Re: cli()/sti() clarification

[Kasper Dupont <kasperd@daimi.au.dk>]

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irfan_hamid@softhome.net wrote:
> 
> Hi,
> 
> I added two system calls, blockintr() and unblockintr() to give cli()/sti()
> control to userland programs (yes I know its not advisable) but I only want
> to do it as a test. My test program looks like this:
> 
>         blockintr();
>         /* Some long calculations */
>         unblockintr();
> 
> The problem is that if I press Ctrl+C during the calculation, the program
> terminates. So I checked the _syscallN() and __syscall_return() macros to
> see if they explicitly call sti() before returning to userspace, but they
> dont.

The flag register is saved on the stack and restored by the INT 0x80
and IRET instructions. You would want to change the interrupt flag
in that register. Here is an ugly piece of code that is known to
work on some kernel version, I don't remember which.

Another approach would be to use ioperm to get access to the IRQ
controller, and block interrupts there.

-- 
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/* This piece of code is ugly and inefficient. */

#include <stdlib.h>
#include <linux/module.h>
#include <asm/current.h>
#include "cli.h"

struct my_module {
  struct module m;
  char name[64];
  unsigned long jmp;
  unsigned long delta;
} my_module;

static int do_cli()
{
	(*(unsigned long*)(((char*)current)+0x1FF4)) &= ~0x200;
	return 0;
}

static int do_sti()
{
        (*(unsigned long*)(((char*)current)+0x1FF4)) |= 0x200;
        return 0;
}

void init_struct(int (*func)(),struct my_module *dst, struct my_module *p)
{
  dst->m.size_of_struct = sizeof(struct module);
  dst->m.next=NULL;
  dst->m.name=p->name;
  dst->m.size=sizeof(my_module);
  dst->m.flags=0;
  dst->m.nsyms=0;
  dst->m.ndeps=0;
  dst->m.syms=NULL;
  dst->m.deps=NULL;
  dst->m.refs=NULL;
  dst->m.init=(void*)&(p->jmp);
  dst->m.cleanup=NULL;
  dst->m.ex_table_start=NULL;
  dst->m.ex_table_end=NULL;
  dst->m.persist_start=NULL;
  dst->m.persist_end=NULL;
  dst->m.can_unload=NULL;
  sprintf(dst->name,"hack_%d_hack",getpid());
  dst->jmp=0xE9909090;
  dst->delta=(unsigned long)func-(unsigned long)(p+1);
}

static void kernel_call(int (*func)())
{
  struct my_module my_module;
  init_struct(func,&my_module,&my_module);
  init_struct(func,&my_module,(void*)create_module(my_module.name,my_module.m.size));
  init_module(my_module.name,&my_module.m);
  delete_module(my_module.name);
}

void cli()
{
  kernel_call(do_cli);
}

void sti()
{
  kernel_call(do_sti);
}