There are many things to prepare before the kernel compilation can be started. The main point here is to find and configure
The type of compilation, to parse command line arguments that are passed to `make`, etc... So let's dive into the top `Makefile` of the Linux kernel.
the type of compilation, to parse command line arguments that are passed to `make`, etc... So let's dive into the top `Makefile` of Linux kernel.
The Linux kernel top `Makefile` is responsible for building two major products: [vmlinux](https://en.wikipedia.org/wiki/Vmlinux) (the resident kernel image) and the modules (any module files). The [Makefile](https://github.com/torvalds/linux/blob/master/Makefile) of the Linux kernel starts with the definition of the following variables:
The top `Makefile` of Linux kernel is responsible for building two major products: [vmlinux](https://en.wikipedia.org/wiki/Vmlinux) (the resident kernel image) and the modules (any module files). The [Makefile](https://github.com/torvalds/linux/blob/master/Makefile) of the Linux kernel starts with the definition of following variables:
```Makefile
VERSION = 4
@ -30,13 +30,13 @@ EXTRAVERSION = -rc3
NAME = Hurr durr I'ma sheep
```
These variables determine the current version of the Linux kernel and are used in the different places, for example in the forming of the `KERNELVERSION` variable:
These variables determine the current version of Linux kernel and are used in different places, for example in the forming of the `KERNELVERSION` variable in the same `Makefile`:
After this we can see a couple of `ifeq` conditionals that check some of the parameters passed to `make`. The Linux kernel `makefiles` provides a special `make help` target that prints all available targets and some of the command line arguments that can be passed to `make`. For example: `make V=1` - provides verbose builds. The first `ifeq` checks if the `V=n` option is passed to make:
After this we can see a couple of `ifeq` conditions that check some of the parameters passed to `make`. The Linux kernel `makefiles` provides a special `make help` target that prints all available targets and some of the command line arguments that can be passed to `make`. For example : `make V=1` => verbose build. The first `ifeq` checks whether the `V=n` option is passed to `make`:
```Makefile
ifeq ("$(origin V)", "command line")
@ -57,7 +57,7 @@ endif
export quiet Q KBUILD_VERBOSE
```
If this option is passed to `make` we set the `KBUILD_VERBOSE` variable to the value of the`V` option. Otherwise we set the `KBUILD_VERBOSE` variable to zero. After this we check value of the `KBUILD_VERBOSE` variable and set values of the `quiet` and `Q` variables depends on the `KBUILD_VERBOSE` value. The `@` symbols suppress the output of the command and if it is present before a command the output will be something like this: `CC scripts/mod/empty.o` instead of `Compiling .... scripts/mod/empty.o`. In the end we just export all of these variables. The next `ifeq` statement checks that `O=/dir` option was passed to the `make`. This option allows to locate all output files in the given `dir`:
If this option is passed to `make`, we set the `KBUILD_VERBOSE` variable to the value of `V` option. Otherwise we set the `KBUILD_VERBOSE` variable to zero. After this we check the value of `KBUILD_VERBOSE` variable and set values of the `quiet` and `Q` variables depending on the value of `KBUILD_VERBOSE` variable. The `@` symbols suppress the output of command. And if it is present before a command the output will be something like this: `CC scripts/mod/empty.o` instead of `Compiling .... scripts/mod/empty.o`. In the end we just export all of these variables. The next `ifeq` statement checks that `O=/dir` option was passed to the `make`. This option allows to locate all output files in the given `dir`:
We check the `KBUILD_SRC` that represents the top directory of the kernel source code and if it is empty (it is empty when the makefile is executed for the first timea.) We then set the `KBUILD_OUTPUT` variable to the value that passed with the `O` option (if this option was passed). In the next step we check this `KBUILD_OUTPUT` variable and if it is set, we do following things:
We check the `KBUILD_SRC` that represents the top directory of the kernel source code and whether it is empty (it is empty when the makefile is executed for the first time). We then set the `KBUILD_OUTPUT` variable to the value passed with the `O` option (if this option was passed). In the next step we check this `KBUILD_OUTPUT` variable and if it is set, we do following things:
* Store value of the`KBUILD_OUTPUT` in the temp `saved-output` variable;
* Try to create given output directory;
* Check that directory created, in other way print error;
* Store the value of `KBUILD_OUTPUT` in the temporary`saved-output` variable;
* Try to create the given output directory;
* Check that directory created, in other way print error message;
* If the custom output directory was created successfully, execute `make` again with the new directory (see the `-C` option).
