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-v1-0-08ba9197f...@google.com/ [1] Link: https://r.android.com/3110088 [2] Signed-off-by: Alice Ryhl <alicer...@google.com> --- 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 <alicer...@google.com>