linux-kernel - Re: [PATCH v11 6/8] mm: rust: add VmAreaNew for f

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Message-ID: <a1afdb55-29e2-4214-bdc7-8a0e6df65c14@lucifer.local>
Date: Fri, 10 Jan 2025 16:09:31 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Frederick Mayle <fmayle@...gle.com>, Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Andreas Hindborg <a.hindborg@...nel.org>,
        Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, rust-for-linux@...r.kernel.org,
        Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, John Hubbard <jhubbard@...dia.com>,
        "Liam R. Howlett" <liam.howlett@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, Christian Brauner <brauner@...nel.org>,
        Jann Horn <jannh@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v11 6/8] mm: rust: add VmAreaNew for f_ops->mmap()

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@...gle.com> 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@...cle.com> (for mm bits)
> > Reviewed-by: Jann Horn <jannh@...gle.com>
> > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
>
> [...]
>
> > +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