This is the fifth part of the `Kernel booting process` series. We saw transition to the 64-bit mode in the previous [part](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-4.md#transition-to-the-long-mode) and we will continue from this point in this part. We will see the last steps before we jump to the kernel code as preparation for kernel decompression, relocation and directly kernel decompression. So... let's start to dive in the kernel code again.
We stopped right before the jump on the 64-bit entry point - `startup_64` which is located in the [arch/x86/boot/compressed/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/head_64.S) source code file. We already saw the jump to the `startup_64` in the `startup_32`:
in the previous part, `startup_64` starts to work. Since we loaded the new Global Descriptor Table and there was CPU transition in other mode (64-bit mode in our case), we can see the setup of the data segments:
in the beginning of the `startup_64`. All segment registers besides `cs` now point to the `ds` which is `0x18` (if you don't understand why it is `0x18`, read the previous part).
`rbp` contains the decompressed kernel start address and after this code executes `rbx` register will contain address to relocate the kernel code for decompression. We already saw code like this in the `startup_32` ( you can read about it in the previous part - [Calculate relocation address](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-4.md#calculate-relocation-address)), but we need to do this calculation again because the bootloader can use 64-bit boot protocol and `startup_32` just will not be executed in this case.
As you can see above, the `rbx` register contains the start address of the kernel decompressor code and we just put this address with `boot_stack_end` offset to the `rsp` register which represents pointer to the top of the stack. After this step, the stack will be correct. You can find definition of the `boot_stack_end` in the end of [arch/x86/boot/compressed/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/head_64.S) assembly source code file:
It located in the end of the `.bss` section, right before the `.pgtable`. If you will look into [arch/x86/boot/compressed/vmlinux.lds.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/vmlinux.lds.S) linker script, you will find Definition of the `.bss` and `.pgtable` there.
As we set the stack, now we can copy the compressed kernel to the address that we got above, when we calculated the relocation address of the decompressed kernel. Before details, let's look at this assembly code:
First of all we push `rsi` to the stack. We need preserve the value of `rsi`, because this register now stores a pointer to the `boot_params` which is real mode structure that contains booting related data (you must remember this structure, we filled it in the start of kernel setup). In the end of this code we'll restore the pointer to the `boot_params` into `rsi` again.
The next two `leaq` instructions calculates effective addresses of the `rip` and `rbx` with `_bss - 8` offset and put it to the `rsi` and `rdi`. Why do we calculate these addresses? Actually the compressed kernel image is located between this copying code (from `startup_32` to the current code) and the decompression code. You can verify this by looking at the linker script - [arch/x86/boot/compressed/vmlinux.lds.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/vmlinux.lds.S):
And `.rodata..compressed` contains the compressed kernel image. So the `rsi` will contain `rip` relative address of the `_bss - 8` and `rdi` will contain relocation relative address of the `_bss - 8`. As we store these addresses in registers, we put the address of `_bss` to the `rcx` register. As you can see in the `vmlinux.lds.S` linker script, it located in the end of all sections with the setup/kernel code. Now we can start to copy data from the `rsi` to `rdi` by `8` bytes with `movsq` instruction.
Note that there is `std` instruction before data copying, it sets `DF` flag and it means that `rsi` and `rdi` will be decremented or in other words, we will copy bytes in backwards. In the end we clear `DF` flag with `cld` instruction and restore `boot_params` structure to the `rsi`.
In the previous paragraph we saw that the `.text` section starts with the `relocated` label. For the start there is clearing of the `bss` section with:
We need to initialze the `.bss` section, because soon we will jump to the [C](https://en.wikipedia.org/wiki/C_%28programming_language%29) code. Here we just clear `eax`, put RIP relative address of the `_bss` to the `rdi` and `_ebss` to `rcx` and fill it with zeros with `rep stosq` instructions.
Again we save `rsi` with a pointer to the `boot_params` structure and call `decompress_kernel` from the [arch/x86/boot/compressed/misc.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/misc.c) with seven arguments:
*`boot_param` - pointer to the [boot_params](https://github.com/torvalds/linux/blob/master//arch/x86/include/uapi/asm/bootparam.h#L114) structure which is filled by bootloader or during early kernel initialzation;
*`heap` - pointer to the `boot_heap` which represents start address of the early boot heap;
*`input_data` - pointer to the start of the compressed kernel or in other words pointer to the `arch/x86/boot/compressed/vmlinux.bin.bz2`;
*`input_len` - size of the compressed kernel;
*`output` - start address of the future decompressed kernel;
*`output_len` - size of decompressed kernel;
*`run_size` - amount of space needed to run the kernel including `.bss` and `.brk` sections.
