Re: [PATCH v2] rust: add PidNamespace

[Alice Ryhl]

On Tue, Oct 1, 2024 at 11:44 AM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> The lifetime of `PidNamespace` is bound to `Task` and `struct pid`.
>
> The `PidNamespace` of a `Task` doesn't ever change once the `Task` is
> alive. A `unshare(CLONE_NEWPID)` or `setns(fd_pidns/pidfd, CLONE_NEWPID)`
> will not have an effect on the calling `Task`'s pid namespace. It will
> only effect the pid namespace of children created by the calling `Task`.
> This invariant guarantees that after having acquired a reference to a
> `Task`'s pid namespace it will remain unchanged.
>
> When a task has exited and been reaped `release_task()` will be called.
> This will set the `PidNamespace` of the task to `NULL`. So retrieving
> the `PidNamespace` of a task that is dead will return `NULL`. Note, that
> neither holding the RCU lock nor holding a referencing count to the
> `Task` will prevent `release_task()` being called.
>
> In order to retrieve the `PidNamespace` of a `Task` the
> `task_active_pid_ns()` function can be used. There are two cases to
> consider:
>
> (1) retrieving the `PidNamespace` of the `current` task (2) retrieving
> the `PidNamespace` of a non-`current` task
>
> From system call context retrieving the `PidNamespace` for case (1) is
> always safe and requires neither RCU locking nor a reference count to be
> held. Retrieving the `PidNamespace` after `release_task()` for current
> will return `NULL` but no codepath like that is exposed to Rust.
>
> Retrieving the `PidNamespace` from system call context for (2) requires
> RCU protection. Accessing `PidNamespace` outside of RCU protection
> requires a reference count that must've been acquired while holding the
> RCU lock. Note that accessing a non-`current` task means `NULL` can be
> returned as the non-`current` task could have already passed through
> `release_task()`.
>
> To retrieve (1) the `current_pid_ns!()` macro should be used which
> ensure that the returned `PidNamespace` cannot outlive the calling
> scope. The associated `current_pid_ns()` function should not be called
> directly as it could be abused to created an unbounded lifetime for
> `PidNamespace`. The `current_pid_ns!()` macro allows Rust to handle the
> common case of accessing `current`'s `PidNamespace` without RCU
> protection and without having to acquire a reference count.
>
> For (2) the `task_get_pid_ns()` method must be used. This will always
> acquire a reference on `PidNamespace` and will return an `Option` to
> force the caller to explicitly handle the case where `PidNamespace` is
> `None`, something that tends to be forgotten when doing the equivalent
> operation in `C`. Missing RCU primitives make it difficult to perform
> operations that are otherwise safe without holding a reference count as
> long as RCU protection is guaranteed. But it is not important currently.
> But we do want it in the future.
>
> Note for (2) the required RCU protection around calling
> `task_active_pid_ns()` synchronizes against putting the last reference
> of the associated `struct pid` of `task->thread_pid`. The `struct pid`
> stored in that field is used to retrieve the `PidNamespace` of the
> caller. When `release_task()` is called `task->thread_pid` will be
> `NULL`ed and `put_pid()` on said `struct pid` will be delayed in
> `free_pid()` via `call_rcu()` allowing everyone with an RCU protected
> access to the `struct pid` acquired from `task->thread_pid` to finish.
>
> Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>

Overall looks good to me, but a few comments below.

Also, I think it would be fine to send the next version without it
being a reply to the file bindings thread.

