Porting to DragonFly BSD

Tyson Whitehead twhitehead at gmail.com
Mon Nov 16 17:32:00 EST 2009

On November 15, 2009 07:42:46 Goetz Isenmann wrote:
> 001e5565 <__hscore_get_errno>:
>   1e5565:       55                      push   %ebp
>   1e5566:       89 e5                   mov    %esp,%ebp
>   1e5568:       65 a1 00 00 00 00       mov    %gs:0x0,%eax
>   1e556e:       8b 15 00 00 00 00       mov    0x0,%edx
>   1e5574:       8b 04 10                mov    (%eax,%edx,1),%eax
>   1e5577:       c9                      leave
>   1e5578:       c3                      ret
> The corresponding relocation info is
> 001e5570 R_386_TLS_IE      errno
> > After some googling, and learning very little, I speclated that simply
> > adding a third line for R_386_TLS_IE with the same action as the first
> > line - just storing value, might work:
> >
> > case R_386_TLS_IE: *pP = value; break;
> >
> > This seems to work up to a point. That is I get no crashes in ghci.

According to "ELF Handling For Thread-Local Storage", R_386_TLS_IE "resolves 
to the absolute address of the GOT [global offset table] slot" that is assigned 
to the TLS (thread local storage) symbol.

http://people.redhat.com/drepper/tls.pdf   (middle of page 37)

The GOT slot is assigned a R_386_TLS_TPOFF relocation for the symbol.

So, what you are suppose to get is:

1- While linking, the linker resolves the R_386_TLS_IE relocation by updating 
the instruction with the absolute address of the corresponding GOT slot.

2- While loading, the dynamic linker resolves the R_386_TLS_TPOFF relocation 
by updating the GOT slot to contain the offset of the symbol relative to the 
TLS pointer (which is stored at %gs:0x0).

3- The program then computes the address of the TLS variable by loading the 
TLS pointer (the "mov %gs:0x0,%eax" instruction) and adding to it the symbols 
TLS offset as stored in the GOT table (the updated "mov 0x0,%edx" instruction).

Cheers!  -Tyson
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