c - Object file to binary code

Question

Let's suppose I have a C file with no external dependency, and only const data section. I would like to compile this file, and then get a binary blob I can load in another program, where the function would be used through a function pointer.

Let's take an example, here is a fictionnal binary module, f1.c

static const unsigned char mylut[256] = {
    [0 ... 127] = 0,
    [128 ... 255] = 1,
};

void f1(unsigned char * src, unsigned char * dst, int len)
{
    while(len) {
        *dst++ = mylut[*src++];
        len--;
    }
}

I would like to compile it to f1.o, then f1.bin, and use it like this in prog.c

int somefunc() {
    unsigned char  * codedata;
    f1_type_ptr  f1_ptr;
    /* open f1.bin, and read it into codedata */

    /* set function pointer to beginning of loaded data */
    f1_ptr =(f1_type_ptr)codedata;

    /* call !*/
    f1_ptr(src, dst, len);
}

I suppose going from f1.c to f1.o involves -fPIC to get position independance. What are the flags or linker script that I can use to go from f1.o to f1.bin ?

Clarification :

I know about dynamic linking. dynamic linking is not possible in this case. The linking step has to be cast func pointer to loaded data, if it is possible.

Please assume there is no OS support. If I could, I would for example write f1 in assembly with PC related adressing.

Answer 1

First of all, as other said you should consider using a DLL or SO.

That said, if you really want to do this, you need to replace the linker script. Something like this (not very well tested, but I think it works):

ENTRY(_dummy_start)
SECTIONS
{
    _dummy_start = 0;
    _GLOBAL_OFFSET_TABLE_ = 0;
    .all : { 
        _all = .;
        LONG(f1 - _all);
        *( .text .text.* .data .data.* .rodata .rodata.* ) 
    }
}

Then compile with:

$ gcc -c -fPIC test.c

Link with:

$ ld -T script.ld test.o -o test.elf

And extract the binary blob with:

$ objcopy -j .all -O binary test.elf test.bin

Probably some explanation of the script is welcome:

• ENTRY(_dummy_start) That just avoids the warning about the program not having an entry point.
• _dummy_start = 0; That defines the symbol used in the previous line. The value is not used.
• _GLOBAL_OFFSET_TABLE_ = 0; That prevents another linker error. I don't think you really need this symbol, so it can be defined as 0.
• .all That's the name of the section that will collect all the bytes of your blob. In this sample it will be all the .text, .data and .rodata sections together. You may need some more if you have complicated functions, in this case objdump -x test.o is your friend.
• LONG(f1 - _all) Not really needed, but you want to know the offset of your function into the blob, don't you? You cannot assume that it will be at offset 0. With this line the very first 4 bytes in the blob will be the offset of the symbol f1 (your function). Change LONG with QUAD if using 64-bit pointers.

UPDATE: And now a quick'n'dirty test (it works!):

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>

typedef void (*f1_t)(char *a, char *b, int len);
f1_t f1;

int main()
{
    char *blob = (char*)valloc(4096);
    FILE *f = fopen("test.bin", "rb");
    fread(blob, 1, 4096, f);
    fclose(f);

    unsigned offs = *(unsigned*)blob;
    f1 = (f1_t)(blob + offs);
    mprotect(blob, 4096, PROT_READ | PROT_WRITE | PROT_EXEC);
    char txt[] = "?hello world!";
    char txt2[sizeof(txt)] = "";
    f1(txt, txt2, sizeof(txt) - 1);
    printf("%s
%s
", txt, txt2);
    return 0;

}