The next `ifeq` statements checks that the `C` or `M` options were passed to `make`:
The next `ifeq` statements check that the `C` or `M` options passed to `make`:
The `C` option tells the `makefile` that we need to check all `c` source code with a tool provided by the `$CHECK` environment variable, by default it is [sparse](https://en.wikipedia.org/wiki/Sparse). The second `M` option provides build for the external modules (will not see this case in this part). We also check if the `KBUILD_SRC` variable is set, and if it isn't we set the `srctree` variable to `.`:
The `C` option tells the `makefile` that we need to check all `c` source code with a tool provided by the `$CHECK` environment variable, by default it is [sparse](https://en.wikipedia.org/wiki/Sparse). The second `M` option provides build for the external modules (will not see this case in this part). We also check whether the `KBUILD_SRC` variable is set, and if it isn't, we set the `srctree` variable to `.`:
```Makefile
ifeq ($(KBUILD_SRC),)
@ -118,7 +118,7 @@ obj := $(objtree)
export srctree objtree VPATH
```
That tells to `Makefile` that the kernel source tree will be in the current directory where `make` was executed. We then set `objtree` and other variables to this directory and export them. The next step is the getting value for the `SUBARCH` variable that represents what the underlying architecture is:
That tells `Makefile` that the kernel source tree will be in the current directory where `make` was executed. We then set `objtree` and other variables to this directory and export them. The next step is to get value for the `SUBARCH` variable that represents what the underlying architecture is:
As you can see it executes the [uname](https://en.wikipedia.org/wiki/Uname) util that prints information about machine, operating system and architecture. As it gets the output of `uname`, it parses it and assigns the result to the `SUBARCH` variable. Now that we have `SUBARCH`, we set the `SRCARCH` variable that provides the directory of the certain architecture and `hfr-arch` that provides directory for the header files:
As you can see, it executes the [uname](https://en.wikipedia.org/wiki/Uname) util that prints information about machine, operating system and architecture. As it gets the output of `uname`, it parses the ouput and assigns the result to the `SUBARCH` variable. Now that we have `SUBARCH`, we set the `SRCARCH` variable that provides the directory of the certain architecture and `hfr-arch` that provides the directory for the header files:
```Makefile
ifeq ($(ARCH),i386)
@ -142,7 +142,7 @@ endif
hdr-arch := $(SRCARCH)
```
Note that `ARCH` is an alias for `SUBARCH`. In the next step we set the `KCONFIG_CONFIG` variable that represents path to the kernel configuration file and if it was not set before, it is set to `.config` by default:
Note `ARCH` is an alias for `SUBARCH`. In the next step we set the `KCONFIG_CONFIG` variable that represents path to the kernel configuration file and if it was not set before, it is set to `.config` by default:
Next we get to the `CC` variable that represents compiler too, so why do we need the `HOST*` variables? `CC` is the target compiler that will be used during kernel compilation, but `HOSTCC` will be used during compilation of the set of the `host` programs (we will see it soon). After this we can see definition of the`KBUILD_MODULES` and `KBUILD_BUILTIN` variables that are used to determine what to compile (kernel, modules or both):
Next we get to the `CC` variable that represents compiler too, so why do we need the `HOST*` variables? `CC` is the target compiler that will be used during kernel compilation, but `HOSTCC` will be used during compilation of the set of the `host` programs (we will see it soon). After this we can see the definition of `KBUILD_MODULES` and `KBUILD_BUILTIN` variables that are used to determine what to compile (kernel, modules or both):
Here we can see definition of these variables and the value of the `KBUILD_BUILTIN` will depend on the `CONFIG_MODVERSIONS` kernel configuration parameter if we pass only `modules` to `make`. The next step is including of the:
Here we can see definition of these variables and the value of `KBUILD_BUILTIN` variable will depend on the `CONFIG_MODVERSIONS` kernel configuration parameter if we pass only `modules` to `make`. The next step is to include the `kbuild` file.
