Re: [PATCH v11 6/8] mm: rust: add VmAreaNew for f_ops->mmap()

[Alice Ryhl]

On Wed, Dec 11, 2024 at 11:37 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> This type will be used when setting up a new vma in an f_ops->mmap()
> hook. Using a separate type from VmAreaRef allows us to have a separate
> set of operations that you are only able to use during the mmap() hook.
> For example, the VM_MIXEDMAP flag must not be changed after the initial
> setup that happens during the f_ops->mmap() hook.
>
> To avoid setting invalid flag values, the methods for clearing
> VM_MAYWRITE and similar involve a check of VM_WRITE, and return an error
> if VM_WRITE is set. Trying to use `try_clear_maywrite` without checking
> the return value results in a compilation error because the `Result`
> type is marked #[must_use].
>
> For now, there's only a method for VM_MIXEDMAP and not VM_PFNMAP. When
> we add a VM_PFNMAP method, we will need some way to prevent you from
> setting both VM_MIXEDMAP and VM_PFNMAP on the same vma.
>
> Acked-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> (for mm bits)
> Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>

[...]

> +impl VmAreaNew {
> +    /// Access a virtual memory area given a raw pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that `vma` is undergoing initial vma setup for the duration of 'a.
> +    #[inline]
> +    pub unsafe fn from_raw<'a>(vma: *const bindings::vm_area_struct) -> &'a Self {
> +        // SAFETY: The caller ensures that the invariants are satisfied for the duration of 'a.
> +        unsafe { &*vma.cast() }
> +    }

It was suggested at https://r.android.com/3389887 that this should
take a mutable raw pointer for better intent. That's fine with me
(Rust doesn't care). Lorenzo, what do you think?

Alice