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@xxxxxxxxxxxx> 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