```Makefile
include scripts/Kbuild.include
```
`kbuild` file. The [Kbuild](https://github.com/torvalds/linux/blob/master/Documentation/kbuild/kbuild.txt) or `Kernel Build System` is the special infrastructure to manage the build of the kernel and its modules. The `kbuild` files has the same syntax that makefiles do. The [scripts/Kbuild.include](https://github.com/torvalds/linux/blob/master/scripts/Kbuild.include) file provides some generic definitions for the `kbuild` system. As we included this `kbuild` files we can see definition of the variables that are related to the different tools that will be used during kernel and modules compilation (like linker, compilers, utils from the [binutils](http://www.gnu.org/software/binutils/), etc...):
The [Kbuild](https://github.com/torvalds/linux/blob/master/Documentation/kbuild/kbuild.txt) or `Kernel Build System` is the special infrastructure to manage the build of the kernel and its modules. The `kbuild` files has the same syntax that makefiles do. The [scripts/Kbuild.include](https://github.com/torvalds/linux/blob/master/scripts/Kbuild.include) file provides some generic definitions for the `kbuild` system. As we included this `kbuild` files we can see definition of the variables that are related to the different tools that will be used during kernel and modules compilation (like linker, compilers, utils from the [binutils](http://www.gnu.org/software/binutils/), etc...):
ELF (Executable and Linkable Format) is a standard file format for executable files and shared libraries. Linux, as well as, many UNIX-like operating systems uses this format. Let's look on structure of the ELF-64 Object File Format and some defintions in the linux kernel source code related with it.
ELF (Executable and Linkable Format) is a standard file format for executable files, object code, shared libraries, and core dumps. Linux, as well as, many other UNIX-like operating systems uses this format. Let's look on the structure of ELF-64 File Format and some defintions in the linux kernel source code related with it.
An ELF object file consists of the following parts:
An ELF file consists of the following parts:
* ELF header - describes the main characteristics of the object file: type, CPU architecture, the virtual address of the entry point, the size and offset the remaining parts, etc...;
* Program header table - listing the available segments and their attributes. Program header table need loaders for placing sections of the file as virtual memory segments;
* Section header table - contains description of the sections.
* ELF header - describes the main characteristics of the object file: type, CPU architecture, virtual address of the entry point, size and offset of the remaining parts, etc...;
* Program header table - lists the available segments and their attributes. Program header table needs loaders for placing sections of this file as virtual memory segments;
* Section header table - contains the description of sections.
Now let's look closer on these components.
**ELF header**
It's located in the beginning of the object file. It's main point is to locate all other parts of the object file. File header contains following fields:
It's located in the beginning of the object file. Its main point is to locate all other parts of the object file. ELF header contains following fields:
* ELF identification - array of bytes which helps to identify the file as an ELF object file and also provides information about general object file characteristic;
* Object file type - identifies the object file type. This field can describe that ELF file is a relocatable object file, executable file, etc...;
* ELF identification - array of bytes which helps identify this file as an ELF file and also provides information about general object file characteristics;
* Object file type - identifies the object file type. This field can describe whether this file is a relocatable file or executable file, etc...;
* Target architecture;
* Version of the object file format;
* Virtual address of the program entry point;
* File offset of the program header table;
* File offset of the section header table;
* Size of an ELF header;
* Size of a program header table entry;
* Size of the ELF header;
* Size of the program header table entry;
* and other fields...
