link 32bit .o with 64bit .o

[GuoMinskey]

Thank you for helping.
I use objcopy to transform 32 bit object into 64 bit, and then link it intoother 64 bit objects.  Now, I can call my function from 32 bit code part(.code32) of head_64.S (mixed 32/64 bit code).
However,  in my C code, I cannot access data which is allocated in .data or .bss section. Since there is an offset between compile addressand load address,  I guess that I need to have my C code deal with theoffset.
In Linux kernel source:  arch/x86/boot/compressed/head_64.S,  that offset is calculated by the following snippet:
        leal    (BP_scratch+4)(%esi), %esp        call    1f1:     popl    %ebp        subl    $1b, %ebp
So, the offset is saved in ebp register.   I think that my Code might workif I can have it aware the offset.  But how can I have the C code add anoffset to all variables in .data and .bss sections ?

-minskey



----------------------------------------
> From: iant@google.com
> To: minskey_guo@hotmail.com
> CC: gcc-help@gcc.gnu.org
> Subject: Re: link 32bit .o with 64bit .o
> Date: Mon, 11 Apr 2011 13:47:58 -0700
>
> GuoMinskey  writes:
>
> > On x86 platform,  is there any means to link 32 bit ELF output into 64 bit  ?
> > In Linux kernel,  arch/x86/boot/compressed/head_64.S,   starts from32-bit code and enables CPU to jump into 64-bit.  So, that file is assembled into 64 bit ELF object with mixed 32/64 bit code.
> > Now, I want to call a routine from somewhere in the 32bit code part of head_64.S.    And I write the routine by C language,  say test.c and compile  it with "gcc -m32 -c -o test.o".    How can I link mytest.o to head_64.o ?  Or I have to use assembly language to rewritemy routine and embed it directly into head_64.S source code ?
>
> On x86, 32-bit code can not call 64-bit code, nor vice-versa. The Linux
> kernel uses some tricks to start up in 32-bit mode, convert to 64-bit
> mode, and branch to 64-bit code as it converts. You can't do the same
> tricks yourself; only the kernel can do them.
>
> So, the answer is that you can link 32-bit and 64-bit code together
> easily enough, using the linker's -b option as the kernel build does,
> but you can't do anything useful with the result.
>
> Ian