why clear interrput flag cause segmentation fault in C?

问题

I am learning some basics about Assembly and C. for learning purpose I decide to write a simple program that disable Interrupts and when user wants to type something in the console he/she can't :

#include <stdio.h>
int main(){
    int a;
    printf("enter your number : ");
    asm ("cli");
    scanf("%d", &a);
    printf("your number is %d\n" , a);     
    return 0;
}

but when I compile this with GCC I got segmentation fault :

Segmentation fault (core dumped)

And when I debug it with gdb I got this message when program reach to the asm("cli"); line:

Program received signal SIGSEGV, Segmentation fault.
main () at cli.c:6
6       asm ("cli");

回答1:

This is happening because You can't disable interrupts from user space program. All interrupts are under the control of kernel. You need to do it from kernel space. Before you do it you need to learn kernel internals first and playing with interrupts are very critical and requires more knowledge on kernel according to my knowledge.

You need to write a kernel module that can interact with user space through /dev/ (or some other) interface. User space code should request kernel module to disable interrupts.

回答2:

cli is a privileged instruction. It raises a #GP(0) exception "If the CPL is greater (has less privilege) than the IOPL of the current program or procedure". This #GP is what causes Linux to deliver a SIGSEGV to your process.

Under Linux, you could make an iopl(3) system call to raise your IO priv level to match your ring 3 CPL, and then you could disable interrupts from user-space. (But don't do this, it's not supported AFAIK. The intended use-case for iopl is to use in and out instructions from user-space with high port numbers, not cli/sti. x86 just happens to use the same permissions for both.)

You'll probably crash your system if you don't re-enable interrupts right away, or maybe even if you do. Or at least screw up that CPU on a multi-core system. Basically don't do this unless you're ready to press the reset button, i.e. shut down X11, saved your files and run sync. Also remount your filesystems read-only.

Or try it in a virtual machine or simulator like BOCHS that will let you break in with a debugger even while interrupts are disabled. Or try it while booted from a USB stick.

Note that disabling interrupts only disables external interrupts. Software-generated interrupts like int $0x80 are still taken, but making system calls with interrupts disabled is probably an even worse idea. (It might work, though. The kernel saves/restores EFLAGS, so it probably won't return to user-space with interrupts re-enabled. Still, leaving interrupts disabled for a long time is a Bad Thing for interrupt latency.)

If you want to play around with disabling interrupts as a beginner, you should probably do it from a toy boot-sector program that uses BIOS calls for I/O. Or just look in the Linux kernel source for some places where it disables/enables interrupts if you're curious why it might do that.

IMO, "normal" asm in user-space is plenty interesting. With performance counters, you can see the details of how the CPU decodes and executes instructions. See links in the x86 tag wiki for manuals, guides, and performance tuning info.

来源：https://stackoverflow.com/questions/20949450/why-clear-interrput-flag-cause-segmentation-fault-in-c

标签

assembly

x86

interrupt