If you read my previous [blog posts](https://0xax.github.io/categories/assembler/), you might have noticed that I have been involved with low-level programming for some time. I wrote some posts about assembly programming for `x86_64` Linux and, at the same time, started to dive into the Linux kernel source code.
I have a great interest in understanding how low-level things work, how programs run on my computer, how they are located in memory, how the kernel manages processes and memory, how the network stack works at a low level, and many many other things. So, I decided to write yet another series of posts about the Linux kernel for the **x86_64** architecture.
Note that I'm not a professional kernel hacker and I don't write code for the kernel at work. It's just a hobby. I just like low-level stuff, and it is interesting for me to see how these things work. So if you notice anything confusing, or if you have any questions/remarks, ping me on Twitter [0xAX](https://twitter.com/0xAX), drop me an [email](mailto:anotherworldofworld@gmail.com) or just create an [issue](https://github.com/0xAX/linux-insides/issues/new). I appreciate it.
All posts will also be accessible at [github repo](https://github.com/0xAX/linux-insides) and, if you find something wrong with my English or the post content, feel free to send a pull request.
Anyway, if you're just starting to learn such tools, I will try to explain some parts during this and the following posts. Alright, this is the end of the simple introduction. Let's start to dive into the Linux kernel and low-level stuff!
I started writing these posts at the time of the `3.18` Linux kernel, and many things have changed since that time. If there are changes, I will update the posts accordingly.
Although this is a series of posts about the Linux kernel, we won't start directly from the kernel code. As soon as you press the magical power button on your laptop or desktop computer, it starts working. The motherboard sends a signal to the [power supply](https://en.wikipedia.org/wiki/Power_supply) device. After receiving the signal, the power supply provides the proper amount of electricity to the computer. Once the motherboard receives the [power good signal](https://en.wikipedia.org/wiki/Power_good_signal), it tries to start the CPU. The CPU resets all leftover data in its registers and sets predefined values for each of them.
The processor starts working in [real mode](https://en.wikipedia.org/wiki/Real_mode). Let's back up a little and try to understand [memory segmentation](https://en.wikipedia.org/wiki/Memory_segmentation) in this mode. Real mode is supported on all x86-compatible processors, from the [8086](https://en.wikipedia.org/wiki/Intel_8086) CPU all the way to the modern Intel 64-bit CPUs. The `8086` processor has a 20-bit address bus, which means that it could work with a `0-0xFFFFF` or `1 megabyte` address space. But it only has `16-bit` registers, which have a maximum address of `2^16 - 1` or `0xffff` (64 kilobytes).
[Memory segmentation](https://en.wikipedia.org/wiki/Memory_segmentation) is used to make use of all the address space available. All memory is divided into small, fixed-size segments of `65536` bytes (64 KB). Since we cannot address memory above `64 KB` with 16-bit registers, an alternate method was devised.
An address consists of two parts: a segment selector, which has a base address; and an offset from this base address. In real mode, the associated base address of a segment selector is `Segment Selector * 16`. Thus, to get a physical address in memory, we need to multiply the segment selector part by `16` and add the offset to it:
which is `65520` bytes past the first megabyte. Since only one megabyte is accessible in real mode, `0x10ffef` becomes `0x00ffef` with the [A20 line](https://en.wikipedia.org/wiki/A20_line) disabled.
The `CS` register consists of two parts: the visible segment selector and the hidden base address. In real-address mode, the base address is normally formed by shifting the 16-bit segment selector value 4 bits to the left to produce a 20-bit base address. However, during a hardware reset the segment selector in the CS register is loaded with `0xf000` and the base address is loaded with `0xffff0000`. The processor uses this special base address until `CS` changes.
