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

[Christian Brauner]

On Wed, Nov 29, 2023 at 01:12:17PM +0000, Alice Ryhl wrote:
> 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@xxxxxxxxxx>
> ---
>  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);

I'm a bit puzzled by all these rust_helper_*() calls. Can you explain
why they are needed? Because they are/can be static inlines and that
somehow doesn't work?

> +
>  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 {

Hm, I wouldn't special-case this. Just expose from_kuid() and let it
take a namespace argument, no? You don't need to provide bindings for
namespaces ofc.

> +        // 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 {}

Do you need that?

> +
>  // 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
>