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24
Booting/linux-bootstrap-1.md
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@ -1,10 +1,10 @@
Kernel booting process. Part 1. Kernel booting process. Part 1.
================================================================================ ================================================================================
From the bootloader to kernel From the bootloader to the kernel
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------
If you have read my previous [blog posts](http://0xax.blogspot.com/search/label/asm), you can see that sometime ago I started to get involved with low-level programming. I wrote some posts about x86_64 assembly programming for Linux. At the same time, I started to dive into the Linux 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 on low-level and many many other things. So, I decided to write yet another series of posts about the Linux kernel for **x86_64**. If you have read my previous [blog posts](http://0xax.blogspot.com/search/label/asm), you can see that sometime ago I started to get involved with low-level programming. I wrote some posts about x86_64 assembly programming for Linux. At the same time, I started to dive into the Linux 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 **x86_64**.
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](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 [linux-insides](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. 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](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 [linux-insides](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.
@ -58,7 +58,7 @@ which is 65519 bytes over first megabyte. Since only one megabyte is accessible
Ok, now we know about real mode and memory addressing. Let's get back to discuss about register values after reset: Ok, now we know about real mode and memory addressing. Let's get back to discuss about register values after reset:
`CS` register consists of two parts: the visible segment selector and hidden base address. We know predefined `CS` base and `IP` value, so the logical address will be: The `CS` register consists of two parts: the visible segment selector and hidden base address. We know the predefined `CS` base and `IP` value, so the logical address will be:
``` ```
0xffff0000:0xfff0 0xffff0000:0xfff0
@ -97,7 +97,7 @@ SECTIONS {
} }
``` ```
Now the BIOS starts: after initializing and checking the hardware, it needs to find a bootable device. A boot order is stored in the BIOS configuration, controlling which devices the kernel 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, the boot sector is stored in the first 446 bytes of the first sector (which is 512 bytes). The final two bytes of the first sector are `0x55` and `0xaa`, which signals the BIOS that this device is bootable. For example: Now the BIOS starts: after initializing and checking the hardware, it 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, the boot sector is stored in the first 446 bytes of the first sector (which is 512 bytes). The final two bytes of the first sector are `0x55` and `0xaa`, which signals the BIOS that this device is bootable. For example:
```assembly ```assembly
; ;
@ -127,7 +127,7 @@ Build and run it with:
nasm -f bin boot.nasm && qemu-system-x86_64 boot nasm -f bin boot.nasm && qemu-system-x86_64 boot
``` ```
This will instruct [QEMU](http://qemu.org) to use the `boot` binary we just built as a disk image. Since the binary generated by the assembly code above fulfills the requirements of the boot sector (the origin is set to `0x7c00`, and we end with the magic sequence), QEMU will treat the binary as the master boot record(MBR) of a disk image. This will instruct [QEMU](http://qemu.org) to use the `boot` binary we just built as a disk image. Since the binary generated by the assembly code above fulfills the requirements of the boot sector (the origin is set to `0x7c00`, and we end with the magic sequence), QEMU will treat the binary as the master boot record (MBR) of a disk image.
You will see: You will see:
@ -144,7 +144,7 @@ objdump -D -b binary -mi386 -Maddr16,data16,intel boot
A real-world boot sector has code to continue the boot process and the partition table instead of a bunch of 0's and an exclamation mark :) From this point onwards, BIOS hands over control to the bootloader. A real-world boot sector has code to continue the boot process and the partition table instead of a bunch of 0's and an exclamation mark :) From this point onwards, BIOS hands over control to the bootloader.
**NOTE**: As you can read above the CPU is in real mode. In real mode, calculating the physical address in memory is done as following: **NOTE**: As you can read above the CPU is in real mode. In real mode, calculating the physical address in memory is done as follows:
``` ```
PhysicalAddress = Segment * 16 + Offset PhysicalAddress = Segment * 16 + Offset
@ -241,7 +241,7 @@ So when the bootloader transfers control to the kernel, it starts at:
0x1000 + X + sizeof(KernelBootSector) + 1 0x1000 + X + sizeof(KernelBootSector) + 1
``` ```
where `X` is the address of the kernel bootsector loaded. In my case `X` is `0x10000`, as we can see in a memory dump: where `X` is the address of the kernel boot sector loaded. In my case `X` is `0x10000`, as we can see in a memory dump:
![kernel first address](http://oi57.tinypic.com/16bkco2.jpg) ![kernel first address](http://oi57.tinypic.com/16bkco2.jpg)
@ -458,27 +458,27 @@ The BSS section is used to store statically allocated, uninitialized data. Linux
rep; stosl rep; stosl
``` ```
First of all the [__bss_start](https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L47) address is moved into `di` and the `_end + 3` address (+3 - aligns to 4 bytes) is moved into `cx`. The `eax` register is cleared (using an `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 repeatedly used, storing the value of `eax` (zero) into the address pointed to by `di`, automatically increasing `di` by four (this occurs 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`: First of all the [__bss_start](https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L47) address is moved into `di` and the `_end + 3` address (+3 - aligns to 4 bytes) is moved into `cx`. The `eax` register is cleared (using a `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 repeatedly used, storing the value of `eax` (zero) into the address pointed to by `di`, automatically increasing `di` by four (this occurs 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`:
![bss](http://oi59.tinypic.com/29m2eyr.jpg) ![bss](http://oi59.tinypic.com/29m2eyr.jpg)
Jump to main Jump to main
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------
That's all, we have the stack, BSS so we can jump to the `main()` C function: That's all, we have the stack and BSS so we can jump to the `main()` C function:
```assembly ```assembly
calll main calll main
``` ```
The `main()` function is located in [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c). What this does, you can read in the next part. The `main()` function is located in [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c). You can read about what this does in the next part.