We get `0xfffffff0`, which is 16 bytes below 4GB. This point is called the [reset vector](https://en.wikipedia.org/wiki/Reset_vector). It's the memory location at which the CPU expects to find the first instruction to execute after reset. It contains a [jump](https://en.wikipedia.org/wiki/JMP_%28x86_instruction%29) (`jmp`) instruction that usually points to the [BIOS](https://en.wikipedia.org/wiki/BIOS) (Basic Input/Output System) entry point. For example, if we look in the [coreboot](https://www.coreboot.org/) source code ([src/cpu/x86/16bit/reset16.inc](https://review.coreboot.org/plugins/gitiles/coreboot/+/refs/heads/4.11_branch/src/cpu/x86/16bit/reset16.inc)), we see:
Here we can see the `jmp` instruction [opcode](http://ref.x86asm.net/coder32.html#xE9), which is `0xe9`, and its destination address at `_start16bit - ( . + 2)`.
We also see that the `reset` section is `16` bytes and is compiled to start from the address `0xfffffff0` ([src/cpu/x86/16bit/reset16.ld](https://review.coreboot.org/plugins/gitiles/coreboot/+/refs/heads/4.11_branch/src/cpu/x86/16bit/reset16.ld)):
Now the BIOS starts. After initializing and checking the hardware, the BIOS needs to find a bootable device. A boot order is stored in the BIOS configuration, controlling which devices the BIOS attempts to boot from. When attempting to boot from a hard drive, the BIOS tries to find a boot sector. On hard drives partitioned with an [MBR partition layout](https://en.wikipedia.org/wiki/Master_boot_record), the boot sector is stored in the first `446` bytes of the first sector, where each sector is `512` bytes. The final two bytes of the first sector are `0x55` and `0xaa`, which designates to the BIOS that this device is bootable. Once the BIOS finds the boot sector, it copies it into a fixed memory location at 0x7c00, jumps to there and start executing it.
This will instruct [QEMU](https://www.qemu.org/) to use the `boot` binary that we just built as a disk image. Since the binary generated by the assembly code above fulfills the requirements of the boot sector (we end it with the magic sequence), QEMU will treat the binary as the master boot record (MBR) of a disk image. Note that when providing a boot binary image to QEMU, setting the origin to 0x7c00 (using `[ORG 0x7c00]`)
In this example, we can see that the code will be executed in `16-bit` real mode. After starting, it calls the [0x10](http://www.ctyme.com/intr/rb-0106.htm) interrupt, which just prints the `!` symbol. The times directive will pad that number of bytes up to 510th byte with zeros and finishes with the two magic bytes `0xaa` and `0x55`.
A real-world boot sector has code for continuing the boot process and a partition table instead of a bunch of 0's and an exclamation mark. :) From this point onwards, the BIOS hands control over to the bootloader.
just as explained above. We have only 16-bit general purpose registers, which has a maximum value of `0xffff`, so if we take the largest values the result will be:
where `0x10ffef` is equal to `(1MB + 64KB - 16B) - 1`. An [8086](https://en.wikipedia.org/wiki/Intel_8086) processor (which was the first processor with real mode), in contrast, has a 20-bit address line. Since `2^20 = 1048576` is 1MB and `2^20 - 1` is the maximum address that could be used, this means that the actual available memory is 1MB.
At the beginning of this post, I wrote that the first instruction executed by the CPU is located at address `0xFFFFFFF0`, which is much larger than `0xFFFFF` (1MB). How can the CPU access this address in real mode? The answer is in the [coreboot](https://www.coreboot.org/Developer_Manual/Memory_map) documentation:
There are a number of bootloaders that can boot Linux, such as [GRUB 2](https://www.gnu.org/software/grub/) and [syslinux](http://www.syslinux.org/wiki/index.php/The_Syslinux_Project). The Linux kernel has a [Boot protocol](https://github.com/torvalds/linux/blob/v4.16/Documentation/x86/boot.txt) which specifies the requirements for a bootloader to implement Linux support. This example will describe GRUB 2.
Continuing from before, now that the BIOS has chosen a boot device and transferred control to the boot sector code, execution starts from [boot.img](http://git.savannah.gnu.org/gitweb/?p=grub.git;a=blob;f=grub-core/boot/i386/pc/boot.S;hb=HEAD). Its code is very simple, due to the limited amount of space available. It contains a pointer which is used to jump to the location of GRUB 2's core image. The core image begins with [diskboot.img](http://git.savannah.gnu.org/gitweb/?p=grub.git;a=blob;f=grub-core/boot/i386/pc/diskboot.S;hb=HEAD), which is usually stored immediately after the first sector in the unused space before the first partition. The above code loads the rest of the core image, which contains GRUB 2's kernel and drivers for handling filesystems, into memory. After loading the rest of the core image, it executes the [grub_main](http://git.savannah.gnu.org/gitweb/?p=grub.git;a=blob;f=grub-core/kern/main.c) function.
