path: root/boot/fake_syscall.c

#include <kjarna/syscall.h>
#include "fake_syscall.h"
#include "kjarna.h"
#include <bits/x86_64/descriptor.h>
#include <kjarna/interface.h>
#include <posix/unistd.h>
#include <asm/x86_64/msr.h>
#include <bits/x86_64/msr.h>
#include <posix/sys/mman.h>
#include <posix/fcntl.h>

/*
 * Fake syscall mechanism -
 *
 * We have a GDT with entries for supervisory mode. These serve as the
 * entries to satisfy the requirements of the SYSCALL instruction, as
 * for some reason (possibly intentionally) the OVMF GDT is not laid out
 * in a way to make use of the SYSCALL instruction possible.
 *
 * This causes us to have to work inside of constraints during loading time
 * - All "user" mode execution entirely blocks interrupt processing. That
 *   means that "user" mode code must not execute "hlt", or the system will
 *   be locked.
 * - When (if?) user input is required, it is always buffered. It is possible
 *   to simulate unbuffered input, at the cost of one syscall per transfer
 *   from "kernel" side to "user" side. This will cause high input latency.
 *   We will not be running Quake in this environment.
 *
 * These constraints are probably fine, as the loading process only needs to 
 * open files, map memory, etc.
 */

void *exit_fake_syscall_stack;

SYSV_ABI void fake_syscall_entry(void);
SYSV_ABI int64_t fake_syscall_begin(void *stack, void **return_stack_save);
SYSV_ABI int64_t fake_syscall_end(int status, void *stack);

void *stack_alloc(void **stack_pointer, size_t alloc_size)
{
	void *block = *(char **)stack_pointer -= alloc_size;

	// maintain alignment
	*(char **)stack_pointer -= alloc_size % sizeof(size_t);

	return block;
}

struct syscall_context_return_state
{
	uint64_t rflags;
	struct descriptor_table_register_long gdtr;
	uint64_t return_ds;
	uintptr_t return_rip;
	uint64_t return_cs;
};

struct syscall_context
{
	uint64_t pad;
	uint64_t syscall_index;
	syscall_delegate_parameters parameters;
	struct syscall_context_return_state state;
};

static struct
{
	uint16_t cs;
	uint16_t ds;
	uint64_t rflags;
	struct descriptor_table_register_long gdtr;
}
system_context;

static void save_system_context()
{
	__asm__ volatile (
			"movw %%cs, %0\n"
			"movw %%ds, %1\n"
			"pushfq\n"
			"popq %3\n"
			"sgdt %2\n"
			:
				"=m"(system_context.cs),
				"=m"(system_context.ds),
				"=m"(system_context.gdtr),
				"=g"(system_context.rflags)
			:
		);
}

static void restore_system_context()
{
	__asm__ volatile (
			"lgdt %0\n"
			"mov %w1, %%ds\n"
			"mov %w1, %%es\n"
			"mov %w1, %%fs\n"
			"mov %w1, %%gs\n"
			"mov %w1, %%gs\n"
			"mov %w1, %%ss\n"
			"push %q2\n"
			"lea .Lflush(%%rip), %%rax\n"
			"push %%rax\n"
			"lretq\n"
			".Lflush:\n"
			"push %q3\n"
			"popfq\n"
			:
			:
				"m"(system_context.gdtr),
				"m"(system_context.ds),
				"m"(system_context.cs),
				"g"(system_context.rflags)
			:
				"rax"
			);

