关于x86_64位系统下的long jump

azhu981712

我自己写了一段汇编程序，其实有一个远跳(long jump)，原本在32位模式下面运行的，但是后来转到64位上面编译能通过，但是运行的时候有错误产生。程序如下：
jmp 8:jmp_ok
这段在32位上运行没有问题，但是在64位上运行就有问题了。我是用nasm编译的，后面加了-f elf64，但后来通过反汇编来看这一段就是（bad），不知道哪位大虾能够帮帮忙，或者能够提供一点Linux内核处理这方面问题的线索也行。

wsgtrsys

There is no jump of the form JMP absaddr to an absolute address in 64 bit mode. The operand of a jump is always a 32 bit relative displacement to rip, which gets sign extended to 64 bit.

The reason you see no consistency is possibly that the offset depends on the the current instruction pointer and you didn't recognize that.

jmp eax isn't allowed either, as addresses are of course always 64 bit wide on a 64 bit architecture. A sequence mov rax, addr + jmp rax is possible, it would look like

48 c7 c0 35 08 40 00            mov rax, 0x00400835
ff e0                           jmp rax
or

48 b8 35 08 40 00 00 00 00 00   mov rax, 0x0000000000400835
ff e0                           jmp rax
How did I know these hex codes? Well, I did ask my compiler. I compiled with gcc -c and disassembled with objdump. I didn't bother to use Intel syntax, because I don't need it. So this is in AT&T syntax.

echo 'asm("mov $400835, %rax\n jmp *%rax\n");' > test.c
gcc -c test.c
objdump -d test.o