Re: Q: -fpic and $_GLOBAL_OFFSET_TABLE_

Re: Q: -fpic and $_GLOBAL_OFFSET_TABLE_

[Oleg Nesterov]

Hello.

hp <lx () lxhp ! in-berlin ! de> wrote:
>
> Oleg Nesterov am Dienstag, 15. Juli 2003 14:19:
> > Hello.
> >
> > I thought, that &GOT == $_GLOBAL_OFFSET_TABLE_ + .
>						^^^^^^^^^^^^^^^^^^^^^
> thus your GOT would be at a different place whenever fetched at a different 
> address in your code - not very likely...

There is R_386_GOTPC relocation item for _GLOBAL_OFFSET_TABLE_.

Look at this code produced by gcc to load &GOT in ebx:

	call .L3
.L3:
	popl %ebx
	addl $_GLOBAL_OFFSET_TABLE_+[.-.L3],%ebx

So &GOT = .L3 + $_GLOBAL_OFFSET_TABLE_+ . - .L3 = $_GLOBAL_OFFSET_TABLE_+ .

Thank you for response,

Oleg.

Q: -fpic and $_GLOBAL_OFFSET_TABLE_

[Oleg Nesterov]

Hello.

I thought, that &GOT == $_GLOBAL_OFFSET_TABLE_ + .

$ cat test.c

void test(void)
{
	asm volatile (
		"addl $_GLOBAL_OFFSET_TABLE_, %eax\n"
		"addl $_GLOBAL_OFFSET_TABLE_, %ecx\n"
	);
}

$ cc -c -fpic -fomit-frame-pointer test.c && ld -shared -o test.so test.o && objdump -d test.so

test.so:     file format elf32-i386

Disassembly of section .text:

0000018c <test>:
 18c:	05 0d 10 00 00       	addl   $0x100d,%eax	==> &GOT = 0x18c + 0x100d = 0x1199
 191:	81 c1 07 10 00 00    	addl   $0x1007,%ecx	==> &GOT = 0x191 + 0x1007 = 0x1198
 197:	c3                   	ret    

How can it be?

Oleg.

Re: Q: -fpic and $_GLOBAL_OFFSET_TABLE_

[hp]

Oleg Nesterov am Dienstag, 15. Juli 2003 14:19:
> Hello.
>
> I thought, that &GOT == $_GLOBAL_OFFSET_TABLE_ + .
						^^^^^^^^^^^^^^^^^^^^^
thus your GOT would be at a different place whenever fetched at a different 
address in your code - not very likely...

and, more in the ELF standard description, re links at
		http://www.lxhp.in-berlin.de/lhplinks.html#elf

>
> $ cat test.c
>
> void test(void)
> {
> 	asm volatile (
> 		"addl $_GLOBAL_OFFSET_TABLE_, %eax\n"
> 		"addl $_GLOBAL_OFFSET_TABLE_, %ecx\n"
> 	);
> }
>
> $ cc -c -fpic -fomit-frame-pointer test.c && ld -shared -o test.so test.o
> && objdump -d test.so
>
> test.so:     file format elf32-i386
>
> Disassembly of section .text:
>
> 0000018c <test>:
>  18c:	05 0d 10 00 00       	addl   $0x100d,%eax	==> &GOT = 0x18c + 0x100d =
> 0x1199 191:	81 c1 07 10 00 00    	addl   $0x1007,%ecx	==> &GOT = 0x191 +
> 0x1007 = 0x1198 197:	c3                   	ret
>
> How can it be?
>
> Oleg.
> -
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-- 
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	FAQ(s) + DOCs at http://linuxassembly.org
  pse, reply to << lx -at- lxhp -dot- in-berlin -dot- de >>

Q: -fpic and $_GLOBAL_OFFSET_TABLE_

[Oleg Nesterov]

Hello.

I thought, that &GOT == $_GLOBAL_OFFSET_TABLE_ + .

$ cat test.c

void test(void)
{
	asm volatile (
		"addl $_GLOBAL_OFFSET_TABLE_, %eax\n"
		"addl $_GLOBAL_OFFSET_TABLE_, %ecx\n"
	);
}

$ cc -c -fpic -fomit-frame-pointer test.c && ld -shared -o test.so test.o && objdump -d test.so

test.so:     file format elf32-i386

Disassembly of section .text:

0000018c <test>:
 18c:	05 0d 10 00 00       	addl   $0x100d,%eax	==> &GOT = 0x18c + 0x100d = 0x1199
 191:	81 c1 07 10 00 00    	addl   $0x1007,%ecx	==> &GOT = 0x191 + 0x1007 = 0x1198
 197:	c3                   	ret    

How can it be?

Oleg.