The `grub_main` function initializes the console, gets the base address for modules, sets the root device, loads/parses the grub configuration file, loads modules, etc. At the end of execution, the `grub_main` function moves grub to normal mode. The `grub_normal_execute` function (from the `grub-core/normal/main.c` source code file) completes the final preparations and shows a menu to select an operating system. When we select one of the grub menu entries, the `grub_menu_execute_entry` function runs, executing the grub `boot` command and booting the selected operating system.
As we can read in the kernel boot protocol, the bootloader must read and fill some fields of the kernel setup header, which starts at offset `0x01f1` from the kernel setup code. You may look at the boot [linker script](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/setup.ld) to confirm the value of this offset. The kernel header [arch/x86/boot/header.S](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/header.S) starts from:
The bootloader must fill this and the rest of the headers (which are only marked as being type `write` in the Linux boot protocol, such as in [this example](https://github.com/torvalds/linux/blob/v4.16/Documentation/x86/boot.txt#L354)) with values either received from the command line or calculated during booting. (We will not go over full descriptions and explanations for all fields of the kernel setup header for now, but we shall do so when discussing how the kernel uses them. You can find a description of all fields in the [boot protocol](https://github.com/torvalds/linux/blob/v4.16/Documentation/x86/boot.txt#L156).)
Here we can see the memory address of the entry point, which is `0x0000000001000000`. Let's go ahead.
Before trying to debug the kernel, please see [Booting a Custom Linux Kernel in QEMU and Debugging It With GDB](http://nickdesaulniers.github.io/blog/2018/10/24/booting-a-custom-linux-kernel-in-qemu-and-debugging-it-with-gdb/)
The bootloader has now loaded the Linux kernel into memory, filled the header fields, and then jumped to the corresponding memory address. We now move directly to the kernel setup code.
Finally, we are in the kernel! Technically, the kernel hasn't run yet. First, the kernel setup part must configure stuff such as the decompressor and some memory management related things, to name a few. After all these things are done, the kernel setup part will decompress the actual kernel and jump to it. Execution of the setup part starts from [arch/x86/boot/header.S](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/header.S) at the [_start](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/header.S#L292) symbol.
It may look a bit strange at first sight, as there are several instructions before it. A long time ago, the Linux kernel had its own bootloader. Now, however, if you run, for example,
Actually, the file `header.S` starts with the magic number [MZ](https://en.wikipedia.org/wiki/DOS_MZ_executable) (see image above), the error message that displays and, following that, the [PE](https://en.wikipedia.org/wiki/Portable_Executable) header:
It needs this to load an operating system with [UEFI](https://en.wikipedia.org/wiki/Unified_Extensible_Firmware_Interface) support. We won't be looking into its inner workings right now but will cover it in upcoming chapters.
The bootloader (GRUB 2 and others) knows about this point (at an offset of `0x200` from `MZ`) and jumps directly to it, despite the fact that `header.S` starts from the `.bstext` section, which prints an error message:
Here we can see a `jmp` instruction opcode (`0xeb`) that jumps to the `start_of_setup-1f` point. In `Nf` notation, `2f`, for example, refers to the local label `2:`. In our case, it's label `1:` that is present right after the jump, and contains the rest of the setup [header](https://github.com/torvalds/linux/blob/v4.16/Documentation/x86/boot.txt#L156). Right after the setup header, we see the `.entrytext` section, which starts at the `start_of_setup` label.
