Build your own Operating System #9
User Modes
Hello everyone!
This is the ninth article of the “Build your own Operating System“ article series. In the previous articles of this article series, we knew lots of things like how we setting up a development environment, segmentation, input-output handling, virtual memory and paging, and how to implement them. In the sixth article, we covered up the road to user modes. I like to suggest, please refer to previous articles, before reading this. It will help you to a better understanding of this article. Let’s move to today's topic.
Usually, a kernel is not supposed to do the application logic itself, but leave that for applications. The kernel creates the proper abstractions (for memory, files, devices) to make application development easier, performs tasks on behalf of applications (system calls), and schedules processes.
User mode, in contrast with kernel mode, is the environment in which the user’s programs execute. This environment is less privileged than the kernel, and will prevent (badly written) user programs from messing with other programs or the kernel. Badly written kernels are free to mess up what they want.
We knew how the beginning of User Mode. I explained it step by step in the sixth article of this article series. I link below that article and you can get some idea about User Modes and how we beginning User Modes by referring to the below article.
In this article, we learn more further about User Modes.
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 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.
Setting Up For User Mode
There are a few things every user mode process needs:
- 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. Don’t worry about setting up a stack that can be grow and shrink at this point in time, focus on getting a basic implementation work first.
- The binary from the GRUB module has to be copied to the page frames used for the programs code.
- A page directory and page tables are needed to map the page frames described above into memory. At least two page tables are needed, because the code and data should be mapped in at 0x00000000 and increasing, and the stack should start just below the kernel, at 0xBFFFFFFB, growing towards lower addresses. The U/S flag has to be set to allow PL3 access.
It might be convenient to store this information in a struct representing a process. This process struct can be dynamically allocated with the kernel’s malloc function.
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 0x18USER_MODE_DATA_SEGMENT_SELECTOR equ 0x20mov cs, USER_MODE_CODE_SEGMENT_SELECTOR | 0x3mov 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.
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 as the file format for the kernel executable is because 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.
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. To do this add start.s file to your working directory with the given code.
Make sure to update your Makefile OBJECTS with start.o.
Then the following code show 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 -nodefaultlibsFor linking, the followings flags should be used:
-T link.ld -melf_i386 # emulate 32 bits ELF, the binary output is specified in the linker scriptThe 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
References
https://manybutfinite.com/post/cpu-rings-privilege-and-protection/
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