CODEWITHSUNDEEP
assemblyx86
user4344762
user4344762

Reputation:

Can I read label value in assembly?

A label represents the address of the first byte following it, but can I read its value? For example can I do the following?

some_label:
    mov eax, some_label    ; this will move the address of mov into eax?

Upvotes: 1

Views: 2963

Answers (1)

Jonathon Reinhart
Jonathon Reinhart

Reputation: 137408

What you show will work in e.g. nasm, but the result may not be what you expect.

A little more information into what you're writing would help this answer be more specific. (Typical user-mode ELF application? Stand-alone binary blob? Something else?)

Because the assembler doesn't know where (in the address space) your code may be running, the instruction you've written will cause a relocation to be generated by the assembler (if your output format supports it).

If your environment does support relocations, (e.g. an ELF object file produced by nasm -f elf) then that should be fine; eax will match what eip was prior to executing that instruction. This works because that instruction was fixed-up by the loader - a relocation was applied.

If your environment does not support relocations, (e.g. a raw binary file, or other non-hosted environment) then you have a problem. If that instruction is at offset 0x100 in your binary blob, then eax will get the value 0x100 - certainly not the actual address you're executing at. To remedy this, you need to do the relocation yourself. Unfortunately, there's no easy way to do this on x86 (32-bit). You have to do something like this:

    call .getbase
.getbase:
    pop ebx
    sub ebx, .getbase

    ;; Now ebx has the base address of your linked application/blob

some_label:
    mov eax, some_label
    add eax, ebx

    ;; Now eax has the (run-time) address of some_label

The x86_64 instruction set added a RIP-relative addressing scheme, which makes this much easier:

some_lable:
    lea rax, [rel some_label]

Upvotes: 5

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