为什么在NASM中使用相对RIP寻址? [英] Why use RIP-relative addressing in NASM?

问题描述

我有一个适用于Mac OS X的程序集hello world程序，如下所示:

I have an assembly hello world program for Mac OS X that looks like this:

global _main


section .text

_main:
    mov rax, 0x2000004
    mov rdi, 1
    lea rsi, [rel msg]
    mov rdx, msg.len
    syscall

    mov rax, 0x2000001
    mov rdi, 0
    syscall


section .data

msg:    db  "Hello, World!", 10
.len:   equ $ - msg

我想知道行 lea rsi，[rel msg] .为什么NA​​SM强迫我这样做?据我了解， msg 只是指向可执行文件中某些数据的指针，并且执行 mov rsi，msg 会将该地址放入 rsi 中.但是，如果我将 lea rsi，[rel msg] 行替换为，NASM会引发此错误(注意:我正在使用命令 nasm -f macho64 hello.asm ):

I was wondering about the line lea rsi, [rel msg]. Why does NASM force me to do that? As I understand it, msg is just a pointer to some data in the executable and doing mov rsi, msg would put that address into rsi. But if I replace the line lea rsi, [rel msg] with , NASM throws this error (note: I am using the command nasm -f macho64 hello.asm):

hello.asm:9: fatal: No section for index 2 offset 0 found

为什么会这样? lea 有什么特别之处，而 mov 无法做到?我怎么知道什么时候使用每个?

Why does this happen? What is so special about lea that mov can't do? How would I know when to use each one?

推荐答案

lea 有什么特别之处，而 mov 无法做到?

What is so special about lea that mov can't do?

mov reg，imm 将 immediate 常量加载到其目标操作数中.立即常数直接在操作码中编码，例如如果 someVar 的地址为 0x00ABCDEF ，则 mov eax，someVar 将被编码为 B8 EF CD AB 00 .IE.要使用 imm 作为 msg 的地址对此类指令进行编码，您需要知道 msg 的确切地址.在与位置无关的代码中，您不会先验地知道它.

mov reg,imm loads an immediate constant into its destination operand. Immediate constant is encoded directly in the opcode, e.g. mov eax,someVar would be encoded as B8 EF CD AB 00 if address of someVar is 0x00ABCDEF. I.e. to encode such an instruction with imm being address of msg you need to know exact address of msg. In position-independent code you don't know it a priori.

mov reg，[expression] 加载位于 expression 描述的地址处的值.x86指令的复杂编码方案允许具有非常复杂的 expression :通常是 reg1 + reg2 * s + displ ，其中 s 可以是0、1、2、4， reg1 和 reg2 可以是通用寄存器，也可以是零，而 displ 是立即移位.在64位模式下， expression 可以具有另一种形式: RIP + displ ，即，相对于下一条指令计算地址.

mov reg,[expression] loads the value located at address described by expression. The complex encoding scheme of x86 instructions allows to have quite complex expression: in general it's reg1+reg2*s+displ, where s can be 0,1,2,4, reg1 and reg2 can be general-purpose registers or zero, and displ is immediate displacement. In 64-bit mode expression can have one more form: RIP+displ, i.e. the address is calculated relative to the next instruction.

lea reg，[expression] 使用所有复杂的地址计算方式将地址本身加载到 reg 中(与不同mov ，它取消对计算出的地址的引用).因此，在编译时不可用的信息，即在 RIP 中的绝对地址，可以在不知道其值的情况下在指令中进行编码.nasm表达式 lea rsi，[rel msg] 被翻译成类似

lea reg,[expression] uses all this complex way of calculating addresses to load the address itself into reg (unlike mov, which dereferences the address calculated). Thus the information, unavailable at compilation time, namely absolute address which would be in RIP, can be encoded in the instruction without knowing its value. The nasm expression lea rsi,[rel msg] gets translated into something like

    lea rsi,[rip+(msg-nextInsn)]
nextInsn:

使用相对地址 msg-nextInsn 而不是 msg 的绝对地址，从而使汇编器不知道实际地址，但仍对指令进行编码.

which uses the relative address msg-nextInsn instead of absolute address of msg, thus allowing the assembler to not know the actual address but still encode the instruction.

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