Conclusion Conclusion
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------
This is the end of the first part about Linux kernel internals. If you have questions or suggestions, ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](anotherworldofworld@gmail.com) or just create [issue](https://github.com/0xAX/linux-internals/issues/new). In the next part we will see first C code which executes in Linux kernel setup, implementation of memory routines as `memset`, `memcpy`, `earlyprintk` implementation and early console initialization and many more. This is the end of the first part about Linux kernel insides. If you have questions or suggestions, ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](anotherworldofworld@gmail.com) or just create [issue](https://github.com/0xAX/linux-internals/issues/new). In the next part we will see first C code which executes in Linux kernel setup, implementation of memory routines as `memset`, `memcpy`, `earlyprintk` implementation and early console initialization and many more.
**Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me PR to [linux-internals](https://github.com/0xAX/linux-internals).** **Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me PR to [linux-insides](https://github.com/0xAX/linux-internals).**
Links Links
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------

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Booting/linux-bootstrap-2.md
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@ -4,7 +4,7 @@ Kernel booting process. Part 2.
First steps in the kernel setup First steps in the kernel setup
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------
We started to dive into linux kernel internals in the previous [part](linux-bootstrap-1.md) and saw the initial part of the kernel setup code. We stopped at the first call to the `main` function (which is the first function written in C) from [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c). We started to dive into linux kernel insides in the previous [part](linux-bootstrap-1.md) and saw the initial part of the kernel setup code. We stopped at the first call to the `main` function (which is the first function written in C) from [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c).
In this part we will continue to research the kernel setup code and In this part we will continue to research the kernel setup code and
* see what `protected mode` is, * see what `protected mode` is,
@ -522,11 +522,11 @@ where `0x80` is the first hard drive and the value of `EDD_MBR_SIG_MAX` macro is
Conclusion Conclusion
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------
This is the end of the second part about Linux kernel internals. In the next part we will see video mode setting and the rest of preparations before transition to protected mode and directly transitioning into it. This is the end of the second part about Linux kernel insides. In the next part we will see video mode setting and the rest of preparations before transition to protected mode and directly transitioning into it.
If you have any questions or suggestions write me a comment or ping me at [twitter](https://twitter.com/0xAX). If you have any questions or suggestions write me a comment or ping me at [twitter](https://twitter.com/0xAX).
**Please note that English is not my first language, And I am really sorry for any inconvenience. If you found any mistakes please send me a PR to [linux-internals](https://github.com/0xAX/linux-internals).** **Please note that English is not my first language, And I am really sorry for any inconvenience. If you found any mistakes please send me a PR to [linux-insides](https://github.com/0xAX/linux-internals).**
Links Links
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------

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Booting/linux-bootstrap-3.md
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@ -567,11 +567,11 @@ That's all. We're in the protected mode and stop at it's entry point. We will se
Conclusion Conclusion
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------
This is the end of the third part about linux kernel internals. In next part we will see first steps in the protected mode and transition into the [long mode](http://en.wikipedia.org/wiki/Long_mode). This is the end of the third part about linux kernel insides. In next part we will see first steps in the protected mode and transition into the [long mode](http://en.wikipedia.org/wiki/Long_mode).
If you have any questions or suggestions write me a comment or ping me at [twitter](https://twitter.com/0xAX). If you have any questions or suggestions write me a comment or ping me at [twitter](https://twitter.com/0xAX).
