This involves renaming the descriptor_from_fd function to the more appropriately named
open_descriptor_from_fd (since we check if the descriptor was opened and error out otherwise),
and creating a new function that does not verify that the file descriptor was opened.
Some pages, such as framebuffer pages, are not physical memory frames reserved for the current process.
Some, such as the framebuffer, may be shared between all processes.
Yet, on exit() or on exec(), a process frees all frames mapped into its address spaces.
And on fork(), it copies all data between frames. So how could we map framebuffers.
Simple: we use one of the bits in page table entries which are available to the OS, and mark whether that page is owned by the current process.
If it is owned, it will be:
- Freed on address space destruction
- Its data will be copied to a new page owned by the child process on fork()
If it is not owned, it will be:
- Left alone on address space destruction
- On fork(), the child's virtual page will be mapped to the same physical frame as the parent
This still needs a bit more work, such as keeping a reference of how many processes use a page to free it when all processes using it exit/exec.
This should be done for MAP_SHARED mappings, for example, since they are not permanent forever,
unlike the framebuffer for example.
Not part of C or POSIX, but since there is no procfs right now, I thought it would be nice to have an interface to query process information.
It works like this: you pass the process ID and a pointer to a struct pstat (can be null).
If the process ID is -1, the kernel picks the process with the highest PID.
Then, if the pointer to a pstat struct is not null, the kernel fills it in with the process's information, and returns the process's PID.
This function is a Luna alternative to fork() and exec().
Why? Simply because I can't figure out for the life of me how to implement a working fork().
So meanwhile, we have spawn() as a replacement. exec() still exists, though.
Very bare-bones for now. Doesn't support arguments or environment (we don't have that stuff right now), and the executable is not a valid ELF, it terminates the task.
But it's a start!
We were previously looking at its segment registers to see if they were user-like, but this method is bad.
What is the task was executing a system call?
So now, we store that value at creation time.
We should start to drop the old InitRD API, which only allows for files to be loaded from the initrd, and which forces pathnames to be relative (bin/init)
With VFS, we can load any kind of file from any kind of filesystem, and using paths that make sense (/bin/init)
The exit() libc function already accepted an integer, but didn't pass it on to the kernel since we had no mechanism for it to do that.
Now, the kernel stores a task's exit status to display it later (and in the future, return it to userspace via wait()/waitpid())
This struct allows us to keep track of what memory is used by the loaded executable. For some reason, freeing this memory when the task exits triggers a kernel page fault, so I'm not doing that right now.
IT ACTUALLY WORKS NOW.
Why wasn't it working? Oh, because I was not setting already present page tables's permissions to user mode. Just a little bug. THAT I SPENT DAYS TRYING TO FIND
Anyways, it works now. Such a relief...
