Re: [PATCH] rust_binder: add Rust Binder driver

[Danilo Krummrich]

(Far from a full review, but two minor drive-by comments.)

On Thu Sep 18, 2025 at 12:19 PM CEST, Alice Ryhl wrote:
> +    /// Register a vma with this page range. Returns the size of the region.
> +    pub(crate) fn register_with_vma(&self, vma: &virt::VmaNew) -> Result<usize> {
> +        let num_bytes = usize::min(vma.end() - vma.start(), bindings::SZ_4M as usize);
> +        let num_pages = num_bytes >> PAGE_SHIFT;
> +
> +        if !ptr::eq::<Mm>(&*self.mm, &**vma.mm()) {
> +            pr_debug!("Failed to register with vma: invalid vma->vm_mm");
> +            return Err(EINVAL);
> +        }
> +        if num_pages == 0 {
> +            pr_debug!("Failed to register with vma: size zero");
> +            return Err(EINVAL);
> +        }
> +
> +        let layout = Layout::array::<PageInfo>(num_pages).map_err(|_| ENOMEM)?;
> +        let pages = KVmalloc::alloc(layout, GFP_KERNEL)?.cast::<PageInfo>();
> +
> +        // SAFETY: This just initializes the pages array.
> +        unsafe {
> +            let self_ptr = self as *const ShrinkablePageRange;
> +            for i in 0..num_pages {
> +                let info = pages.as_ptr().add(i);
> +                ptr::addr_of_mut!((*info).range).write(self_ptr);
> +                ptr::addr_of_mut!((*info).page).write(None);
> +                let lru = ptr::addr_of_mut!((*info).lru);
> +                ptr::addr_of_mut!((*lru).next).write(lru);
> +                ptr::addr_of_mut!((*lru).prev).write(lru);

I think we should prefer &raw in new code.

> +            }
> +        }
> +
> +        let mut inner = self.lock.lock();
> +        if inner.size > 0 {
> +            pr_debug!("Failed to register with vma: already registered");
> +            drop(inner);
> +            // SAFETY: The `pages` array was allocated with the same layout.
> +            unsafe { KVmalloc::free(pages.cast(), layout) };

NIT: If you use KVBox instead, even though you convert to a raw pointer later,
you can probably safe one unsafe.

> +            return Err(EBUSY);
> +        }
> +
> +        inner.pages = pages.as_ptr();
> +        inner.size = num_pages;
> +        inner.vma_addr = vma.start();
> +
> +        Ok(num_pages)
> +    }