How to call machine code stored in char array?

I'm trying to call native machine-language code. Here's what I have so far (it gets a bus error):

char prog[] = {'\xc3'}; // x86 ret instruction

int main()
{
    typedef double (*dfunc)();

    dfunc d = (dfunc)(&prog[0]);
    (*d)();
    return 0;
}

It does correctly call the function and it gets to the ret instruction. But when it tries to execute the ret instruction, it has a SIGBUS error. Is it because I'm executing code on a page that is not cleared for execution or something like that?

So what am I doing wrong here?

Horia Coman

One first problem might be that the location where the prog data is stored is not executable.

On Linux at least, the resulting binary will place the contents of global variables in the "data" segment or here, which is not executable in most normal cases.

The second problem might be that the code you are invoking is invalid in some way. There's a certain procedure to calling a method in C, called the calling convention (you might be using the "cdecl" one, for example). It might not be enough for the called function to just "ret". It might also need to do some stack cleanup etc. otherwise the program will behave unexpectedly. This might prove an issue once you get past the first problem.

Rudi

You need to call memprotect in order to make the page where prog lives executable. The following code does make this call, and can execute the text in prog.

#include <unistd.h>
#include <stdio.h>
#include <malloc.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/mman.h>

char prog[] = {
   0x55,             // push   %rbp
   0x48, 0x89, 0xe5, // mov    %rsp,%rbp
   0xf2, 0x0f, 0x10, 0x05, 0x00, 0x00, 0x00,
       //movsd  0x0(%rip),%xmm0        # c <x+0xc>
   0x00,
   0x5d,             // pop    %rbp
   0xc3,             // retq
};

int main()
{
    long pagesize = sysconf(_SC_PAGE_SIZE);
    long page_no = (long)prog/pagesize;
    int res = mprotect((void*)(page_no*pagesize), (long)page_no+sizeof(prog), PROT_EXEC|PROT_READ|PROT_WRITE);
    if(res)
    {
        fprintf(stderr, "mprotect error:%d\n", res);
        return 1;
    }
    typedef double (*dfunc)(void);

    dfunc d = (dfunc)(&prog[0]);
    double x = (*d)();
    printf("x=%f\n", x);
    fflush(stdout);
    return 0;
}

As everyone already said, you must ensure prog[] is executable, however the proper way to do it, unless you're writing a JIT compiler, is to put the symbol in an executable area, either by using a linker script or by specifying the section in the C code if the compiler allows , e.g.:

const char prog[] __attribute__((section(".text"))) = {...}

Virtually all C compilers will let you do this by embedding regular assembly language in your code. Of course it's a non-standard extension to C, but compiler writers recognise that it's often necessary. As a non-standard extension, you'll have to read your compiler manual and check how to do it, but the GCC "asm" extension is a fairly standard approach.

 void DoCheck(uint32_t dwSomeValue)
 {
    uint32_t dwRes;

    // Assumes dwSomeValue is not zero.
    asm ("bsfl %1,%0"
      : "=r" (dwRes)
      : "r" (dwSomeValue)
      : "cc");

    assert(dwRes > 3);
 }

Since it's easy to trash the stack in assembler, compilers often also allow you to identify registers you'll use as part of your assembler. The compiler can then ensure the rest of that function steers clear of those registers.

If you're writing the assembler code yourself, there is no good reason to set up that assembler as an array of bytes. It's not just a code smell - I'd say it is a genuine error which could only happen by being unaware of the "asm" extension which is the right way to embed assembler in your C.

Essentially this has been clamped down on because it was an open invitation to virus writers. But you can allocate and buffer and set it up with native machinecode in straight C - that's no problem. The issue is calling it. Whilst you can try setting up a function pointer with the address of the buffer and calling it, that's highly unlikely to work, and highly likely to break on the next version of the compiler if somehow you do manage to coax it into doing what you want. So the best bet is to simply resort to a bit of inline assembly, to set up the return and jump to the automatically generated code. But if the system protects against this, you'll have to find methods of circumventing the protection, as Rudi described in his answer (but very specific to one particular system).

One obvious error is that \xc3 is not returning the double that you claim it's returning.

moooeeeep

You can eliminate the crash by allowing the compiler to store the array in the read-only section of your process memory (if it's known at compile time). For example by declaring the array const.

Example:

const char prog[] = {'\xc3'}; // x86 ret instruction

int main()
{
    typedef double (*dfunc)();

    dfunc d = (dfunc)(&prog[0]);
    (*d)();
    return 0;
}

Alternatively you can compile the code with disabled stack protection gcc -z execstack.