rust: file: add Kuid wrapper

Adds a wrapper around `kuid_t` called `Kuid`. This allows us to define
various operations on kuids such as equality and current_euid. It also
lets us provide conversions from kuid into userspace values.

Rust Binder needs these operations because it needs to compare kuids for
equality, and it needs to tell userspace about the pid and uid of
incoming transactions.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20231206-alice-file-v2-5-af617c0d9d94@google.com

Author: Darksonn

rust/bindings/bindings_helper.h

@@ -11,6 +11,7 @@
 #include <linux/errname.h>
 #include <linux/file.h>
 #include <linux/fs.h>
+#include <linux/pid_namespace.h>
 #include <linux/security.h>
 #include <linux/slab.h>
 #include <linux/refcount.h>

rust/helpers.c

@@ -142,6 +142,51 @@ void rust_helper_put_task_struct(struct task_struct *t)
 }
 EXPORT_SYMBOL_GPL(rust_helper_put_task_struct);
 
+kuid_t rust_helper_task_uid(struct task_struct *task)
+{
+	return task_uid(task);
+}
+EXPORT_SYMBOL_GPL(rust_helper_task_uid);
+
+kuid_t rust_helper_task_euid(struct task_struct *task)
+{
+	return task_euid(task);
+}
+EXPORT_SYMBOL_GPL(rust_helper_task_euid);
+
+#ifndef CONFIG_USER_NS
+uid_t rust_helper_from_kuid(struct user_namespace *to, kuid_t uid)
+{
+	return from_kuid(to, uid);
+}
+EXPORT_SYMBOL_GPL(rust_helper_from_kuid);
+#endif /* CONFIG_USER_NS */
+
+bool rust_helper_uid_eq(kuid_t left, kuid_t right)
+{
+	return uid_eq(left, right);
+}
+EXPORT_SYMBOL_GPL(rust_helper_uid_eq);
+
+kuid_t rust_helper_current_euid(void)
+{
+	return current_euid();
+}
+EXPORT_SYMBOL_GPL(rust_helper_current_euid);
+
+struct user_namespace *rust_helper_current_user_ns(void)
+{
+	return current_user_ns();
+}
+EXPORT_SYMBOL_GPL(rust_helper_current_user_ns);
+
+pid_t rust_helper_task_tgid_nr_ns(struct task_struct *tsk,
+				  struct pid_namespace *ns)
+{
+	return task_tgid_nr_ns(tsk, ns);
+}
+EXPORT_SYMBOL_GPL(rust_helper_task_tgid_nr_ns);
+
 struct kunit *rust_helper_kunit_get_current_test(void)
 {
 	return kunit_get_current_test();

rust/kernel/cred.rs

@@ -8,6 +8,7 @@
 
 use crate::{
     bindings,
+    task::Kuid,
     types::{AlwaysRefCounted, Opaque},
 };
 
@@ -52,9 +53,9 @@ impl Credential {
     }
 
     /// Returns the effective UID of the given credential.
-    pub fn euid(&self) -> bindings::kuid_t {
+    pub fn euid(&self) -> Kuid {
         // SAFETY: By the type invariant, we know that `self.0` is valid.
-        unsafe { (*self.0.get()).euid }
+        Kuid::from_raw(unsafe { (*self.0.get()).euid })
     }
 }
 

rust/kernel/task.rs

@@ -5,7 +5,12 @@
 //! C header: [`include/linux/sched.h`](../../../../include/linux/sched.h).
 
 use crate::{bindings, types::Opaque};
-use core::{marker::PhantomData, ops::Deref, ptr};
+use core::{
+    cmp::{Eq, PartialEq},
+    marker::PhantomData,
+    ops::Deref,
+    ptr,
+};
 
 /// Returns the currently running task.
 #[macro_export]
@@ -78,6 +83,12 @@ unsafe impl Sync for Task {}
 /// The type of process identifiers (PIDs).
 type Pid = bindings::pid_t;
 
+/// The type of user identifiers (UIDs).
+#[derive(Copy, Clone)]
+pub struct Kuid {
+    kuid: bindings::kuid_t,
+}
+
 impl Task {
     /// Returns a task reference for the currently executing task/thread.
     ///
@@ -132,12 +143,32 @@ impl Task {
         unsafe { *ptr::addr_of!((*self.0.get()).pid) }
     }
 
+    /// Returns the UID of the given task.
+    pub fn uid(&self) -> Kuid {
+        // SAFETY: By the type invariant, we know that `self.0` is valid.
+        Kuid::from_raw(unsafe { bindings::task_uid(self.0.get()) })
+    }
+
+    /// Returns the effective UID of the given task.
+    pub fn euid(&self) -> Kuid {
+        // SAFETY: By the type invariant, we know that `self.0` is valid.
+        Kuid::from_raw(unsafe { bindings::task_euid(self.0.get()) })
+    }
+
     /// Determines whether the given task has pending signals.
     pub fn signal_pending(&self) -> bool {
         // SAFETY: By the type invariant, we know that `self.0` is valid.
         unsafe { bindings::signal_pending(self.0.get()) != 0 }
     }
 
+    /// Returns the given task's pid in the current pid namespace.
+    pub fn pid_in_current_ns(&self) -> Pid {
+        // SAFETY: Calling `task_active_pid_ns` with the current task is always safe.
+        let namespace = unsafe { bindings::task_active_pid_ns(bindings::get_current()) };
+        // SAFETY: We know that `self.0.get()` is valid by the type invariant.
+        unsafe { bindings::task_tgid_nr_ns(self.0.get(), namespace) }
+    }
+
     /// Wakes up the task.
     pub fn wake_up(&self) {
         // SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid.
@@ -147,6 +178,41 @@ impl Task {
     }
 }
 
+impl Kuid {
+    /// Get the current euid.
+    pub fn current_euid() -> Kuid {
+        // SAFETY: Just an FFI call.
+        Self::from_raw(unsafe { bindings::current_euid() })
+    }
+
+    /// Create a `Kuid` given the raw C type.
+    pub fn from_raw(kuid: bindings::kuid_t) -> Self {
+        Self { kuid }
+    }
+
+    /// Turn this kuid into the raw C type.
+    pub fn into_raw(self) -> bindings::kuid_t {
+        self.kuid
+    }
+
+    /// Converts this kernel UID into a userspace UID.
+    ///
+    /// Uses the namespace of the current task.
+    pub fn into_uid_in_current_ns(self) -> bindings::uid_t {
+        // SAFETY: Just an FFI call.
+        unsafe { bindings::from_kuid(bindings::current_user_ns(), self.kuid) }
+    }
+}
+
+impl PartialEq for Kuid {
+    fn eq(&self, other: &Kuid) -> bool {
+        // SAFETY: Just an FFI call.
+        unsafe { bindings::uid_eq(self.kuid, other.kuid) }
+    }
+}
+
+impl Eq for Kuid {}
+
 // SAFETY: The type invariants guarantee that `Task` is always ref-counted.
 unsafe impl crate::types::AlwaysRefCounted for Task {
     fn inc_ref(&self) {