[PATCH 0/3] Tracepoints and static branch/call in Rust

[Alice Ryhl]

An important part of a production ready Linux kernel driver is
tracepoints. So to write production ready Linux kernel drivers in Rust,
we must be able to call tracepoints from Rust code. This patch series
adds support for calling tracepoints declared in C from Rust.

To use the tracepoint support, you must:

1. Declare the tracepoint in a C header file as usual.
2. Make sure that the header file is visible to bindgen so that Rust
   bindings are generated for the symbols that the tracepoint macro
   emits.
3. Use the declare_trace! macro in your Rust code to generate Rust
   functions that call into the tracepoint.

For example, the kernel has a tracepoint called `sched_kthread_stop`. It
is declared like this:

	TRACE_EVENT(sched_kthread_stop,
		TP_PROTO(struct task_struct *t),
		TP_ARGS(t),
		TP_STRUCT__entry(
			__array(	char,	comm,	TASK_COMM_LEN	)
			__field(	pid_t,	pid			)
		),
		TP_fast_assign(
			memcpy(__entry->comm, t->comm, TASK_COMM_LEN);
			__entry->pid	= t->pid;
		),
		TP_printk("comm=%s pid=%d", __entry->comm, __entry->pid)
	);

To call the above tracepoint from Rust code, you would add the relevant
header file to rust/bindings/bindings_helper.h and add the following
invocation somewhere in your Rust code:

	declare_trace! {
	    fn sched_kthread_stop(task: *mut task_struct);
	}

This will define a Rust function of the given name that you can call
like any other Rust function. Since these tracepoints often take raw
pointers as arguments, it may be convenient to wrap it in a safe
wrapper:

	mod raw {
	    declare_trace! {
	        fn sched_kthread_stop(task: *mut task_struct);
	    }
	}
	
	#[inline]
	pub fn trace_sched_kthread_stop(task: &Task) {
	    // SAFETY: The pointer to `task` is valid.
	    unsafe { raw::sched_kthread_stop(task.as_raw()) }
	}

A future expansion of the tracepoint support could generate these safe
versions automatically, but that is left as future work for now.

This is intended for use in the Rust Binder driver, which was originally
sent as an RFC [1]. The RFC did not include tracepoint support, but you
can see how it will be used in Rust Binder at [2]. The author has
verified that the tracepoint support works on Android devices.

This implementation implements support for static keys in Rust so that
the actual static branch will end up in the Rust object file. However,
it would also be possible to just wrap the trace_##name generated by
__DECLARE_TRACE in an extern C function and then call that from Rust.
This will simplify the Rust code by removing the need for static
branches and calls, but it places the static branch behind an external
call, which has performance implications.

A possible middle ground would be to place just the __DO_TRACE body in
an extern C function and to implement the Rust wrapper by doing the
static branch in Rust, and then calling into C the code that contains
__DO_TRACE when the tracepoint is active. However, this would need some
changes to include/linux/tracepoint.h to generate and export a function
containing the body of __DO_TRACE when the tracepoint should be callable
from Rust.

So in general, there is a tradeoff between placing parts of the
tracepoint (which is perf sensitive) behind an external call, and having
code duplicated in both C and Rust (which must be kept in sync when
changes are made). This is an important point that I would like feedback
on from the C maintainers.

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-0-08ba9197f637@xxxxxxxxxx/ [1]
Link: https://r.android.com/3110088 [2]
Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
---
Alice Ryhl (3):
      rust: add static_call support
      rust: add static_key_false
      rust: add tracepoint support

 rust/bindings/bindings_helper.h |  1 +
 rust/bindings/lib.rs            | 15 +++++++
 rust/helpers.c                  | 24 +++++++++++
 rust/kernel/lib.rs              |  3 ++
 rust/kernel/static_call.rs      | 92 +++++++++++++++++++++++++++++++++++++++++
 rust/kernel/static_key.rs       | 87 ++++++++++++++++++++++++++++++++++++++
 rust/kernel/tracepoint.rs       | 92 +++++++++++++++++++++++++++++++++++++++++
 scripts/Makefile.build          |  2 +-
 8 files changed, 315 insertions(+), 1 deletion(-)
---
base-commit: 1613e604df0cd359cf2a7fbd9be7a0bcfacfabd0
change-id: 20240606-tracepoint-31e15b90e471

Best regards,
-- 
Alice Ryhl <aliceryhl@xxxxxxxxxx>