}

static struct segment_descriptor const fake_syscall_gdt[] =
{
	{ 0 },
	{ 0xffff, 0, 0, 0x9a, 0xaf, 0},
	{ 0xffff, 0, 0, 0x92, 0xcf, 0}
};

static int64_t syscall_delegate_open(syscall_delegate_parameters params)
{
	return open((char *)params[0], (int)params[1], (int)params[2]);
}

static int64_t syscall_delegate_close(syscall_delegate_parameters params)
{
	return close((int)params[0]);
}

static int64_t syscall_delegate_lseek(syscall_delegate_parameters params)
{
	return lseek((int)params[0], (off_t)params[1], (int)params[2]);
}

static int64_t syscall_delegate_read(syscall_delegate_parameters params)
{
	return read((int)params[0], (void *)params[1], (size_t)params[2]);
}

static int64_t syscall_delegate_write(syscall_delegate_parameters params)
{
	return write((int)params[0], (void *)params[1], (size_t)params[2]);
}

static int64_t syscall_delegate_mmap(syscall_delegate_parameters params)
{
	return (int64_t) mmap((void *)params[0], (size_t)params[1], (int)params[2], (int)params[3], (int)params[4], (off_t)params[5]);
}

static int64_t syscall_delegate_munmap(syscall_delegate_parameters params)
{
	return munmap((void *)params[0], (size_t)params[1]);
}

static int64_t syscall_delegate_exit(syscall_delegate_parameters params)
{
	fake_syscall_end((int)params[0], exit_fake_syscall_stack);
	return -1;
}

static syscall_delegate *syscall_handler_delegates[NR_SYSCALLS] =
{
	[SYS_OPEN] = syscall_delegate_open,
	[SYS_CLOSE] = syscall_delegate_close,
	[SYS_LSEEK] = syscall_delegate_lseek,
	[SYS_READ] = syscall_delegate_read,
	[SYS_WRITE] = syscall_delegate_write,
	[SYS_MMAP] = syscall_delegate_mmap,
	[SYS_MUNMAP] = syscall_delegate_munmap,
	[SYS_EXIT] = syscall_delegate_exit
};

SYSV_ABI int64_t fake_syscall_handler(struct syscall_context *context)
{
	int index = (int)context->syscall_index;
	if (index > NR_SYSCALLS)
	{
		// TODO: ENOSYS
		return -1;
	}

	syscall_delegate *delegate = syscall_handler_delegates[index];

	if (delegate == nullptr)
	{
		// TODO: ENOSYS
		return -1;
	}

	restore_system_context();
	int64_t result = delegate(context->parameters);
	save_system_context();
	
	return result;
}

static void install_syscall_handler(void)
{
	union msr_lstar lstar = { (uintptr_t)fake_syscall_entry };
	union msr_star star = { { 0, 1 << 3, 1 << 3 | 3 } };

	msr_write(MSR_INDEX_LSTAR, lstar.value);
	msr_write(MSR_INDEX_STAR, star.value);

	uint64_t efer = msr_read(MSR_INDEX_EFER);
	efer |= 1;
	msr_write(MSR_INDEX_EFER, efer);
}

static void *create_fake_syscall_stack(void *entry_addr, struct linear_buffer *stack_buffer)
{
	stack_buffer->length = KJARNA_BOOT_STACK_SIZE;
	stack_buffer->base = mmap(nullptr, stack_buffer->length, 0, 0, -1, 0);
	void *stack_pointer_head = stack_buffer->base + stack_buffer->length;

	struct syscall_context *context = stack_alloc(&stack_pointer_head, sizeof(*context));

	context->state.rflags = 0;
	context->state.gdtr.base = (uintptr_t)&fake_syscall_gdt;
	context->state.gdtr.limit = sizeof(fake_syscall_gdt) - 1;
	context->state.return_ds = 16;
	context->state.return_cs = 8;
	context->state.return_rip = (uintptr_t)entry_addr;

	return stack_pointer_head;
}

int64_t fake_syscall_start(struct kjarna_boot_image *image)
{
	install_syscall_handler();
	void *fake_stack = create_fake_syscall_stack((void *)image->entry, &image->stack_buffer);
	save_system_context();
	int64_t result = fake_syscall_begin(fake_stack, &exit_fake_syscall_stack);
	return result;
	while(true);
}