linux-insides/Booting/linux-bootstrap-5.md

Kernel booting process. Part 5.

Kernel decompression

This is the fifth part of the Kernel booting process series. We saw transition to the 64-bit mode in the previous part 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.

Preparation before kernel decompression

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 source code file. We already saw the jump to the startup_64 in the startup_32:

	pushl	$__KERNEL_CS
	leal	startup_64(%ebp), %eax
	...
	...
	...
	pushl	%eax
	...
	...
	...
	lret

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:

	.code64
	.org 0x200
ENTRY(startup_64)
	xorl	%eax, %eax
	movl	%eax, %ds
	movl	%eax, %es
	movl	%eax, %ss
	movl	%eax, %fs
	movl	%eax, %gs

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).

The next step is computation of difference between where the kernel was compiled and where it was loaded:

#ifdef CONFIG_RELOCATABLE
	leaq	startup_32(%rip), %rbp
	movl	BP_kernel_alignment(%rsi), %eax
	decl	%eax
	addq	%rax, %rbp
	notq	%rax
	andq	%rax, %rbp
	cmpq	$LOAD_PHYSICAL_ADDR, %rbp
	jge	1f
#endif
	movq	$LOAD_PHYSICAL_ADDR, %rbp
1:
	leaq	z_extract_offset(%rbp), %rbx

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), 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.

In the next step we can see setup of the stack and reset of the flags register:

	leaq	boot_stack_end(%rbx), %rsp

 	pushq	$0
	popfq

As you can see above rbx register contains the start address of the decompressing kernel code and we just put this address with boot_stack_end offset to the rsp register. After this stack will be correct. You can find definition of the boot_stack_end in the end of compressed/head_64.S file:

	.bss
	.balign 4
boot_heap:
	.fill BOOT_HEAP_SIZE, 1, 0
boot_stack:
	.fill BOOT_STACK_SIZE, 1, 0
boot_stack_end:

It located in the .bss section right before .pgtable. You can look at arch/x86/boot/compressed/vmlinux.lds.S to find it.

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. Let's look at this code:

	pushq	%rsi
	leaq	(_bss-8)(%rip), %rsi
	leaq	(_bss-8)(%rbx), %rdi
	movq	$_bss, %rcx
	shrq	$3, %rcx
	std
	rep	movsq
	cld
	popq	%rsi

First of all we push rsi to the stack. We need save the value of rsi, because this register now stores a pointer to the boot_params real mode structure (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 address 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:

	. = 0;
	.head.text : {
		_head = . ;
		HEAD_TEXT
		_ehead = . ;
	}
	.rodata..compressed : {
		*(.rodata..compressed)
	}
	.text :	{
		_text = .; 	/* Text */
		*(.text)
		*(.text.*)
		_etext = . ;
	}

Note that .head.text section contains startup_32. You can remember it from the previous part:

	__HEAD
	.code32
ENTRY(startup_32)
...
...
...

.text section contains decompression code:

assembly

	.text
relocated:
...
...
...
/*
 * Do the decompression, and jump to the new kernel..
 */
...

And .rodata..compressed contains the compressed kernel image.

So 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 register, we put the address of _bssto thercxregister. As you can see in thevmlinux.lds.S, it located in the end of all sections with the setup/kernel code. Now we can start to copy data from rsitordiby 8 bytes withmovsq` 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.

After this we get .text section address and jump to it:

	leaq	relocated(%rbx), %rax
	jmp	*%rax

Last preparation before kernel decompression

The .text section starts with the relocated label. For the start there is clearing of the bss section with:

	xorl	%eax, %eax
	leaq    _bss(%rip), %rdi
	leaq    _ebss(%rip), %rcx
	subq	%rdi, %rcx
	shrq	$3, %rcx
	rep	stosq

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.

In the end we can see the call of the decompress_kernel routine:

	pushq	%rsi
	movq	$z_run_size, %r9
	pushq	%r9
	movq	%rsi, %rdi
	leaq	boot_heap(%rip), %rsi
	leaq	input_data(%rip), %rdx
	movl	$z_input_len, %ecx
	movq	%rbp, %r8
	movq	$z_output_len, %r9
	call	decompress_kernel
	popq	%r9
	popq	%rsi

Again we save rsi with a pointer to the boot_params structure and call decompress_kernel from the arch/x86/boot/compressed/misc.c with seven arguments. All arguments will be passed through the registers. We finished all preparation and now can look on the kernel decompression.

Kernel decompression

As I wrote above, decompress_kernel function is in the arch/x86/boot/compressed/misc.c source code file. This function starts with the video/console initialization that we saw in the previous parts. This call needs to know if bootloader used 32 or 64-bit protocols. After this we store pointers to the start of the free memory and to the end of it:

	free_mem_ptr     = heap;
	free_mem_end_ptr = heap + BOOT_HEAP_SIZE;

where heap is the second parameter of the decompress_kernel function which we got with:

leaq	boot_heap(%rip), %rsi

As you saw above, boot_heap is defined as:

boot_heap:
	.fill BOOT_HEAP_SIZE, 1, 0

where BOOT_HEAP_SIZE is 0x400000 if the kernel compressed with bzip2 or 0x8000 if not.

In the next step we call the choose_kernel_location function from arch/x86/boot/compressed/aslr.c. As we can understand from the function name it chooses the memory location where the kernel image will be decompressed. Let's look at this function.

