diff options
Diffstat (limited to 'boot/main.c')
| -rw-r--r-- | boot/main.c | 198 |
1 files changed, 2 insertions, 196 deletions
diff --git a/boot/main.c b/boot/main.c index ed18578..55eab00 100644 --- a/boot/main.c +++ b/boot/main.c @@ -1,196 +1,5 @@ #include "kjarna.h" -#include <stdnoreturn.h> -#include <asm/x86_64/msr.h> -#include <bits/x86_64/msr.h> -#include <bits/x86_64/descriptor.h> -#include <libc/stdio.h> -#include <posix/sys/mman.h> -#include <posix/unistd.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 *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; -} - -static struct segment_descriptor const fake_syscall_gdt[] = -{ - { 0 }, - { 0xffff, 0, 0, 0x9a, 0xaf, 0}, - { 0xffff, 0, 0, 0x92, 0xcf, 0} -}; - -typedef uint64_t syscall_delegate_parameters[6]; - -static struct -{ - uint16_t cs; - uint16_t ds; - uint64_t rflags; - struct descriptor_table_register_long gdtr; -} -uefi_context; - -static void save_uefi_context() -{ - __asm__ volatile ( - "movw %%cs, %0\n" - "movw %%ds, %1\n" - "pushfq\n" - "popq %3\n" - "sgdt %2\n" - : - "=m"(uefi_context.cs), - "=m"(uefi_context.ds), - "=m"(uefi_context.gdtr), - "=g"(uefi_context.rflags) - : - ); -} - -static void restore_uefi_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"(uefi_context.gdtr), - "m"(uefi_context.ds), - "m"(uefi_context.cs), - "g"(uefi_context.rflags) - : - "rax" - ); - -} - -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; -}; - - -typedef int64_t (syscall_delegate)(syscall_delegate_parameters params); - -int64_t syscall_delegate_write(syscall_delegate_parameters params) -{ - return write((int)params[0], (void *)params[1], (size_t)params[2]); -} - -constexpr int NR_SYSCALLS = 10; -static syscall_delegate *syscall_handler_delegates[NR_SYSCALLS] = -{ - nullptr, - syscall_delegate_write -}; - -SYSV_ABI int64_t fake_syscall_handler(struct syscall_context *context) -{ - if (context->syscall_index > NR_SYSCALLS) - { - // TODO: ENOSYS - return -1; - } - - syscall_delegate *delegate = syscall_handler_delegates[context->syscall_index]; - - if (delegate == nullptr) - { - // TODO: ENOSYS - return -1; - } - - restore_uefi_context(); - int64_t result = delegate(context->parameters); - save_uefi_context(); - - return result; -} - -SYSV_ABI void fake_syscall_entry(void); -SYSV_ABI void fake_syscall_start(void *stack); - -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; -} +#include "fake_syscall.h" int main(int argc, char **argv) { @@ -198,9 +7,6 @@ int main(int argc, char **argv) (void)argv; struct kjarna_boot_image boot_image = get_boot_image(); - install_syscall_handler(); - void *fake_syscall_stack = create_fake_syscall_stack((void *)boot_image.entry); - save_uefi_context(); - fake_syscall_start(fake_syscall_stack); + fake_syscall_start(&boot_image); } |