inline assembly and segment registers

[Mihai Donțu <mihai.dontu@xxxxxxxxx>]

Hi,

I know this isn't the proper list to ask this, but I'm desperate :)

Does anyone know why the code below dies with a 'general protection fault'?:

DWORD __readfsdword( int index )
{
	DWORD v;
	
	__asm__ __volatile__ (
		"mov %%fs:(%1),%0\n"
		:"=a"(v)
		:"a"(&index)
	);

	return v;
}

or, why does this work:
    mov %fs:18,%eax
    ret
and this doesn't (asm of the C code above):
    lea 0x4(%esp),%eax
    mov %fs:(%eax),%eax <-- crash here
    ret

gdb -c core bin
$gdb p *(int*)$eax
$1 = 18

I know I'm lacking some basic x86 know-how, but google doesn't want to help me
with this one.

Oh, %fs and the LDT entry are all properly set and I prefer a __readfsdword()
function instead of a macro (because of the type checking and such), so:

#define __readfsdword( __index ) ({ DWORD v; __asm__ __volatile__ ( "mov %%fs:" #__index ",%0" : "=r"(v) : ); v; })

will not do :)

Thanks,

-- 
Mihai Donțu