Switch to ring 3

Reverted · **Joined:** Sat Feb 28, 2015 7:25 am **Posts:** 3

Hello, I've (at last) successfully created basic of os - software interrupts in ring 3. Now I am aiming to crate hardware handling for for ex. PIT interrupt. I get back to user mode switch code and pushed corrected eflags on to stack, so the iret will pop ant set the register. After doing this my code fails.

Code:

.global usermode_switch
.type usermode_switch, @function
usermode_switch:
    #Data segment setup
    mov $0x13, %ax
    mov %ax, %ds
    mov %ax, %gs
    mov %ax, %fs
    mov %ax, %es
    #Stack save
    mov %esp, %eax
    #stack setup for iret and user space return
    pushl $0x13
    pushl %eax
    pushf
    #enable ints after switch to ring 3
    pop %eax
    or $0x200, %eax
    pushl %eax
    #CS selector
    pushl $0x0b
    pushl $end_switch
    iret
.extern upperKernelCode
end_switch:
    call upperKernelCode

bochs log says only :

Code:

interrupt(): gate descriptor is not valid sys seg (vector=0x08)

and the two more faults.
Edit: I should mention that

Code:

call upperKernelCode

generates GPF

SpyderTL · **Joined:** Sun Sep 19, 2010 10:05 pm **Posts:** 1074

I dont think 13 (or even 0x13, for that matter) are valid segment numbers. They start at 0x08, and go up by 0x08, and you need at least two entries in the GDT.

So, you should probably be using segments 0x08 and 0x10.

gerryg400 · **Posted:** Mon Jun 01, 2015 9:01 pm

SpyderTL wrote:

I dont think 13 (or even 0x13, for that matter) are valid segment numbers. They start at 0x08, and go up by 0x08, and you need at least two entries in the GDT.

So, you should probably be using segments 0x08 and 0x10.

The lowest 2 bits are the RPL so 0x13 could be a valid selector.

GreaseMonkey · **Joined:** Tue May 26, 2015 11:53 pm **Posts:** 5

First things first, have you set up a TSS? You need SS0 and ESP0 at the very least, and I believe there's a few other things.

Another thing is that when transitioning to ring 3, IRET not only pops EIP, CS, and EFLAGS, but it also pops the user-mode ESP that you will be using.

I would highly recommend using a separate stack, even if you need to set one up purely for returning to ring 0.

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