Luna/kernel/src/sys/exec.cpp

#define MODULE "exec"

#include "assert.h"
#include "errno.h"
#include "interrupts/Interrupts.h"
#include "memory/MemoryManager.h"
#include "std/stdlib.h"
#include "std/string.h"
#include "sys/elf/ELFLoader.h"
#include "thread/Scheduler.h"

void sys_exec(Context* context, const char* pathname)
{
    if (!pathname)
    {
        context->rax = -EINVAL; // FIXME: This should probably return EFAULT.
        return;
    }

    kinfoln("exec(): executing %s", pathname);

    VFS::Node* program = VFS::resolve_path(pathname);
    if (!program)
    {
        context->rax = -ENOENT;
        return;
    }

    uint64_t allocated_stack = (uint64_t)MemoryManager::get_pages(TASK_PAGES_IN_STACK, MAP_READ_WRITE | MAP_USER);
    if (!allocated_stack)
    {
        context->rax = -ENOMEM;
        return;
    }

    char* kpathname = strdup(
        pathname); // Since we are going to free the original task's memory, we cannot use anything coming from it.

    // FIXME: This should be done later, but since there is a very high chance that loading the executed program's
    // ELF image will overwrite ours, we have to do it here.
    ELFLoader::release_elf_image(Scheduler::current_task()->image);

    // FIXME: Check the ELF image is valid before loading it into memory. This will allow us to first check, then free
    // the previous image, then load, which should reduce the chances of loading failing to almost zero.
    ELFImage* image = ELFLoader::load_elf_from_filesystem(kpathname);
    if (!image)
    {
        MemoryManager::release_pages((void*)allocated_stack, TASK_PAGES_IN_STACK);
        Scheduler::current_task()->image = nullptr;
        kfree(kpathname);
        kerrorln("exec(): ERROR: Failed to load program. Previous program has already been freed, thus cannot "
                 "return to it.");
        return Scheduler::task_exit(context, -255);
    }

    kfree(kpathname);

    Interrupts::disable();
    ASSERT(!Interrupts::are_enabled()); // This part is pretty sensitive.

    Task* task = Scheduler::current_task();
    ASSERT(task);

    // At this point, pretty much nothing can fail.

    MemoryManager::release_pages((void*)task->allocated_stack, TASK_PAGES_IN_STACK);

    task->allocated_stack = allocated_stack;

    Scheduler::reset_task(task, image);

    set_context_from_task(*task, context);

    return;
}