Kernel Loader

The Kernel Loader ("KernelLdr"/"Kernelldr") was added in 8.0.0. It is responsible for applying relocations to the Kernel, and mapping the Kernel's .text/.rodata/.data/.bss at a random slide.

Kernel Loader

KernelLdr is called immediately by the Kernel's crt0 (after it deprivileges from EL2 to EL1, if required), with the following signature:

   void KernelLoader_Main(uintptr_t kernel_base_address, KernelMap *kernel_map, uintptr_t ini1_base_address);

KernelLdr_Main

First, it clears BSS, and then sets SP = <BSS end>.

    for (uint64_t *i = __bss_start; i != __bss_end; i++) {
        *i = 0;
    }
    SP = __bss_end;

Next, it applies relocations to itself and calls its init array.

    KernelLdr_ApplyRelocations(&KernelLdr_Main, __dynamic_start);
    KernelLdr_libc_init_array();

Then, it calls the function which relocates the kernel, and jumps back to the kernel entrypoint.

    // KernelLdr_LoadKernel returns (relocated_kernel_base - original_kernel_base).
    uintptr_t kernel_relocation_offset = KernelLdr_LoadKernel(kernel_base, kernel_map, ini_base);
    
    // dtor called for static page allocator.
    g_InitialPageAllocator.~KInitialPageAllocator();
    
    // Jumps back to the kernel code that called KernelLdr_Main.
    ((void (*)(void))(kernel_relocation_offset + LR))();

KernelLdr_ApplyRelocations

This does standard ELF relocation using .dynamic.

First, it iterates over all entries in .dynamic, extracting .rel.dyn, .rela.dyn, relent, relatent, relcount, relacount from the relevant entries.

Then it does the following two loops to apply R_AARCH64_RELATIVE relocations:

    for (size_t i = 0; i < rel_count; i++) {
        const Elf64_Rel *rel = dyn_rel_start + rel_ent * i;
        while (uint32_t(rel->r_info) != R_AARCH64_RELATIVE) { /* Invalid entry, infloops */ }
        *((Elf64_Addr *)(base_address + rel->r_offset)) += base_address;
    }

    for (size_t i = 0; i < rela_count; i++) {
        const Elf64_Rela *rela = dyn_rela_start + rela_ent * i;
        while (uint32_t(rela->r_info) != R_AARCH64_RELATIVE) { /* Invalid entry, infloops */ }
        *((Elf64_Addr *)(base_address + rela->r_offset)) = base_address + rela->r_addend;
    }

KernelLdr_libc_init_array()

This is just standard libc init array code. .init_array is empty in all available binaries.

KernelLdr_LoadKernel

First, it backs up the original kernel base, and then relocates the kernel for

    // Backup kernel_base argument for use later
    original_kernel_base = kernel_base;
    
    // Move kernel elsewhere in DRAM if needed (unused in practice?)
    // This is maybe to support reserving unused memory for a second OS/hypervisor?
    KernelLdr_RelocateKernelPhysically(&kernel_base, &kernel_map);

TODO: More stuff

KernelLdr_RelocateKernelPhysically

This retrieves memory layout information from the secure monitor, and adjusts the kernel's physical location if necessary.

    adjusted_kernel_base = KernelLdr_GetAdjustedKernelPhysicalBase(*p_kernel_base);

    if (adjusted_kernel_base != *p_kernel_base) {
        // Copy data to adjusted destination
        memmove(adjusted_kernel_base, *p_kernel_base, (*p_kernel_map)->data_end_offset);

        // Adjust pointers.
        kernel_base_diff = adjusted_kernel_base - *p_kernel_base;
        *p_kernel_base = (uintptr_t)*p_kernel_base + kernel_base_diff;
        *p_kernel_map  = (uintptr_t)*p_kernel_map  + kernel_base_diff;
    }

KernelLdr_GetAdjustedKernelPhysicalBase

TODO: secmon calls here

KInitialPageAllocator::KInitialPageAllocator

This sets the allocator's next address to 0 (guessed, since this is done statically in KernelLoader).

    constexpr KInitialPageAllocator::KInitialPageAllocator : next_address(0) {}

KInitialPageAllocator::~KInitialPageAllocator

This just clears the allocator's next address.

    this->next_address = 0;

KInitialPageAllocator::Initialize

This sets the allocator's next address (function inferred as it is (presumably) inlined and next_address is (presumably) private).

    this->next_address = address;

KInitialPageAllocator::Allocate

This linearly allocates a page.

    virtual void *KInitialPageAllocator::Allocate() {
        void *address = reinterpret_cast<void *>(this->next_address);
        if (address == nullptr) {
            // If called on uninitialized allocator, panic by infinite looping
            while (true) {}
        }
        this->next_address += 0x1000;
        memset(address, 0, 0x1000);
        return address;
    }

KInitialPageAllocator::Free

This frees a page (implemented as noop in KernelLoader)

    virtual void KInitialPageAllocator::Free(void *address) {
        // Does Nothing
    }

Structures

KernelMap

Offset	Size	Description
0x0	4	.text offset
0x4	4	.text end offset
0x8	4	.rodata end offset
0xC	4	.rodata end offset
0x10	4	.rwdata offset
0x14	4	.rwdata end offset
0x18	4	.bss offset
0x1C	4	.bss end offset
0x20	4	INI1 load offset
0x24	4	.dynamic end offset
0x28	4	.init_array end offset
0x2C	4	.init_array end offset

KInitialPageAllocator

KInitialPageAllocator is just a simple linear allocator.

Offset	Size	Description
0x0	8	vtable;
0x8	8	Next Address;

KInitialPageAllocator::vtable

Offset	Size	Description
0x0	8	void *(*Allocate)(KInitialPageAllocator *this);
0x8	8	void (*Free)(KInitialPageAllocator *this, void *address);