At the start choose_kernel_location tries to find kaslr option in the command line if CONFIG_HIBERNATION is set and nokaslr option if this configuration option CONFIG_HIBERNATION is not set:

#ifdef CONFIG_HIBERNATION
	if (!cmdline_find_option_bool("kaslr")) {
		debug_putstr("KASLR disabled by default...\n");
		goto out;
	}
#else
	if (cmdline_find_option_bool("nokaslr")) {
		debug_putstr("KASLR disabled by cmdline...\n");
		goto out;
	}
#endif

If there is no kaslr or nokaslr in the command line it jumps to out label:

out:
	return (unsigned char *)choice;

which just returns the output parameter which we passed to the choose_kernel_location without any changes. Let's try to understand what kaslr is. We can find information about it in the documentation:

kaslr/nokaslr [X86]

Enable/disable kernel and module base offset ASLR
(Address Space Layout Randomization) if built into
the kernel. When CONFIG_HIBERNATION is selected,
kASLR is disabled by default. When kASLR is enabled,
hibernation will be disabled.

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).

Let's consider the case when kernel's command line contains the kaslr option.

There is the call of the mem_avoid_init function from the same aslr.c source code file. This function gets the unsafe memory regions (initrd, kernel command line and etc...). We need to know about these memory regions to not overlap them with the kernel after decompression. For example:

	initrd_start  = (u64)real_mode->ext_ramdisk_image << 32;
	initrd_start |= real_mode->hdr.ramdisk_image;
	initrd_size  = (u64)real_mode->ext_ramdisk_size << 32;
	initrd_size |= real_mode->hdr.ramdisk_size;
	mem_avoid[1].start = initrd_start;
	mem_avoid[1].size = initrd_size;

Here we can see calculation of the initrd start address and size. ext_ramdisk_image is high 32-bits of the ramdisk_image field from boot header and ext_ramdisk_size is high 32-bits of the ramdisk_size field from boot protocol:

Offset	Proto	Name		Meaning
/Size
...
...
...
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:

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 which is defined as:

#define MEM_AVOID_MAX 5
static struct mem_vector mem_avoid[MEM_AVOID_MAX];

where mem_vector structure is:

struct mem_vector {
	unsigned long start;
	unsigned long size;
};

The next step after we collect 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:

minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);

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 and collect regions which are good for decompressed kernel image:

for (i = 0; i < real_mode->e820_entries; i++) {
	process_e820_entry(&real_mode->e820_map[i], minimum, size);
}

Recall that we collected e820_entries in the second part of the Kernel booting process part 2.

First of all process_e820_entry function does some checks that 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:

struct mem_vector region, img;

if (entry->type != E820_RAM)
	return;

if (entry->addr >= CONFIG_RANDOMIZE_BASE_MAX_OFFSET)
	return;

if (entry->addr + entry->size < minimum)
	return;

After this, we store e820 memory region start address and the size in the mem_vector structure (we saw definition of this structure above):

region.start = entry->addr;
region.size = entry->size;

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:

region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);

if (region.start > entry->addr + entry->size)
	return;

Next we 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:

region.size -= region.start - entry->addr;

if (region.start + region.size > CONFIG_RANDOMIZE_BASE_MAX_OFFSET)
		region.size = CONFIG_RANDOMIZE_BASE_MAX_OFFSET - region.start;

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...:

for (img.start = region.start, img.size = image_size ;
	     mem_contains(&region, &img) ;
	     img.start += CONFIG_PHYSICAL_ALIGN) {
		if (mem_avoid_overlap(&img))
			continue;
		slots_append(img.start);
	}

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:

	slots[slot_max++] = addr;

which is defined as:

static unsigned long slots[CONFIG_RANDOMIZE_BASE_MAX_OFFSET /
			   CONFIG_PHYSICAL_ALIGN];
static unsigned long slot_max;

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:

if (slot_max == 0)
	return 0;

return slots[get_random_long() % slot_max];

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. 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.

After all these checks will see the familiar message:

Decompressing Linux... 

and call decompress function which will decompress the kernel. decompress function depends on what decompression algorithm was chosen during kernel compilation:

#ifdef CONFIG_KERNEL_GZIP
#include "../../../../lib/decompress_inflate.c"
#endif

#ifdef CONFIG_KERNEL_BZIP2
#include "../../../../lib/decompress_bunzip2.c"
#endif

#ifdef CONFIG_KERNEL_LZMA
#include "../../../../lib/decompress_unlzma.c"
#endif

#ifdef CONFIG_KERNEL_XZ
#include "../../../../lib/decompress_unxz.c"
#endif

#ifdef CONFIG_KERNEL_LZO
#include "../../../../lib/decompress_unlzo.c"
#endif

#ifdef CONFIG_KERNEL_LZ4
#include "../../../../lib/decompress_unlz4.c"
#endif

After kernel will be decompressed, the last function handle_relocations will relocate the kernel to the address that we got from choose_kernel_location. After the kernel is relocated we return from the decompress_kernel to head_64.S. The address of the kernel will be in the rax register and we jump to it:

jmp	*%rax

That's all. Now we are in the kernel!

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 insides.

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.

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.

Links

• address space layout randomization
• initrd
• long mode
• bzip2
• RDdRand instruction
• Time Stamp Counter
• Programmable Interval Timers
• Previous part