full flow implemented

1 files changed, 245 insertions, 0 deletions

diff --git a/boot/fake_syscall.c b/boot/fake_syscall.c
new file mode 100644
index 0000000..e2aaeca
--- /dev/null
+++ b/boot/fake_syscall.c
@@ -0,0 +1,245 @@
+#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)
+{
+	constexpr size_t fake_syscall_stack_size = 0x20000; // 128KiB stack
+	char *stack_pointer_base = mmap(nullptr, fake_syscall_stack_size, 0, 0, -1, 0);
+	void *stack_pointer_head = stack_pointer_base + fake_syscall_stack_size;
+
+	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);
+	save_system_context();
+	int64_t result = fake_syscall_begin(fake_stack, &exit_fake_syscall_stack);
+	return result;
+	while(true);
+}
+