**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 a PR with corrections at [linux-internals](https://github.com/0xAX/linux-internals).** **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 a PR with corrections at [linux-insides](https://github.com/0xAX/linux-internals).**
Links Links
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------

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Booting/linux-bootstrap-4.md
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@ -153,7 +153,7 @@ and if `KEEP_SEGMENTS` is not set, we need to set `ds`, `ss` and `es` registers
movl %eax, %ss movl %eax, %ss
``` ```
remember that `__BOOT_DS` is `0x18` (index of data segment in the Global Descriptor Table). If `KEEP_SEGMENTS` is not set, we jump to the label `1f` or update segment registers with `__BOOT_DS` if this flag is set. remember that `__BOOT_DS` is `0x18` (index of data segment in the Global Descriptor Table). If `KEEP_SEGMENTS` is set, we jump to the label `1f` or update segment registers with `__BOOT_DS` if this flag is not set.
If you read the previous [part](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-3.md), you can remember that we already updated segment registers in the [arch/x86/boot/pmjump.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/pmjump.S), so why do we need to set up it again? Actually linux kernel also has the 32-bit boot protocol, so `startup_32` can be the first function which will be executed right after a bootloader transfers control to the kernel. If you read the previous [part](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-3.md), you can remember that we already updated segment registers in the [arch/x86/boot/pmjump.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/pmjump.S), so why do we need to set up it again? Actually linux kernel also has the 32-bit boot protocol, so `startup_32` can be the first function which will be executed right after a bootloader transfers control to the kernel.
@ -482,11 +482,11 @@ That's all!
Conclusion Conclusion
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------
This is the end of the fourth part linux kernel booting process. If you have questions or suggestions, ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](anotherworldofworld@gmail.com) or just create an [issue](https://github.com/0xAX/linux-internals/issues/new). This is the end of the fourth part linux kernel booting process. If you have questions or suggestions, ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](anotherworldofworld@gmail.com) or just create an [issue](https://github.com/0xAX/linux-insides/issues/new).
In the next part we will see kernel decompression and many more. In the next part we will see kernel decompression and many more.
**Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me PR to [linux-internals](https://github.com/0xAX/linux-internals).** **Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me PR to [linux-insides](https://github.com/0xAX/linux-internals).**
Links Links
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------

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Booting/linux-bootstrap-5.md
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@ -457,13 +457,13 @@ That's all. Now we are in the kernel!
Conclusion Conclusion
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------
This is the end of the fifth and the last part about linux kernel booting process. We will not see posts about kernel booting anymore (maybe only updates in this and previous posts), but there will be many posts about other kernel internals. This is the end of the fifth and the last part about linux kernel booting process. We will not see posts about kernel booting anymore (maybe only updates in this and previous posts), but there will be many posts about other kernel insides.
Next chapter will be about kernel initialization and we will see the first steps in the linux kernel initialization code. Next chapter will be about kernel initialization and we will see the first steps in the linux kernel initialization code.
If you will have any questions or suggestions write me a comment or ping me in [twitter](https://twitter.com/0xAX). If you will have any questions or suggestions write me a comment or ping me in [twitter](https://twitter.com/0xAX).
**Please note that English is not my first language, And I am really sorry for any inconvenience. If you will find any mistakes please send me PR to [linux-internals](https://github.com/0xAX/linux-internals).** **Please note that English is not my first language, And I am really sorry for any inconvenience. If you will find any mistakes please send me PR to [linux-insides](https://github.com/0xAX/linux-internals).**
Links Links
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------

10
Misc/contribute.md
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@ -293,7 +293,7 @@ $ git pull upstream master
After this my local repository with the Linux kernel source code is synced with the [mainline](https://github.com/torvalds/linux) repository. Now we can make some changes in the source code. As I already wrote, I have no advice for you where you can start and what `TODO` in the Linux kernel. But the best place for newbies is `staging` tree. In other words the set of drivers from the [drivers/staging](https://github.com/torvalds/linux/tree/master/drivers/staging). The maintainer of the `staging` tree is [Greg Kroah-Hartman](https://en.wikipedia.org/wiki/Greg_Kroah-Hartman) and the `staging` tree is that place where your trivial patch can be accepted. Let's look on a simple example that describes how to generate patch, check it and send to the [Linux kernel mail listing](https://lkml.org/). After this my local repository with the Linux kernel source code is synced with the [mainline](https://github.com/torvalds/linux) repository. Now we can make some changes in the source code. As I already wrote, I have no advice for you where you can start and what `TODO` in the Linux kernel. But the best place for newbies is `staging` tree. In other words the set of drivers from the [drivers/staging](https://github.com/torvalds/linux/tree/master/drivers/staging). The maintainer of the `staging` tree is [Greg Kroah-Hartman](https://en.wikipedia.org/wiki/Greg_Kroah-Hartman) and the `staging` tree is that place where your trivial patch can be accepted. Let's look on a simple example that describes how to generate patch, check it and send to the [Linux kernel mail listing](https://lkml.org/).
