Re: [PATCH v3 2/9] rust: cred: add Rust abstraction for `struct cred`

[Benno Lossin <benno.lossin@proton.me>]

On 1/18/24 15:36, Alice Ryhl wrote:
> diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
> new file mode 100644
> index 000000000000..ccec77242dfd
> --- /dev/null
> +++ b/rust/kernel/cred.rs
> @@ -0,0 +1,65 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Credentials management.
> +//!
> +//! C header: [`include/linux/cred.h`](../../../../include/linux/cred.h)

IIRC you can use `srctree/include/..` to avoid the `../..` madness.

> +//!
> +//! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html>
> +
> +use crate::{
> +    bindings,
> +    types::{AlwaysRefCounted, Opaque},
> +};
> +
> +/// Wraps the kernel's `struct cred`.
> +///
> +/// # Invariants
> +///
> +/// Instances of this type are always ref-counted, that is, a call to `get_cred` ensures that the
> +/// allocation remains valid at least until the matching call to `put_cred`.
> +#[repr(transparent)]
> +pub struct Credential(Opaque<bindings::cred>);
> +
> +// SAFETY: By design, the only way to access a `Credential` is via an immutable reference or an
> +// `ARef`. This means that the only situation in which a `Credential` can be accessed mutably is
> +// when the refcount drops to zero and the destructor runs. It is safe for that to happen on any
> +// thread, so it is ok for this type to be `Send`.

IMO the only important part is that calling `drop`/`dec_ref` is OK from
any thread.

In general I think it might be a good idea to make
`AlwaysRefCounted: Send + Sync`. But that is outside the scope of this
patch.

> +unsafe impl Send for Credential {}
> +
> +// SAFETY: It's OK to access `Credential` through shared references from other threads because
> +// we're either accessing properties that don't change or that are properly synchronised by C code.
> +unsafe impl Sync for Credential {}
> +
> +impl Credential {
> +    /// Creates a reference to a [`Credential`] from a valid pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the
> +    /// returned [`Credential`] reference.
> +    pub unsafe fn from_ptr<'a>(ptr: *const bindings::cred) -> &'a Credential {
> +        // SAFETY: The safety requirements guarantee the validity of the dereference, while the
> +        // `Credential` type being transparent makes the cast ok.
> +        unsafe { &*ptr.cast() }
> +    }
> +
> +    /// Returns the effective UID of the given credential.
> +    pub fn euid(&self) -> bindings::kuid_t {
> +        // SAFETY: By the type invariant, we know that `self.0` is valid.

Is `euid` an immutable property, or why does this memory access not race
with something?

-- 
Cheers,
Benno

> +        unsafe { (*self.0.get()).euid }
> +    }
> +}
> +
> +// SAFETY: The type invariants guarantee that `Credential` is always ref-counted.
> +unsafe impl AlwaysRefCounted for Credential {
> +    fn inc_ref(&self) {
> +        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> +        unsafe { bindings::get_cred(self.0.get()) };
> +    }
> +
> +    unsafe fn dec_ref(obj: core::ptr::NonNull<Credential>) {
> +        // SAFETY: The safety requirements guarantee that the refcount is nonzero. The cast is okay
> +        // because `Credential` has the same representation as `struct cred`.
> +        unsafe { bindings::put_cred(obj.cast().as_ptr()) };
> +    }
> +}