one more query. bss is not a part of the executable but the linker script defines
an output bss  section.  This means that the linker script only causes the symbols and section header for the bss section 
to be added to executable. Am I right ?
 
Thanks
Vaibhav Jain

On Mon, Mar 26, 2012 at 4:54 AM, Pranay Kumar Srivastava <Pranay.Shrivastava@xxxxxxx> wrote:

> -----Original Message-----
> From: Vaibhav Jain [mailto:vjoss197@xxxxxxxxx]
> Sent: Monday, March 26, 2012 4:31 PM
> To: Pranay Kumar Srivastava
> Cc: kernelnewbies@xxxxxxxxxxxxxxxxx
> Subject: Re: Query on linker scripts
>
> Thanks a lot for the explanation and the link!! I have just one more
> question about
> linker scripts. I am not clear about alignment of sections. Is it
> necessary to align sections?

Its not mandatory but helps you to know in terms of pages how many are in use, otherwise while creating ptes for the kernel you'll need to calculate from the number of bytes the kernel has consumed and find out pages required. Otherwise its just a simple shift operation to find out the corresponding page from the page's beginning address.


> Can the alignment be different from 4096?
> In the script that I provided the text and data sections are aligned
> while the bss section is not. Is there
> a reason for it ?

The alignment generally corresponds to page granularity in the case of linker scripts. It's not about aligning data in C which can be done on say 4byte 8 byte etc... This alignment is done so that when the time comes to protect kernel and start user space applications you'll know which pages corresponds to kernel and can't be swapped and must be protected. Instead of thinking about protecting how many bytes kernel uses, think about how many pages the kernel uses.

Well, bss isn't part of your executable. It's stack and hence it doesn't make sense to have ALIGN there since you'll be responsible of setting up the stack for the kernel and using it. The data/text are part of the executable so you would want to have them aligned on page boundary so that when you load the kernel those sections would be loaded at page aligned address and end at page boundary.


>
> Thanks
> Vaibhav Jain
>
>
>
> On Sun, Mar 25, 2012 at 11:41 PM, Pranay Kumar Srivastava
> <Pranay.Shrivastava@xxxxxxx> wrote:
>
>
> > -----Original Message-----
> > From: Vaibhav Jain [mailto:vjoss197@xxxxxxxxx]
> > 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@xxxxxxxxxxxxxxxxx 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
> > <Pranay.Shrivastava@xxxxxxx> wrote:
> > On 03/24/2012 11:52 PM, Pranay Kumar Srivastava wrote:
> > >
> > > ________________________________________
> > > From: kernelnewbies-
> > bounces+pranay.shrivastava=hcl.com@xxxxxxxxxxxxxxxxx [kernelnewbies-
> > bounces+pranay.shrivastava=hcl.com@xxxxxxxxxxxxxxxxx] On Behalf Of
> > kernelnewbies-request@xxxxxxxxxxxxxxxxx [kernelnewbies-
> > request@xxxxxxxxxxxxxxxxx]
> > > Sent: Saturday, March 24, 2012 9:30 PM
> > > To: kernelnewbies@xxxxxxxxxxxxxxxxx
> > > 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@xxxxxxxxx>
> > > Subject: Query on linker scripts
> > > To: kernelnewbies@xxxxxxxxxxxxxxxxx
> > > Message-ID:
> > >
> >  <CAKuUYSw=_zZykPWeTbJsGEYPPSroWK+whm0o5L_PnCManVcrng@xxxxxxxxxxxxxx>
> > > 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
> > > -------------- next part --------------
> > > An HTML attachment was scrubbed...
> > > URL:
> >
> http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/2012
> > 0323/6e1741da/attachment-0001.html
> > >
> > > ------------------------------
> > >
> > > Message: 2
> > > Date: Sat, 24 Mar 2012 16:26:38 +0100
> > > From: Carlo Caione<carlo.caione@xxxxxxxxx>
> > > Subject: Re: Query on linker scripts
> > > To: Vaibhav Jain<vjoss197@xxxxxxxxx>
> > > Cc: kernelnewbies@xxxxxxxxxxxxxxxxx
> > > Message-ID:<4F6DE7AE.9070808@xxxxxxxxx>
> > > 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
> > >
> > >
> > >
> > > ------------------------------
> > >
> > > _______________________________________________
> > > Kernelnewbies mailing list
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> > >
> > >
> > > End of Kernelnewbies Digest, Vol 16, Issue 29
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