If we will look in the driver for the [Digi International EPCA PCI](https://github.com/torvalds/linux/tree/master/drivers/staging/dgap) based devices, we will see the `dgap_sindex` function on line 295: If we look in the driver for the [Digi International EPCA PCI](https://github.com/torvalds/linux/tree/master/drivers/staging/dgap) based devices, we will see the `dgap_sindex` function on line 295:
```C ```C
static char *dgap_sindex(char *string, char *group) static char *dgap_sindex(char *string, char *group)
@ -369,7 +369,7 @@ We've passed name of the branch (`master` in this case) to the `format-patch` co
$ git format-patch master --stdout > dgap-patch-1.patch $ git format-patch master --stdout > dgap-patch-1.patch
``` ```
The last step after we have generated our patch is just to send it to the Linux kernel mail listing. Of course you can use any email client, but the `Git` provides special command for this: `git send-email`. Before you will send your patch, you need to know where to send it. Yes, you can send it just to the Linux kernel mail listing address which is `linux-kernel@vger.kernel.org`, but there is a high probability that the patch will be ignored, because as you may already know there is the large flow of messages on the Linux kernel mail listing. The better way will be send to a maintainer of subsystem where you have made changes. We can find maintainer and other related guys who has touched the code with the `get_maintainer.pl` script. All of you need is just pass file or directory where you wrote a code. Go to the root directory with source code of the Linux kernel and execute it: The last step after we have generated our patch is to send it to the Linux kernel mailing list. Of course, you can use any email client, `git` provides a special command for this: `git send-email`. Before you send your patch, you need to know where to send it. Yes, you can just send it to the Linux kernel mailing list address which is `linux-kernel@vger.kernel.org`, but it is very likely that the patch will be ignored, because of the large flow of messages. The better choice would be to send the patch to the maintainers of the subsystem where you have made changes. To find the names of these maintainers use the `get_maintainer.pl` script. All you need to do is pass the file or directory where you wrote code.
``` ```
$ ./scripts/get_maintainer.pl -f drivers/staging/dgap/dgap.c $ ./scripts/get_maintainer.pl -f drivers/staging/dgap/dgap.c
@ -394,7 +394,7 @@ $ git send-email --to "Lidza Louina <lidza.louina@gmail.com>" \
--cc "linux-kernel@vger.kernel.org" --cc "linux-kernel@vger.kernel.org"
``` ```
That's all. The patch is sent and now only have to wait feedback from the Linux kernel developers. After you will sent a patch and a maintainer accepted it, you will find it in the maintainer's repository (for example [patch](https://git.kernel.org/cgit/linux/kernel/git/gregkh/staging.git/commit/?h=staging-testing&id=b9f7f1d0846f15585b8af64435b6b706b25a5c0b) that you saw in this part) and after some time a maintainer will send pull request to Linus and you will see your patch in the mainline repository. That's all. The patch is sent and now you only have to wait for feedback from the Linux kernel developers. After you send a patch and a maintainer accepts it, you will find it in the maintainer's repository (for example [patch](https://git.kernel.org/cgit/linux/kernel/git/gregkh/staging.git/commit/?h=staging-testing&id=b9f7f1d0846f15585b8af64435b6b706b25a5c0b) that you saw in this part) and after some time the maintainer will send a pull request to Linus and you will see your patch in the mainline repository.
That's all. That's all.
@ -403,7 +403,7 @@ Some advice
In the end of this part I want to give you some advice that will describe what to do and what not to do during development of the Linux kernel: In the end of this part I want to give you some advice that will describe what to do and what not to do during development of the Linux kernel:
* Think, Think, Think. And think again before you decided to send a patch. * Think, Think, Think. And think again before you decide to send a patch.
* Each time when you have changed something in the Linux kernel source code - compile it. After any changes. Again and again. Nobody likes changes that don't even compile. * Each time when you have changed something in the Linux kernel source code - compile it. After any changes. Again and again. Nobody likes changes that don't even compile.
@ -452,7 +452,7 @@ It's important that your email be in the [plain text](https://en.wikipedia.org/w
[PATCH v2] staging/dgap: Use strpbrk() instead of dgap_sindex() [PATCH v2] staging/dgap: Use strpbrk() instead of dgap_sindex()
``` ```
Also it must contain changelog that will describe all changes changes from previous patch versions. Of course, this is not an exhaustive list of requirements for Linux kernel development, but some of the most important items were addressed. Also it must contain a changelog that describes all changes from previous patch versions. Of course, this is not an exhaustive list of requirements for Linux kernel development, but some of the most important items were addressed.
Happy Hacking! Happy Hacking!