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

[Lorenzo Stoakes]

On Fri, Jan 10, 2025 at 02:34:48PM +0100, Alice Ryhl wrote:
> 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?

Yeah sounds reasonable, in C it's mutable right up until... well ok it
never stops being that :P

>
> Alice