On Tue, Nov 26, 2024 at 6:15 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> On Fri, Nov 22, 2024 at 4:41 PM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
> > +impl CurrentTask {
> > + /// Access the address space of this task.
> > + ///
> > + /// To increment the refcount of the referenced `mm`, you can use `ARef::from`.
> > + #[inline]
> > + pub fn mm(&self) -> Option<&MmWithUser> {
> > + let mm = unsafe { (*self.as_ptr()).mm };
> > +
> > + if mm.is_null() {
> > + None
> > + } else {
> > + // SAFETY: If `current->mm` is non-null, then it references a valid mm with a non-zero
> > + // value of `mm_users`. The returned `&MmWithUser` borrows from `CurrentTask`, so the
> > + // `&MmWithUser` cannot escape the current task, meaning `mm_users` can't reach zero
> > + // while the reference is still live.
> > + Some(unsafe { MmWithUser::from_raw(mm) })
>
> Maybe also add safety comments for these nitpicky details:
>
> kthreads can use kthread_use_mm()/kthread_unuse_mm() to change
> current->mm (which allows kthreads to access arbitrary userspace
> address spaces with copy_from_user/copy_to_user), but as long as you
> can't call into kthread_use_mm()/kthread_unuse_mm() from Rust code,
> this should be correct. If you do want to allow calls into
> kthread_use_mm()/kthread_unuse_mm() later on, you might have to gate
> this on a check for PF_KTHREAD, or something like that.
Huh ... is it possible to use kthread_use_mm() to create a situation
where current->mm has mm_users equal to zero? If not, then I don't
think it's a problem.
> Binary formats' .load_binary implementations can change current->mm by
> calling begin_new_exec(), but that's not an issue as long as no binary
> format loaders are implemented in Rust.
I think we can allow such loaders by having them involve an unsafe
operation asserting that you're not holding any references into
current when you start the new process.
Alice
