How to find the address & length of a C++ function at runtime (MinGW)

As this is my first post to stackoverflow I want to thank you all for your valuable posts that helped me a lot in the past.

I use MinGW (gcc 4.4.0) on Windows-7(64) - more specifically I use Nokia Qt + MinGW but Qt is not involved in my Question.

I need to find the address and -more important- the length of specific functions of my application at runtime, in order to encode/decode these functions and implement a software protection system.

I already found a solution on how to compute the length of a function, by assuming that static functions placed one afte开发者_如何学JAVAr each other in a source-file, it is logical to be also sequentially placed in the compiled object file and subsequently in memory.

Unfortunately this is true only if the whole CPP file is compiled with option: "g++ -O0" (optimization level = 0). If I compile it with "g++ -O2" (which is the default for my project) the compiler seems to relocate some of the functions and as a result the computed function length seems to be both incorrect and negative(!).

This is happening even if I put a "#pragma GCC optimize 0" line in the source file, which is supposed to be the equivalent of a "g++ -O0" command line option.

I suppose that "g++ -O2" instructs the compiler to perform some global file-level optimization (some function relocation?) which is not avoided by using the #pragma directive.

Do you have any idea how to prevent this, without having to compile the whole file with -O0 option? OR: Do you know of any other method to find the length of a function at runtime?

I prepare a small example for you, and the results with different compilation options, to highlight the case.

The Source:

// ===================================================================
// test.cpp
//
// Intention: To find the addr and length of a function at runtime
// Problem:   The application output is correct when compiled with: "g++ -O0"
//            but it's erroneous when compiled with "g++ -O2"
//            (although a directive "#pragma GCC optimize 0" is present)
// ===================================================================

#include <stdio.h>
#include <math.h>

#pragma GCC optimize 0

static int test_01(int p1)
{
    putchar('a');
    putchar('\n');
    return 1;
}

static int test_02(int p1)
{
    putchar('b');
    putchar('b');
    putchar('\n');
    return 2;
}

static int test_03(int p1)
{
    putchar('c');
    putchar('\n');
    return 3;
}

static int test_04(int p1)
{
    putchar('d');
    putchar('\n');
    return 4;
}

// Print a HexDump of a specific address and length
void HexDump(void *startAddr, long len)
{
    unsigned char *buf = (unsigned char *)startAddr;
    printf("addr:%ld, len:%ld\n", (long )startAddr, len);
    len = (long )fabs(len);
    while (len)
    {
        printf("%02x.", *buf);
        buf++;
        len--;
    }
    printf("\n");
}


int main(int argc, char *argv[])
{
    printf("======================\n");
    long fun_len = (long )test_02 - (long )test_01;
    HexDump((void *)test_01, fun_len);

    printf("======================\n");
    fun_len = (long )test_03 - (long )test_02;
    HexDump((void *)test_02, fun_len);

    printf("======================\n");
    fun_len = (long )test_04 - (long )test_03;
    HexDump((void *)test_03, fun_len);

    printf("Test End\n");
    getchar();

    // Just a trick to block optimizer from eliminating test_xx() functions as unused
    if (argc > 1)
    {
      test_01(1);
      test_02(2);
      test_03(3);
      test_04(4);
    }
}

The (correct) Output when compiled with "g++ -O0":

[note the 'c3' byte (= assembly 'ret') at the end of all functions]

======================
addr:4199344, len:37
55.89.e5.83.ec.18.c7.04.24.61.00.00.00.e8.4e.62.00.00.c7.04.24.0a.00.00.00.e8.42
.62.00.00.b8.01.00.00.00.c9.c3.
======================
addr:4199381, len:49
55.89.e5.83.ec.18.c7.04.24.62.00.00.00.e8.29.62.00.00.c7.04.24.62.00.00.00.e8.1d
.62.00.00.c7.04.24.0a.00.00.00.e8.11.62.00.00.b8.02.00.00.00.c9.c3.
======================
addr:4199430, len:37
55.89.e5.83.ec.18.c7.04.24.63.00.00.00.e8.f8.61.00.00.c7.04.24.0a.00.00.00.e8.ec
.61.00.00.b8.03.00.00.00.c9.c3.
Test End

The erroneous Output when compiled with "g++ -O2": (a) function test_01 addr & len seem correct (b) functions test_02, test_03 have negative lengths,

and fun. test_02 length is also incorrect.

======================
addr:4199416, len:36
83.ec.1c.c7.04.24.61.00.00.00.e8.c5.61.00.00.c7.04.24.0a.00.00.00.e8.b9.61.00.00
.b8.01.00.00.00.83.c4.1c.c3.
======================
addr:4199452, len:-72
83.ec.1c.c7.04.24.62.00.00.00.e8.a1.61.00.00.c7.04.24.62.00.00.00.e8.95.61.00.00
.c7.04.24.0a.00.00.00.e8.89.61.00.00.b8.02.00.00.00.83.c4.1c.c3.57.56.53.83.ec.2
0.8b.5c.24.34.8b.7c.24.30.89.5c.24.08.89.7c.24.04.c7.04.
======================
addr:4199380, len:-36
83.ec.1c.c7.04.24.63.00.00.00.e8.e9.61.00.00.c7.04.24.0a.00.00.00.e8.dd.61.00.00
.b8.03.00.00.00.83.c4.1c.c3.
Test End

This is happening even if I put a "#pragma GCC optimize 0" line in the source file, which is supposed to be the equivalent of a "g++ -O0" command line option.

I don't believe this is true: it is supposed to be the equivalent of attaching __attribute__((optimize(0))) to subsequently defined functions, which causes those functions to be compiled with a different optimisation level. But this does not affect what goes on at the top level, whereas the command line option does.

If you really must do horrible things that rely on top level ordering, try the -fno-toplevel-reorder option. And I suspect that it would be a good idea to add __attribute__((noinline)) to the functions in question as well.