Build your own operating system

Part 09

Hello everyone…

This is part 9 of Implement Your Own Operating System. In this article, we will discuss how to run a simple program in user mode.

User Mode

User mode is now almost within our reach, there are just a few more steps required to get there. Although these steps might seem easy the way they are presented in this chapter, they can be tricky to implement, since there are a lot of places where small errors will cause bugs that are hard to find.

1. Segments for User Mode

To enable user mode we need to add two more segments to the GDT. They are very similar to the kernel segments we added when we set up the GDT in the article about segmentation:

The segment descriptors are needed for user mode.

The difference is the DPL, which allows code to execute in PL3. The segments can still be used to address the entire address space, just using these segments for user-mode code will not protect the kernel. For that we need paging.

2. Setting Up For User Mode

Every user-mode process requires a few things:

• Page frames for code, data, and stack. At the moment it suffices to allocate one-page frame for the stack and enough page frames to fit the program’s code.
• The binary from the GRUB module has to be copied to the page frames used for the program's code.
• A page directory and page tables are needed to map the page frames described above into memory. Because the code and data should be mapped in at 0x00000000 and rising, and the stack should start immediately below the kernel, at 0xBFFFFFFB, and expand towards lower addresses, at least two-page tables are required.

3. Entering User Mode

The only way to execute code with a lower privilege level than the current privilege level (CPL) is to execute an iret or lret instruction — interrupt return or long return, respectively.

To enter user mode we set up the stack as if the processor had raised an inter-privilege level interrupt. The stack should look like the following:

[esp + 16]  ss      ; the stack segment selector we want for        user mode
[esp + 12]  esp     ; the user mode stack pointer
[esp +  8]  eflags  ; the control flags we want to use in user mode
[esp +  4]  cs      ; the code segment selector
[esp +  0]  eip     ; the instruction pointer of user mode code to execute

The instruction iret will then read these values from the stack and fill in the corresponding registers.

Before we execute iret we need to change to the page directory we set up for the user-mode process.

In order to continue executing kernel code after we’ve switched PDT, the kernel needs to be mapped in. By maintaining a separate PDT for the kernel, which maps all data at 0xC0000000 and above, and merge it with the user PDT which only maps below 0xC0000000 we can accomplish this. Note that the physical address of the PDT should be used when setting the cr3 register.

The register eflags contains a set of different flags and for us the most important one is the interrupt enable (IF) flag. If interrupts are disabled when entering user mode, then interrupts can’t be enabled once user mode is entered. Setting the IF flag in the eflags entry on the stack will enable interrupts in user mode, since the assembly code instruction iret will set the register eflags to the corresponding value on the stack.

For now, we should have interrupts disabled, as it is somewhat difficult to get inter-privilege level interrupts to work properly but we will implement this in an upcoming article.

The value eip on the stack should point to the entry point for the user code — 0x00000000 and the value esp on the stack should be where the stack starts — 0xBFFFFFFB (0x00000000–4) in our implementation.

The values cs and ss on the stack should be the segment selectors for the user code and user data segments, respectively. The lowest two bits of a segment selector is the RPL (Requested Privilege Level). When using iret to enter PL3, the RPL of cs and ss should be 0x3. The following code shows an example:

    USER_MODE_CODE_SEGMENT_SELECTOR equ 0x18
    USER_MODE_DATA_SEGMENT_SELECTOR equ 0x20
    mov cs, USER_MODE_CODE_SEGMENT_SELECTOR | 0x3
    mov ss, USER_MODE_DATA_SEGMENT_SELECTOR | 0x3

The register ds, and the other data segment registers, should be set to the same segment selector as ss. They can be set with the mov assembly code instruction which is the ordinary way we have used.

We have now set all the features needed to execute iret. If everything of the above process has been set up right, we should now have a kernel that can enter user mode.

4. Using C for User Mode Programs

When C is used as the programming language for user-mode programs, it is important to think about the structure of the file that will be the result of the compilation.

The reason we can use ELF [18] as the file format for the kernel executable is that GRUB knows how to parse and interpret the ELF file format. If we implemented an ELF parser, we could compile the user mode programs into ELF binaries as well. We leave this as an exercise for the reader.

One thing we can do to make it easier to develop user-mode programs is to allow the programs to be written in C, but compile them to flat binaries instead of ELF binaries. In C the layout of the generated code is more unpredictable and the entry point, main, might not be at offset 0 in the binary. One common way to work around this is to add a few assembly code lines placed at offset 0 which calls main:

    extern main

    section .text
        ; push argv
        ; push argc
        call main
        ; main has returned, eax is return value
        jmp  $    ; loop forever

If this code is saved in a file called start.s, then the following code shows an example of a linker script that places these instructions first in executable (remember that start.s gets compiled to start.o):

OUTPUT_FORMAT("binary")    /* output flat binary */

    SECTIONS
    {
        . = 0;                 /* relocate to address 0 */

        .text ALIGN(4):
        {
            start.o(.text)     /* include the .text section of start.o */
            *(.text)           /* include all other .text sections */
        }

        .data ALIGN(4):
        {
            *(.data)
        }

        .rodata ALIGN(4):
        {
            *(.rodata*)
        }
    }

Note: *(.text) will not include the .text section of start.o again.

With this script, we can write programs in C or assembler (or any other language that compiles to object files linkable with ld), and it is easy to load and map for the kernel (.rodata will be mapped in as writeable, though).

When we compile user programs we want the following GCC flags:

-m32 -nostdlib -nostdinc -fno-builtin -fno-stack-protector -nostartfiles-nodefaultlibs

For linking, the followings flags should be used:

-T link.ld -melf_i386  
# emulate 32 bits ELF, the binary output is specified
# in the linker script

The option -T instructs the linker to use the linker script link.ld.

I hope you got a more understanding of User Modes. Let’s meet with the next article of the ‘Build your own Operating System’ series. Thank you so much for reading!

The Little OS Book: https://littleosbook.github.io/book.pdf

Thank you for reading and hope to see you in the next article as well!