CODEWITHSUNDEEP
clinuxassemblyx86-64
haste
haste

Reputation: 1461

Calling main from assembly

I'm writing a small library intended to be used in place of libc in a small application. I've read the source of the major libc alternatives, but I am unable to get the parameter passing to work for the x86_64 architecture on Linux.

The library does not require any initialization step in between _start and main. Since the libc and its alternatives do use a initialization step, and my assembly knowledge being limited, I suspect the parameter reordering is causing me troubles.

This is what I've got, which contains assembly inspired from various implementations:

.text
.global _start

_start:
    /* Mark the outmost frame by clearing the frame pointer. */
    xorl %ebp, %ebp

    /* Pop the argument count of the stack and place it
     * in the first parameter-passing register. */
    popq %rdi

    /* Place the argument array in the second parameter-passing register. */
    movq %rsi, %rsp

    /* Align the stack at a 16-byte boundary. */
    andq $~15, %rsp

    /* Invoke main (defined by the host program). */
    call main

    /* Request process termination by the kernel. This
     * is x86 assembly but it works for now. */
    mov  %ebx, %eax
    mov  %eax, 1
    int  $80

And the entry point is the ordinary main signature: int main(int argc, char* argv[]). Environment variables etc. are not required for this particular project.

The AMD64 ABI says rdi should be used for the first parameter, and rsi for the second.

How do I correctly setup the stack and pass the parameters to main on Linux x86_64? Thanks!

References:
http://www.eglibc.org/cgi-bin/viewvc.cgi/trunk/libc/sysdeps/x86_64/elf/start.S?view=markup
http://git.uclibc.org/uClibc/tree/libc/sysdeps/linux/x86_64/crt1.S

Upvotes: 2

Views: 2658

Answers (2)

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

main is called by crt0.o; see also this question

The kernel is setting up the initial stack and process environment after execve as specified in the ABI document (architecture specific); the crt0 (and related) code is in charge of calling main.

Upvotes: 0

Gunther Piez
Gunther Piez

Reputation: 30449

I think you got

 /* Place the argument array in the second parameter-passing register. */
    movq %rsi, %rsp

wrong. It should be

movq %rsp, %rsi      # move argv to rsi, the second parameter in x86_64 abi

Upvotes: 1

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