why do we need reloc_offset ??

2009-09-02 Thread HongWoo Lee 
Hi everyone~

In ther linux kernel code, I found the reloc_offset.

{{{
// file : misc.S
/* Returns (address we are running at) - (address we were linked at)
 * for use before the text and data are mapped to KERNELBASE.
 */
_GLOBAL(reloc_offset)
}}}

I couldn't understand the comment saying Returns (address we are running
at) - (address we were linked at).
For now, I'm studying each instruction.

And below is best comment I can explain for each instruction.

_GLOBAL(reloc_offset)
mflrr0// move from link register, save the
return address
bl  1f // bl 1f
1: mflrr3// move from link register, r3 is just
return address pointing itself
LOAD_REG_IMMEDIATE(r4,1b)// get the 1b address, r4 is the
address
subfr3,r4,r3// r3 = r3 – r4
mtlrr0// restore return address
blr

After this, I still don't know why r3-r4 is the offset.
And what does it mean ??

Please explain to me the reason why we need reloc_offset and the code.
Or let me know any helpful reference about this.

Thank in advance.

HongWoo.
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Re: why do we need reloc_offset ??

2009-09-02 Thread Michael Ellerman 
On Wed, 2009-09-02 at 15:33 +0900, HongWoo Lee wrote:
 Hi everyone~ 
 
 In ther linux kernel code, I found the reloc_offset. 
 
 {{{
 // file : misc.S 
 /* Returns (address we are running at) - (address we were linked at)
  * for use before the text and data are mapped to KERNELBASE.
  */
 _GLOBAL(reloc_offset)
 }}}
 
 I couldn't understand the comment saying Returns (address we are
 running at) - (address we were linked at). 
 For now, I'm studying each instruction. 
 
 And below is best comment I can explain for each instruction. 
 
 _GLOBAL(reloc_offset)
 mflrr0// move from link register, save the return 
 address
 bl  1f // bl 1f
 1: mflrr3// move from link register, r3 is just 
 return address pointing itself 

At this point r3 contains the value of LR based on the branch we just
did. So it's the address of the current instruction, based on where the
code is _running_.

 LOAD_REG_IMMEDIATE(r4,1b)// get the 1b address, r4 is the address 

Here we load into r4 the address of the previous instruction, but based
on the label 1b. The address of the label is calculated by the linker,
so r4 contains the address the instruction was linked at.

 subfr3,r4,r3// r3 = r3 – r4 

So here we calculate any difference between the address the code was
linked at and the address it's running at.

 mtlrr0// restore return address 
 blr
 
 After this, I still don't know why r3-r4 is the offset. 
 And what does it mean ?? 

The offset is just the difference between the address the code was
linked at and the address it's running it. It's used in places where the
code might be (or is always) running at an address other than the
address it was linked at.

cheers


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