Re: How to make stack executable on demand?

["H. J. Lu" <hjl@lucon.org>]

On Fri, Apr 16, 2004 at 10:02:58PM +0200, Arjan van de Ven wrote:
> >  But it will either fail if
> > kernel is set with non-executable stack,
> 
> eh no. mprotect with prot_exec is still supposed to work. The stacks
> still have MAY_EXEC attribute, just not the actual EXEC attribute

Ok. It looks like a bug in Red Hat EL 3 kernel. In fs/exec.c, there
are

                if (executable_stack)
                        mpnt->vm_flags = VM_STACK_FLAGS | VM_MAYEXEC | VM_EXEC;
		else
                        mpnt->vm_flags = VM_STACK_FLAGS & ~(VM_MAYEXEC|VM_EXEC); 

That is if an executabl is not marked with executable stack, the
VM_MAYEXEC bit is turned off. But 2.6.5-mm6 has

                if (unlikely(executable_stack == EXSTACK_ENABLE_X))
                        mpnt->vm_flags = VM_STACK_FLAGS |  VM_EXEC;
                else if (executable_stack == EXSTACK_DISABLE_X)
                        mpnt->vm_flags = VM_STACK_FLAGS & ~VM_EXEC;
                else
                        mpnt->vm_flags = VM_STACK_FLAGS;
  
The VM_MAYEXEC bit is untouched. Now the question is if it is a good
idea for user to change stack permission.


H.J.