Re: relocatable modules' symbols

[Prabhu nath <gprabhunath@xxxxxxxxx>]

In principle, the linker will associate the kernel code/data  to the Kernel virtual address space i.e. between 0xC000_0000 to 0xFFFF_FFFF and
the same linker will associate the Application's code/data to the user virtual address space i.e. between 0x0000_0000 to 0xBFFF_FFFF.

Linker itself cannot distinguish between the kernel code/data and user code/data but it is the linker script that will guide the linker to associate the code/data to the respective addresses.

What you are seeing is the linker script provided to the linker with -T option when building the kernel.

To build the user application the linker uses its default linker script which you get it by executing

$ ld --verbose.

To compare between the two just juxtapose the readelf output of both kernel executable (vmlinux) and any user application executable (a.out)

$ readelf -a vmlinux

$ readelf -a a.out

Just inspect the section header and program header of both the outputs you will get the difference.

Regards,
Prabhu

On Fri, Mar 1, 2013 at 11:07 AM, Arun KS <getarunks@xxxxxxxxx> wrote:

Hi Horseriver,

On Sun, Feb 24, 2013 at 7:26 AM, horseriver <horserivers@xxxxxxxxx> wrote:
> On Sun, Feb 24, 2013 at 04:00:37PM +0700, Mulyadi Santosa wrote:
>> On Sat, Feb 23, 2013 at 6:45 AM, horseriver <horserivers@xxxxxxxxx> wrote:
>> > hi:
>> >
>> >   I have built vmlinux at the top dir of kernel source ,then I use objdump to look into
>> >   its section information.I find the statup_32 which is the start routine of kernel ,
>> >   locats at 0xc0100000. I know the 0x100000 is the defined address for locating
>> >   protect-mode code .But which I can not understand is the 0xc prefix .where does it come from?
>>
>> If my guess is right, that's because kernel mode code start at
>> 0xc000000 a.k.a a bit above 3 GiB on x86 32 bit machine
>
>   I have find this answer .
>   It is defined in lds script file .
>
> here is the code :
>   SECTIONS
>   {
>   . = 0xC0000000 + 0x100000;
>   /* read-only */
>   _text = .;            /* Text and read-only data */
>
>
>   why use 0xC0000000 as its start ? why not  just  use 0x100000 only ?
>   if use 0xC0000000,every linked symbole will be prefixed by 0xc , what is the purpose ?

Usually, user kernel space split is 3G:1G.
ie 1G for kernel space. And this starts from 0xC000_0000 till 0xFFFF_FFFF

All the address below 0xC000_0000(ie 3G from 0x0 to 0xBFFF_FFFF) is
used for user space.

Thanks,
Arun

>
> thanks!
>
>
>
> --
>> regards,
>>
>> Mulyadi Santosa
>> Freelance Linux trainer and consultant
>>
>> blog: the-hydra.blogspot.com
>> training: mulyaditraining.blogspot.com
>
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