All arguments will be passed through the registers according to [System V Application Binary Interface](http://www.x86-64.org/documentation/abi.pdf). We finished all preparation and now can look on the kernel decompression.
As we saw in previous paragraph, the `decompress_kernel` function is defined in the [arch/x86/boot/compressed/misc.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/misc.c) source code file and takes seven arguments. This function starts with the video/console initialization that we already saw in the previous parts. Again, we need to do this because we don't know, do we started in the [real mode](https://en.wikipedia.org/wiki/Real_mode) or a bootloader used 32 or 64-bit boot protocols.
After the first initialization steps, we store pointers to the start of the free memory and to the end of it:
where the `heap` is the second parameter of the `decompress_kernel` function which we got in the [arch/x86/boot/compressed/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/head_64.S):
where the `BOOT_HEAP_SIZE` is macro which expands to the `0x400000` (in a case of `bzip2` kernel and `0x8000` in other cases) value and represents size of the heap.
After heap pointers initialzation, the next step is the call of the `choose_kernel_location` function from [arch/x86/boot/compressed/aslr.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/aslr.c#L298) source code file. As we can understand from the function name it chooses the memory location where the kernel image will be decompressed. I know, it may look weird, that we need to find or even `choose` location where to decompress the compressed kernel image. But actuall the Linux kernel supports [kASLR](https://en.wikipedia.org/wiki/Address_space_layout_randomization) feature which in simple words allows to decompress the kernel into random address for security reasons. Let's open the [arch/x86/boot/compressed/aslr.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/aslr.c#L298) source code file and will look at the `choose_kernel_location` implementation.
At the start `choose_kernel_location` tries to find `kaslr` option in the Linux kernel command line if the `CONFIG_HIBERNATION` is set and `nokaslr` option if this configuration option otherwise:
If the `CONFIG_HIBERNATION` kernel configuration option is enabled during kernel configuration and if there is no `kASLR` option in the Linux kernel command line, we will see `KASLR disabled by default...` output and will jump to the `out` label:
which just returns the `output` parameter which we passed to the `choose_kernel_location` without any changes. In other case, if the `CONFIG_HIBERNATION` kernel configuration option is disabled and the `nokaslr` option is in the kernel command line we do the same that in previous condition.
For now, let's suppose that kernel was configured with enabled randomization and try to understand what `kASLR` is. We can find information about it in the [documentation](https://github.com/torvalds/linux/blob/master/Documentation/kernel-parameters.txt):
It means that we can pass the `kaslr` option to the kernel's command line and get a random address for the decompressed kernel (you can read more about aslr [here](https://en.wikipedia.org/wiki/Address_space_layout_randomization)). So, our current goal is to find random address where we can `safely` to decompress the Linux kernel. I'm not in vain wrote - `safely`. What does it mean in this context? You may remember that besides the code of decompressor and directly the kernel image, there are some unsafe places in memory. For example [initrd](https://en.wikipedia.org/wiki/Initrd) image is in memory too and we must not overlap it by the decompressed kernel.
The next function will help us to find safe place where we can decompress kernel. This function is the - `mem_avoid_init`. It defined in the same source code [file](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/aslr.c) and takes four arguments that we already saw in the `decompress_kernel` function:
*`input_data` - pointer to the start of the compressed kernel or in other words pointer to the `arch/x86/boot/compressed/vmlinux.bin.bz2`;
*`input_len` - size of the compressed kernel;
*`output` - start address of the future decompressed kernel;
*`output_len` - size of decompressed kernel.
The main point of this function is to fill array of the `mem_vector` structures:
Here we can see calculation of the [initrd](http://en.wikipedia.org/wiki/Initrd) start address and size. The `ext_ramdisk_image` is high `32-bits` of the `ramdisk_image` field from the setup header and `ext_ramdisk_size` is high 32-bits of the `ramdisk_size` field from [boot protocol](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt):
0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
...
```
And `ext_ramdisk_image` and `ext_ramdisk_size` you can find in the [Documentation/x86/zero-page.txt](https://github.com/torvalds/linux/blob/master/Documentation/x86/zero-page.txt):
```
Offset Proto Name Meaning
/Size
...
...
...
0C0/004 ALL ext_ramdisk_image ramdisk_image high 32bits
0C4/004 ALL ext_ramdisk_size ramdisk_size high 32bits
So we're taking `ext_ramdisk_image` and `ext_ramdisk_size`, shifting them left on `32` (now they will contain low 32-bits in the high 32-bit bits) and getting start address of the `initrd` and size of it. After this we store these values in the `mem_avoid` array.