You can find `elf64_hdr` structure which presents ELF64 header in the linux kernel source code:
@ -47,11 +47,11 @@ typedef struct elf64_hdr {
} Elf64_Ehdr;
```
This structure defined in the [elf.h](https://github.com/torvalds/linux/blob/master/include/uapi/linux/elf.h)
This structure defines in the [elf.h](https://github.com/torvalds/linux/blob/master/include/uapi/linux/elf.h)
**Sections**
All data is stored in sections in an Elf object file. Sections identified by index in the section header table. Section header contains following fields:
All data is stored in sections in an Elf file. Sections are identified by index in the section header table. Section header contains following fields:
* Section name;
* Section type;
@ -64,7 +64,7 @@ All data is stored in sections in an Elf object file. Sections identified by ind
* Address alignment boundary;
* Size of entries, if section has table;
And presented with the following `elf64_shdr` structure in the linux kernel:
And presented with the following `elf64_shdr` structure in the linux kernel source code:
```C
typedef struct elf64_shdr {
@ -83,7 +83,7 @@ typedef struct elf64_shdr {
**Program header table**
All sections are grouped into segments in an executable or shared object file. Program header is an array of structures which describe every segment. It looks like:
All sections are grouped into segments in an executable file or shared library. Program header table is an array of structures which describe every segment. It looks like:
```C
typedef struct elf64_phdr {
@ -98,16 +98,14 @@ typedef struct elf64_phdr {
} Elf64_Phdr;
```
in the linux kernel source code.
`elf64_phdr` structure defines in the same [elf.h](https://github.com/torvalds/linux/blob/master/include/uapi/linux/elf.h).
`elf64_phdr` defined in the same [elf.h](https://github.com/torvalds/linux/blob/master/include/uapi/linux/elf.h).
And ELF object file also contains other fields/structures which you can find in the [Documentation](http://www.uclibc.org/docs/elf-64-gen.pdf). Now let's look on the `vmlinux`.
And ELF file also contains other fields/structures which you can find in the [Documentation](http://www.uclibc.org/docs/elf-64-gen.pdf). Now let's look on the `vmlinux`.
`vmlinux` is relocatable ELF object file too. So we can look at it with the `readelf` util. First of all let's look on a header:
`vmlinux` is an ELF file too. So we can look at it with the `readelf` util. First of all, let's look on the elf header of vmlinux:
```
$ readelf -h vmlinux
@ -144,15 +142,15 @@ ffffffff80000000 - ffffffffa0000000 (=512 MB) kernel text mapping, from phys 0
So we can find it in the `vmlinux` with:
```
readelf -s vmlinux | grep ffffffff81000000
$ readelf -s vmlinux | grep ffffffff81000000
1: ffffffff81000000 0 SECTION LOCAL DEFAULT 1
65099: ffffffff81000000 0 NOTYPE GLOBAL DEFAULT 1 _text
90766: ffffffff81000000 0 NOTYPE GLOBAL DEFAULT 1 startup_64
```
Note that here is address of the`startup_64` routine is not `ffffffff80000000`, but `ffffffff81000000` and now i'll explain why.
Note that ,the address of `startup_64` routine is not `ffffffff80000000`, but `ffffffff81000000`. Now I'll explain why.
We can see following definition in the [arch/x86/kernel/vmlinux.lds.S](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/vmlinux.lds.S):
We can see the following definition in the [arch/x86/kernel/vmlinux.lds.S](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/vmlinux.lds.S):
```
. = __START_KERNEL;
@ -176,10 +174,11 @@ Where `__START_KERNEL` is:
`__START_KERNEL_map` is the value from documentation - `ffffffff80000000` and `__PHYSICAL_START` is `0x1000000`. That's why address of the `startup_64` is `ffffffff81000000`.
And the last we can get program headers from `vmlinux` with the following command:
At last we can get program headers from `vmlinux` with the following command:
0xAX
9 years ago