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Posted: **Tue Jul 14, 2015 8:17 pm**

Edit: Found it out, thanks!

Hi All, I am trying to get GCC to produce:

movq %%gs:0x45, xxxx

The 0x45 above will change as my structure changes, but I am having trouble telling asm how to fix the value.
Any help is appreciated.

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#define GetKernelGSPointerData(KernelPointerType, Member, ReturnType, ReturnVariableName) \
	ReturnType ReturnVariableName; \
	{ int ofs = offset(KernelPointerType, Member);  asm volatile ("movq %%gs:%1, %0" : "=q"(ReturnVariableName) : "m"(ofs) ); }

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#define offset(type, member) ((WORD)(QWORD)&((type *)0)->member);

Example calling code:

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GetKernelGSPointerData(tCPU, Sleeping, QWORD, Test);

It currently Produces:

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  30c2cb:	c7 04 24 00 00 00 00 	mov    DWORD PTR [rsp],0x0
  30c2d2:	65 48 8b 34 24       	mov    rsi,QWORD PTR gs:[rsp]

Which is fine, but I would like a

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mov rsi, QWORD PTR gs:0

Posted: **Tue Jul 14, 2015 8:21 pm**

If I change to:

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#define GetKernelGSPointerData(KernelPointerType, Member, ReturnType, ReturnVariableName) \
	ReturnType ReturnVariableName; \
	{ int ofs = offset(KernelPointerType, Member);  asm volatile ("movq %%gs:(%1), %0" : "=q"(ReturnVariableName) : "r"(ofs) ); }

This is produced, which is fine, but still wondering how to produce gs:0x0

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  30c2cb:	31 f6                	xor    esi,esi
  30c2cd:	65 67 4c 8b 06       	mov    r8,QWORD PTR gs:[esi]

Posted: **Tue Jul 14, 2015 8:36 pm**

Figured it out, as usual, just after posting here. LOL

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#define GetKernelGSPointerData(KernelPointerType, Member, ReturnType, ReturnVariableName) \
	ReturnType ReturnVariableName; \
	{ asm volatile ("movq %%gs:%1, %0" : "=q"(ReturnVariableName) : "m"(*(QWORD*)offset(KernelPointerType, Member)) ); }

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#define offset(type, member) ((QWORD)&((type *)0)->member)

Found this: http://f.osdev.org/viewtopic.php?f=13&t=21721

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