Build your own operating system-part09
9.1 Segments for User Mode:-
The segment descriptors needed for user mode.IndexOffsetNameAddress rangeTypeDPL3
0x18user code segment
0x00000000 - 0xFFFFFFFFRXPL34
0x20user data segment
0x00000000 - 0xFFFFFFFFRWPL3
The difference is the DPL, which now 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.
9.2 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
0x00000000and 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
9.3 Entering User Mode:-
The only way to execute code with a lower privilege level than the current privilege level (CPL) is to execute an
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
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 setup for the user mode process. It is important to remember that to continue executing kernel code after we’ve switched PDT, the kernel needs to be mapped in. One way to accomplish this is to have 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) when performing the switch. Remember that physical address of the PDT has to be used when setting the register
eflags contains a set of different flags. Most important for us is the interrupt enable (IF) flag. The assembly code instruction
sti can’t be used in privilege level 3 for enabling interrupts. If interrupts are disabled when entering user mode, then interrupts can’t 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 requires a little more work to get inter-privilege level interrupts to work properly .
eip on the stack should point to the entry point for the user code -
0x00000000 in our case. The value
esp on the stack should be where the stack starts -
0xC0000000 - 4).
ss on the stack should be the segment selectors for the user code and user data segments, respectively. As we saw in the segmentation chapter, the lowest two bits of a segment selector is the RPL - the Requested Privilege Level. When using
iret to enter PL3, the RPL of
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
ds, and the other data segment registers, should be set to the same segment selector as
ss. They can be set the ordinary way, with the
mov assembly code instruction.
We are now ready to execute
iret. If everything has been set up right, we should now have a kernel that can enter user mode.
9.4 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 as the file format for 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. 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
extern main section .text
; push argv
; push argc
; 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 show an example of a linker script that places these instructions first in executable (remember that
start.s gets compiled to
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):
} .rodata ALIGN(4):
*(.text) will not include the
.text section of
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
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
-T instructs the linker to use the linker script