rust: cred: add Rust abstraction for struct cred

Add a wrapper around `struct cred` called `Credential`, and provide
functionality to get the `Credential` associated with a `File`.

Rust Binder must check the credentials of processes when they attempt to
perform various operations, and these checks usually take a
`&Credential` as parameter. The security_binder_set_context_mgr function
would be one example. This patch is necessary to access these security_*
methods from Rust.

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20231206-alice-file-v2-2-af617c0d9d94@google.com

Author: wedsonaf

rust/bindings/bindings_helper.h

@@ -7,6 +7,7 @@
  */
 
 #include <kunit/test.h>
+#include <linux/cred.h>
 #include <linux/errname.h>
 #include <linux/file.h>
 #include <linux/fs.h>

rust/helpers.c

@@ -23,6 +23,7 @@
 #include <kunit/test-bug.h>
 #include <linux/bug.h>
 #include <linux/build_bug.h>
+#include <linux/cred.h>
 #include <linux/err.h>
 #include <linux/errname.h>
 #include <linux/fs.h>
@@ -164,6 +165,18 @@ struct file *rust_helper_get_file(struct file *f)
 }
 EXPORT_SYMBOL_GPL(rust_helper_get_file);
 
+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);
+
 /*
  * `bindgen` binds 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.

rust/kernel/cred.rs

@@ -0,0 +1,64 @@
+// 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,
+    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(pub(crate) 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`.
+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.
+        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<Self>) {
+        // SAFETY: The safety requirements guarantee that the refcount is nonzero.
+        unsafe { bindings::put_cred(obj.cast().as_ptr()) };
+    }
+}

rust/kernel/file.rs

@@ -7,6 +7,7 @@
 
 use crate::{
     bindings,
+    cred::Credential,
     error::{code::*, Error, Result},
     types::{ARef, AlwaysRefCounted, Opaque},
 };
@@ -151,6 +152,21 @@ impl File {
         self.0.get()
     }
 
+    /// Returns the credentials of the task that originally opened the file.
+    pub fn cred(&self) -> &Credential {
+        // SAFETY: Since the caller holds a reference to the file, it is guaranteed that its
+        // refcount does not hit zero during this function call.
+        //
+        // It's okay to read the `f_cred` field without synchronization as `f_cred` is never
+        // changed after initialization of the file.
+        let ptr = unsafe { (*self.as_ptr()).f_cred };
+
+        // SAFETY: The signature of this function ensures that the caller will only access the
+        // returned credential while the file is still valid, and the C side ensures that the
+        // credential stays valid at least as long as the file.
+        unsafe { Credential::from_ptr(ptr) }
+    }
+
     /// Returns the flags associated with the file.
     ///
     /// The flags are a combination of the constants in [`flags`].

rust/kernel/lib.rs

@@ -33,6 +33,7 @@ extern crate self as kernel;
 #[cfg(not(testlib))]
 mod allocator;
 mod build_assert;
+pub mod cred;
 pub mod error;
 pub mod file;
 pub mod init;