在 2019-07-09二的 09:38 +0000，Mahmood Naderan via gcc-help写道：
> BTW, as I run the instruction, I get permission denied error.
You can not run the output of the assembler (as). It's a relocatable object
file, not executable. You should link it.
>
> On Tuesday, July 9, 2019, 1:23:58 PM GMT+4:30, Mahmood Naderan via gcc-
> help <gcc-help@xxxxxxxxxxx> wrote:
>
> >You mean x86-64, instead of x86. On x86 "c5 00" means "lds (%eax),%eax".
> > "c5" itself is not illegal. For example, "c5 00 58 00" means an AVX
> > instruction
> > "vaddps (%rax),%xmm15,%xmm8". The disassembler can only determine that it's
> > an
> > illegal instruction after it has seen "c5 00 00".
>
> Xi,The code looks like
> $ cat machine3.c
> void main() {
> __asm__(".byte 0xc5, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
> 0x00, 0x00, 0x00, 0x00, 0x00");
> }
> $ gcc -masm=intel -g -c machine3.c
> $ objdump -d -M intel -S -z machine3.o
>
> machine3.o: file format elf64-x86-64
>
>
> Disassembly of section .text:
>
> 0000000000000000 <main>:
> void main() {
> 0: 55 push rbp
> 1: 48 89 e5 mov rbp,rsp
> 4: c5 00 00 (bad)
> 7: 00 00 add BYTE PTR [rax],al
> 9: 00 00 add BYTE PTR [rax],al
> b: 00 00 add BYTE PTR [rax],al
> d: 00 00 add BYTE PTR [rax],al
> f: 00 00 add BYTE PTR [rax],al
> 11: 00 00 add BYTE PTR [rax],al
> __asm__(".byte 0xc5, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
> 0x00, 0x00, 0x00, 0x00, 0x00");
> }
> 13: 5d pop rbp
> 14: c3 ret
>
>
>
> Why gcc considers that as a three byte instructions and then tag it as a bad
> instruction?
It's not GCC. Objdump is a part of Binutils.
"c5" may be a prefix of a legal instruction so objdump can't say it's illegal.
"c5 00" may be a prefix of a legal instruction so objdump can't say it's
illegal.
(An example: "c5 00 58 00" is "vaddsubps %xmm0,%xmm1,%xmm2".)
But "c5 00 00" is not a prefix of any known legal instruction so objdump can say
it's definitely illegal.
> Regards,
> Mahmood
--
Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University