The next step after we collected all unsafe memory regions in the `mem_avoid` array will be searching for the random address which does not overlap with the unsafe regions with the `find_random_addr` function. First of all we can see align of the output address in the `find_random_addr` function:
You can remember `CONFIG_PHYSICAL_ALIGN` configuration option from the previous part. This option provides the value to which kernel should be aligned and it is `0x200000` by default. Once we have the aligned output address, we go through the memory regions which we got with the help of the BIOS [e820](https://en.wikipedia.org/wiki/E820) service and collect regions which are good for decompressed kernel image:
Recall that we collected `e820_entries` in the second part of the [Kernel booting process part 2](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-2.md#memory-detection). The `process_e820_entry` function does some checks that an `e820` memory region is not `non-RAM`, that the start address of the memory region is not bigger than maximum allowed `aslr` offset and that memory region is not less than value of kernel alignment:
As we store these values, we align the `region.start` as we did it in the `find_random_addr` function and check that we didn't get an address that is bigger than original memory region:
In the next step we need to get the difference between the original address and aligned and check that if the last address in the memory region is bigger than `CONFIG_RANDOMIZE_BASE_MAX_OFFSET`, we reduce the memory region size so that the end of the kernel image will be less than the maximum `aslr` offset:
In the end we go through all unsafe memory regions and check that each region does not overlap unsafe ares with kernel command line, initrd and etc...:
If the memory region does not overlap unsafe regions we call the `slots_append` function with the start address of the region. `slots_append` function just collects start addresses of memory regions to the `slots` array:
After `process_e820_entry` will be executed, we will have an array of the addresses which are safe for the decompressed kernel. Next we call `slots_fetch_random` function for getting random item from this array:
where `get_random_long` function checks different CPU flags as `X86_FEATURE_RDRAND` or `X86_FEATURE_TSC` and chooses method for getting random number (it can be obtain with RDRAND instruction, Time stamp counter, programmable interval timer and etc...). After retrieving the random address execution of the `choose_kernel_location` is finished.
Now let's back to the [misc.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/misc.c#L404). After getting the address for the kernel image, there need to be some checks to be sure that the retrieved random address is correctly aligned and address is not wrong.
and call the `__decompress` function which will decompress the kernel. The `__decompress` function depends on what decompression algorithm was chosen during kernel compilation:
After kernel will be decompressed, the last two functions are the `parse_elf` and the `handle_relocations`. The main point of these function is to move the uncompressed kernel image to the correct memory place. The fact is that the decompression will decompress compressed part [in-place](https://en.wikipedia.org/wiki/In-place_algorithm) and we still need to move kernel to the correct address. As we already know, the kernel image is [ELF](https://en.wikipedia.org/wiki/Executable_and_Linkable_Format) executable, so the main goal of the `parse_elf` function is to move loadable segments to the correct address. We can see loadable segments in the output of the `readelf` util:
```
readelf -l vmlinux
Elf file type is EXEC (Executable file)
Entry point 0x1000000
There are 5 program headers, starting at offset 64
The goal of the `parse_elf` function is to load these segments to the `output` address that we got from the `choose_kernel_location` function. This function starts from the checkking of the [ELF](https://en.wikipedia.org/wiki/Executable_and_Linkable_Format) signature:
```C
Elf64_Ehdr ehdr;
Elf64_Phdr *phdrs, *phdr;
memcpy(&ehdr, output, sizeof(ehdr));
if (ehdr.e_ident[EI_MAG0] != ELFMAG0 ||
ehdr.e_ident[EI_MAG1] != ELFMAG1 ||
ehdr.e_ident[EI_MAG2] != ELFMAG2 ||
ehdr.e_ident[EI_MAG3] != ELFMAG3) {
error("Kernel is not a valid ELF file");
return;
}
```
and if it does not valid it prints error message and halt. If we got a valid `ELF` file, copy go through all program headers from the given `ELF` file and copies all loadable segments with correct address to the output buffer:
```C
for (i = 0; i <ehdr.e_phnum;i++){
phdr = &phdrs[i];
switch (phdr->p_type) {
case PT_LOAD:
#ifdef CONFIG_RELOCATABLE
dest = output;
dest += (phdr->p_paddr - LOAD_PHYSICAL_ADDR);
#else
dest = (void *)(phdr->p_paddr);
#endif
memcpy(dest,
output + phdr->p_offset,
phdr->p_filesz);
break;
default: /* Ignore other PT_* */ break;
}
}
```
That's all. From now all loadable segments are in the correct place. The last `handle_relocations` function adjusts addresses in the kernel image and called only if the `kASLR` was enabled during kernel configuration.
After the kernel is relocated we return back from the `decompress_kernel` to the [arch/x86/boot/compressed/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/head_64.S). The address of the kernel will be in the `rax` register and we jump to it:
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.
**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).**
