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

[Anibal Caceres Hernando <anibal dot caceres at ericsson dot com>]

   Hi,
   Thank you for your response, Ian. The system is GNU/Linux, so I did 
as you said, and used "-z execstack" option for the linker, and now the 
code works properly. It seems then that GCC 4.x is more restrictive 
about executing things from the stack than GCC 3.x.
   Kind regards,

   Aníbal.

Ian Lance Taylor wrote:
> 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
>