Direct assignment of code to function pointer: not working in gcc 4.x

[Ian Lance Taylor]

Anibal Caceres Hernando <anibal.caceres@ericsson.com> writes:

> int main() {
>    char *code = new char[5];
>    int codeIndex = 0;
>    printf("Start!!\n");
>
>    code[codeIndex++]=0x55; //push %ebp
>    code[codeIndex++]=0x89; //mov    %esp,%ebp
>    code[codeIndex++]=0xe5;    //"
>
>    code[codeIndex++]=0xc9; //LEAVE
>    code[codeIndex++]=0xc3; //RET
>
>    FunctionType invoke=(FunctionType)&code[0];
>    invoke();
>
>    printf("Finish!!\n");
>    return 0;
> }
>
>    If this is compiled with gcc 3.3.3 it executes without any problem:
> prints "Start!!", then executes invoke(), which does nothing, as you
> can see (the assembler is just a call to an empty function), and
> finally it prints "Finish!!".
>    But the problem happens when I compile this using gcc 4.1.2 (and
> the same happens with 4.2.2): prints "Start!!", and it gives a
> Segmentation Violation when it starts executing invoke (I've seen
> using the gdb that the SIGSEGV happens just in the first instruction,
> in the "push %ebp").

This program requires an executable stack.  You neglected to mention
which system you are running on.  If it is GNU/Linux, see the
execstack(8) man page.

Ian