>  rust/helpers/helpers.c       |   1 +
>  rust/helpers/pid_namespace.c |  26 ++++++++++
>  rust/kernel/lib.rs           |   1 +
>  rust/kernel/pid_namespace.rs |  70 +++++++++++++++++++++++++
>  rust/kernel/task.rs          | 119 ++++++++++++++++++++++++++++++++++++++++---
>  5 files changed, 211 insertions(+), 6 deletions(-)
>
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index 62022b18caf5ec17231fd0e7be1234592d1146e3..d553ad9361ce17950d505c3b372a568730020e2f 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -17,6 +17,7 @@
>  #include "kunit.c"
>  #include "mutex.c"
>  #include "page.c"
> +#include "pid_namespace.c"
>  #include "rbtree.c"
>  #include "refcount.c"
>  #include "security.c"
> diff --git a/rust/helpers/pid_namespace.c b/rust/helpers/pid_namespace.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..f41482bdec9a7c4e84b81ec141027fbd65251230
> --- /dev/null
> +++ b/rust/helpers/pid_namespace.c
> @@ -0,0 +1,26 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/pid_namespace.h>
> +#include <linux/cleanup.h>
> +
> +struct pid_namespace *rust_helper_get_pid_ns(struct pid_namespace *ns)
> +{
> +       return get_pid_ns(ns);
> +}
> +
> +void rust_helper_put_pid_ns(struct pid_namespace *ns)
> +{
> +       put_pid_ns(ns);
> +}
> +
> +/* Get a reference on a task's pid namespace. */
> +struct pid_namespace *rust_helper_task_get_pid_ns(struct task_struct *task)
> +{
> +       struct pid_namespace *pid_ns;
> +
> +       guard(rcu)();
> +       pid_ns = task_active_pid_ns(task);
> +       if (pid_ns)
> +               get_pid_ns(pid_ns);
> +       return pid_ns;
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index ff7d88022c57ca232dc028066dfa062f3fc84d1c..0e78ec9d06e0199dfafc40988a2ae86cd5df949c 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -44,6 +44,7 @@
>  #[cfg(CONFIG_NET)]
>  pub mod net;
>  pub mod page;
> +pub mod pid_namespace;
>  pub mod prelude;
>  pub mod print;
>  pub mod sizes;
> diff --git a/rust/kernel/pid_namespace.rs b/rust/kernel/pid_namespace.rs
> new file mode 100644
> index 0000000000000000000000000000000000000000..9a0509e802b4939ad853a802ee6d069a5f00c9df
> --- /dev/null
> +++ b/rust/kernel/pid_namespace.rs
> @@ -0,0 +1,70 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +// Copyright (c) 2024 Christian Brauner <brauner@xxxxxxxxxx>
> +
> +//! Pid namespaces.
> +//!
> +//! C header: [`include/linux/pid_namespace.h`](srctree/include/linux/pid_namespace.h) and
> +//! [`include/linux/pid.h`](srctree/include/linux/pid.h)
> +
> +use crate::{
> +    bindings,
> +    types::{AlwaysRefCounted, Opaque},
> +};
> +use core::{
> +    ptr,
> +};

This doesn't pass the rustfmt check.

$ rustfmt --check rust/kernel/pid_namespace.rs
Diff in /home/aliceryhl/rust-for-linux/rust/kernel/pid_namespace.rs:11:
     bindings,
     types::{AlwaysRefCounted, Opaque},
 };
-use core::{
-    ptr,
-};
+use core::ptr;

 /// Wraps the kernel's `struct pid_namespace`. Thread safe.
 ///

> +    /// Returns a PidNamespace reference for the currently executing task's/thread's pid namespace.
> +    ///
> +    /// This function can be used to create an unbounded lifetime by e.g., storing the returned
> +    /// PidNamespace in a global variable which would be a bug. So the recommended way to get the
> +    /// current task's/thread's pid namespace is to use the [`current_pid_ns`] macro because it is
> +    /// safe.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that the returned object doesn't outlive the current task/thread.
> +    pub unsafe fn current_pid_ns() -> impl Deref<Target = PidNamespace> {
> +        struct PidNamespaceRef<'a> {
> +            task: &'a PidNamespace,
> +            _not_send: NotThreadSafe,
> +        }
> +
> +        impl Deref for PidNamespaceRef<'_> {
> +            type Target = PidNamespace;
> +
> +            fn deref(&self) -> &Self::Target {
> +                self.task
> +            }
> +        }
> +
> +        let pidns = unsafe { bindings::task_active_pid_ns(Task::current_raw()) };
> +        PidNamespaceRef {
> +            // SAFETY: If the current thread is still running, the current task and its associated
> +            // pid namespace are valid. Given that `PidNamespaceRef` is not `Send`, we know it
> +            // cannot be transferred to another thread (where it could potentially outlive the
> +            // current `Task`).
> +            task: unsafe { &*pidns.cast() },

This could use `PidNamespace::from_ptr` instead of the cast.

Also, the safety comment about it not being Send seems incomplete. The
real reason it's okay is that the caller must ensure that the
PidNamespaceRef doesn't outlive the current task/thread.

> +    /// Returns the given task's pid in the provided pid namespace.
> +    pub fn task_tgid_nr_ns(&self, pidns: &PidNamespace) -> Pid {
> +        // SAFETY: We know that `self.0.get()` is valid by the type invariant.
> +        unsafe { bindings::task_tgid_nr_ns(self.0.get(), pidns.as_ptr()) }
>      }

The underlying C function accepts null pointers for the namespace. We
could do the same by accepting `pidns: Option<&PidNamespace>`.

Alice