This is the first code that actually runs (aside from the previous jump instructions, of course). After the kernel setup part receives control from the bootloader, the first `jmp` instruction is located at the `0x200` offset from the start of the kernel real mode, i.e., after the first 512 bytes. This can be seen in both the Linux kernel boot protocol and the GRUB 2 source code:
In my case, the kernel is loaded at the physical address `0x10000`. This means that segment registers have the following values after kernel setup starts:
First of all, the kernel ensures that the `ds` and `es` segment registers point to the same address. Next, it clears the direction flag using the `cld` instruction:
As I wrote earlier, `grub2` loads kernel setup code at address `0x10000` by default and `cs` at `0x1020` because execution doesn't start from the start of the file, but from the jump here:
which is at a `512` byte offset from [4d 5a](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/header.S#L46). We also need to align `cs` from `0x1020` to `0x1000`, as well as all other segment registers. After that, we set up the stack:
which pushes the value of `ds` to the stack, followed by the address of the [6](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/header.S#L602) label and executes the `lretw` instruction. When the `lretw` instruction is called, it loads the address of label `6` into the [instruction pointer](https://en.wikipedia.org/wiki/Program_counter) register and loads `cs` with the value of `ds`. Afterward, `ds` and `cs` will have the same values.
Almost all of the setup code is for preparing the C language environment in real mode. The next [step](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/header.S#L575) is checking the `ss` register's value and setting up a correct stack if `ss` is wrong:
Here we set the alignment of `dx` (which contains the value of `sp` as given by the bootloader) to `4` bytes and check if it is zero. If it is, we set `dx` to `0xfffc` (The last 4-byte aligned address in a 64KB segment). If it is not zero, we continue to use the value of `sp` given by the bootloader (`0xf7f4` in my case). Afterwards, we put the value of `ax` (`0x1000`) into `ss`. We now have a correct stack:
* The second scenario, (`ss` != `ds`). First, we put the value of [_end](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/setup.ld) (the address of the end of the setup code) into `dx` and check the `loadflags` header field using the `testb` instruction to see whether we can use the heap. [loadflags](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/header.S#L320) is a bitmask header defined as:
If the `CAN_USE_HEAP` bit is set, we put `heap_end_ptr` into `dx` (which points to `_end`) and add `STACK_SIZE` (the minimum stack size, `1024` bytes) to it. After this, if `dx` is not carried (it will not be carried, `dx = _end + 1024`), jump to label `2` (as in the previous case) and make a correct stack.
The last two steps that need to happen before we can jump to the main C code are setting up the [BSS](https://en.wikipedia.org/wiki/.bss) area and checking the "magic" signature. First, signature checking:
This simply compares the [setup_sig](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/setup.ld) with the magic number `0x5a5aaa55`. If they are not equal, a fatal error is reported.
If the magic number matches, knowing we have a set of correct segment registers and a stack, we only need to set up the BSS section before jumping into the C code.
The BSS section is used to store statically allocated, uninitialized data. Linux carefully ensures this area of memory is first zeroed using the following code:
First, the [__bss_start](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/setup.ld) address is moved into `di`. Next, the `_end + 3` address (+3 - aligns to 4 bytes) is moved into `cx`. The `eax` register is cleared (using the `xor` instruction), and the bss section size (`cx - di`) is calculated and put into `cx`. Then, `cx` is divided by four (the size of a 'word'), and the `stosl` instruction is used repeatedly, storing the value of `eax` (zero) into the address pointed to by `di`, automatically increasing `di` by four, repeating until `cx` reaches zero. The net effect of this code is that zeros are written through all words in memory from `__bss_start` to `_end`:
The `main()` function is located in [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/main.c). You can read about what this does in the next part.
This is the end of the first part about Linux kernel insides. If you have questions or suggestions, ping me on Twitter [0xAX](https://twitter.com/0xAX), drop me an [email](mailto:anotherworldofworld@gmail.com), or just create an [issue](https://github.com/0xAX/linux-internals/issues/new). In the next part, we will see the first C code that executes in the Linux kernel setup, the implementation of memory routines such as `memset`, `memcpy`, `earlyprintk`, early console implementation and initialization, and much more.
**Please note that English is not my first language and I am really sorry for any inconvenience. If you find any mistakes please send me PR to [linux-insides](https://github.com/0xAX/linux-internals).**
