[PATCH 5/7] rust: file: add `Kuid` wrapper

[Alice Ryhl <aliceryhl@google.com>]

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>
---
 rust/bindings/bindings_helper.h |  1 +
 rust/helpers.c                  | 45 ++++++++++++++++++++++++++
 rust/kernel/cred.rs             |  5 +--
 rust/kernel/task.rs             | 71 ++++++++++++++++++++++++++++++++++++++++-
 4 files changed, 119 insertions(+), 3 deletions(-)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 81b13a953eae..700f01840188 100644
--- a/rust/bindings/bindings_helper.h
+++ b/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>
diff --git a/rust/helpers.c b/rust/helpers.c
index fd633d9db79a..58e3a9dff349 100644
--- a/rust/helpers.c
+++ b/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();
diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
index 3794937b5294..fbc749788bfa 100644
--- a/rust/kernel/cred.rs
+++ b/rust/kernel/cred.rs
@@ -8,6 +8,7 @@
 
 use crate::{
     bindings,
+    task::Kuid,
     types::{AlwaysRefCounted, Opaque},
 };
 
@@ -52,9 +53,9 @@ pub fn get_secid(&self) -> u32 {
     }
 
     /// 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 })
     }
 }
 
diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index b2299bc7ac1f..1a27b968a907 100644
--- a/rust/kernel/task.rs
+++ b/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 +143,34 @@ pub fn pid(&self) -> Pid {
         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: We know that `self.0.get()` is valid by the type invariant. The rest is just FFI
+        // calls.
+        unsafe {
+            let namespace = bindings::task_active_pid_ns(bindings::get_current());
+            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 +180,42 @@ pub fn wake_up(&self) {
     }
 }
 
+impl Kuid {
+    /// Get the current euid.
+    pub fn current_euid() -> Kuid {
+        // SAFETY: Just an FFI call.
+        Self {
+            kuid: 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 UID that userspace understands. 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) {

-- 
2.43.0.rc1.413.gea7ed67945-goog