Why do I get oveflow in printf?

Question

Hey I have to call a function of glibc in assembly for an exercise. So I found this code to call printf.

section .rodata
    format: db 'Hello %s', 10
    name:   db 'Conrad'

section .text
        global main
        extern printf
    main:
        ; printf(format, name)
        mov rdi, format
        mov rsi, name
        call printf
        ; return 0
        mov rax, 0
        ret

But i get the error:

Symbol `printf' causes overflow in R_X86_64_PC32 relocation

Compiled it with:

nasm -f elf64 -o test.o test.asm

gcc -o test test.o

The error occurs after doing

./test

Add `\0` to `format` and `name`. – eanmos Nov 14 '19 at 13:06 — eanmos, Nov 14 '19 at 13:06
@eanmos: That's a separate problem. – R.. GitHub STOP HELPING ICE Nov 14 '19 at 13:09 — R.. GitHub STOP HELPING ICE, Nov 14 '19 at 13:09

score 3 · Answer 1 · answered Nov 14 '19 at 13:11

Change call printf to call printf@PLT. The former only works if the actual definition of printf is withing ±2GB of the call instruction, which can't be known if the definition is in a shared library (it would work if you static link, though). "Overflow" is telling you that the relative address, which would need to be up to 64-bit, overflows in a 32-bit call instruction offset.

By using printf@PLT, you'll instead get a relative address that resolves statically at link time to a thunk in the PLT, which loads and jumps to the address of the function definition, resolved at dynamic-linking time.

As Maxime B. noted, the loads of the addresses of format and name are also not correct for position-independent code. They should be loaded with "rip-relative" form, but it looks like you're using the weird "Intel syntax" for asm and I'm not sure how to write it in that syntax. You could, as Maxime B. suggested, build with -fno-pie, but better would be figuring out the way to fix your code so it doesn't depend on being linked for a particular fixed address.

@JanWolfram: It might be nasm expecting it to be written in some odd way (again, I'm not familiar with the syntax it uses) or it might just be that you introduced extra spaces or something else wrong in the syntax. — R.. GitHub STOP HELPING ICE, Nov 14 '19 at 13:38
@JanWolfram For NASM you may want to consider `call printf wrt ..plt` . `call printf@plt` would be GNU assembler syntax — Michael Petch, Nov 14 '19 at 14:29

score 2 · Answer 2 · edited Nov 14 '19 at 13:11

2

You should compile your with -no-pie

This error is explained here. Quoting the original post:

Debian switched to PIC/PIE binaries in 64-bits mode & GCC in your case is trying to link your object as PIC, but it will encounter absolute address in mov $str, %rdi.

edited Nov 14 '19 at 13:11

Federico klez Culloca

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answered Nov 14 '19 at 13:10

Maxime B.

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Then I only have segmentation fault – Jan Wolfram Nov 14 '19 at 13:15
@JanWolfram Which is probably due to the lack of '\0' at the end of the strings as pointed out by eanmos – Maxime B. Nov 14 '19 at 13:16
:/ no thats not the error. Sorry for my bad knowledge – Jan Wolfram Nov 14 '19 at 13:20
@JanWolfram: and because you didn't align the stack before calling `printf`. (And didn't set AL=0 so it will save the XMM regs to its register-save area with aligned stores, so this ABI violation will likely crash *in practice*). – Peter Cordes Nov 14 '19 at 16:08

Why do I get oveflow in printf?

2 Answers2