On 11/19/13 18:34, Stephen Polkowski wrote:
> Hello,
>
> I'm building a #GP fault handler for a shell application. I named my
> file __gp_fault_handler.S and I added it to my inf file. Everything compiles
> and
> the exception handler actually works.
>
> However, now I want to set a global variable that is declared in a
> separate "C" file. When I assemble my file GAS uses a R_X86_64_32S offset to
> the
> variable. In my other "C" files I see that the compiler uses a R_X86_64_64
> offset
> to my variable instead.
>
> How can I get the GNU assembler to output a R_X86_64_64 offset in the
> EDK2 build?
>
>
> Thanks,
>
> Stephen
>
>
> Here's a shortened version of my assembly code:
>
> .extern EXCEPTION_EXECUTE_HANDLER
> .global __gp_fault_handler
>
> __gp_fault_handler: .code64
>
> # Set the GP_FAULT flag
> movl $1, EXCEPTION_EXECUTE_HANDLER
>
>
>
>
> Here is the GNU assembler output:
>
> Disassembly of section .text:
>
> 0000000000000000 <__gp_fault_handler>:
>
> 16: c7 04 25 00 00 00 00 movl $0x1,0x0
> 1d: 01 00 00 00
> 19: R_X86_64_32S EXCEPTION_EXECUTE_HANDLER
According to what I found on the web, you need to use movabs for 64-bit
displacement. Additionally, you can only use the "Areg (AL/AX/EAX/RAX)"
as source/destination register.
Hence, assuming that you want to write 4 bytes,
.extern EXCEPTION_EXECUTE_HANDLER
.global __gp_fault_handler
__gp_fault_handler: .code64
# Set the GP_FAULT flag
movl $1, %eax
movabs %eax, EXCEPTION_EXECUTE_HANDLER
Which assembles to
0000000000000000 <__gp_fault_handler>:
0: b8 01 00 00 00 mov $0x1,%eax
5: a3 00 00 00 00 00 00 movabs %eax,0x0
c: 00 00
6: R_X86_64_64 EXCEPTION_EXECUTE_HANDLER
Laszlo
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