rust: add abstraction for credentials.

rust/helpers.c

@@ -402,6 +402,17 @@ void rust_helper_chained_irq_exit(struct irq_chip *chip,
 }
 EXPORT_SYMBOL_GPL(rust_helper_chained_irq_exit);
 
+const struct cred *rust_helper_get_cred(const struct cred *cred)
+{
+	return get_cred(cred);
+}
+EXPORT_SYMBOL_GPL(rust_helper_get_cred);
+
+void rust_helper_put_cred(const struct cred *cred) {
+	put_cred(cred);
+}
+EXPORT_SYMBOL_GPL(rust_helper_put_cred);
+
 /* We use bindgen's --size_t-is-usize option to bind the C size_t type
  * as the Rust usize type, so we can use it in contexts where Rust
  * expects a usize like slice (array) indices. usize is defined to be

rust/kernel/cred.rs

@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Credentials management.
+//!
+//! C header: [`include/linux/cred.h`](../../../../include/linux/cred.h)
+//!
+//! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html>
+
+use crate::bindings;
+use core::{marker::PhantomData, mem::ManuallyDrop, ops::Deref};
+
+/// Wraps the kernel's `struct cred`.
+///
+/// # Invariants
+///
+/// The pointer `Credential::ptr` is non-null and valid. Its reference count is also non-zero.
+pub struct Credential {
+    pub(crate) ptr: *const bindings::cred,
+}
+
+impl Clone for Credential {
+    fn clone(&self) -> Self {
+        // SAFETY: The type invariants guarantee that `self.ptr` has a non-zero reference count.
+        let ptr = unsafe { bindings::get_cred(self.ptr) };
+
+        // INVARIANT: We incremented the reference count to account for the new `Credential` being
+        // created.
+        Self { ptr }
+    }
+}
+
+impl Drop for Credential {
+    fn drop(&mut self) {
+        // SAFETY: The type invariants guarantee that `ptr` has a non-zero reference count.
+        unsafe { bindings::put_cred(self.ptr) };
+    }
+}
+
+/// A wrapper for [`Credential`] that doesn't automatically decrement the refcount when dropped.
+///
+/// We need the wrapper because [`ManuallyDrop`] alone would allow callers to call
+/// [`ManuallyDrop::into_inner`]. This would allow an unsafe sequence to be triggered without
+/// `unsafe` blocks because it would trigger an unbalanced call to `put_cred`.
+///
+/// # Invariants
+///
+/// The wrapped [`Credential`] remains valid for the lifetime of the object.
+pub struct CredentialRef<'a> {
+    cred: ManuallyDrop<Credential>,
+    _p: PhantomData<&'a ()>,
+}
+
+impl CredentialRef<'_> {
+    /// Constructs a new [`struct cred`] wrapper that doesn't change its reference count.
+    ///
+    /// # Safety
+    ///
+    /// The pointer `ptr` must be non-null and valid for the lifetime of the object.
+    pub(crate) unsafe fn from_ptr(ptr: *const bindings::cred) -> Self {
+        Self {
+            cred: ManuallyDrop::new(Credential { ptr }),
+            _p: PhantomData,
+        }
+    }
+}
+
+impl Deref for CredentialRef<'_> {
+    type Target = Credential;
+
+    fn deref(&self) -> &Self::Target {
+        self.cred.deref()
+    }
+}

rust/kernel/file.rs

@@ -5,7 +5,7 @@
 //! C headers: [`include/linux/fs.h`](../../../../include/linux/fs.h) and
 //! [`include/linux/file.h`](../../../../include/linux/file.h)
 
-use crate::{bindings, error::Error, Result};
+use crate::{bindings, cred::CredentialRef, error::Error, Result};
 use core::{mem::ManuallyDrop, ops::Deref};
 
 /// Wraps the kernel's `struct file`.
@@ -44,6 +44,16 @@ impl File {
         // SAFETY: `File::ptr` is guaranteed to be valid by the type invariants.
         unsafe { (*self.ptr).f_flags & bindings::O_NONBLOCK == 0 }
     }
+
+    /// Returns the credentials of the task that originally opened the file.
+    pub fn cred(&self) -> CredentialRef<'_> {
+        // SAFETY: `File::ptr` is guaranteed to be valid by the type invariants.
+        let ptr = unsafe { (*self.ptr).f_cred };
+        // SAFETY: The lifetimes of `self` and `CredentialRef` are tied, so it is guaranteed that
+        // the credential pointer remains valid (because the file is still alive, and it doesn't
+        // change over the lifetime of a file).
+        unsafe { CredentialRef::from_ptr(ptr) }
+    }
 }
 
 impl Drop for File {

rust/kernel/lib.rs

@@ -47,6 +47,7 @@ pub mod amba;
 pub mod buffer;
 pub mod c_types;
 pub mod chrdev;
+pub mod cred;
 pub mod device;
 pub mod driver;
 mod error;