[PATCH v8 2/5] rust: add tracepoint support

Alice Ryhl
2024-08-22T12:04:55Z

Make it possible to have Rust code call into tracepoints defined by C
code. It is still required that the tracepoint is declared in a C
header, and that this header is included in the input to bindgen.

Instead of calling __DO_TRACE directly, the exported rust_do_trace_
function calls an inline helper function. This is because the `cond`
argument does not exist at the callsite of DEFINE_RUST_DO_TRACE.

__DECLARE_TRACE always emits an inline static and an extern declaration
that is only used when CREATE_RUST_TRACE_POINTS is set. These should not
end up in the final binary so it is not a problem that they sometimes
are emitted without a user.

Reviewed-by: Carlos Llamas <cmllamas@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
include/linux/tracepoint.h | 22 +++++++++++++++++-
include/trace/define_trace.h | 12 ++++++++++
rust/bindings/bindings_helper.h | 1 +
rust/kernel/lib.rs | 1 +
rust/kernel/tracepoint.rs | 49 +++++++++++++++++++++++++++++++++++++++++
5 files changed, 84 insertions(+), 1 deletion(-)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 6be396bb4297..5042ca588e41 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -237,6 +237,18 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
#define __DECLARE_TRACE_RCU(name, proto, args, cond)
#endif

+/*
+ * Declare an exported function that Rust code can call to trigger this
+ * tracepoint. This function does not include the static branch; that is done
+ * in Rust to avoid a function call when the tracepoint is disabled.
+ */
+#define DEFINE_RUST_DO_TRACE(name, proto, args)
+#define __DEFINE_RUST_DO_TRACE(name, proto, args) \
+ notrace void rust_do_trace_##name(proto) \
+ { \
+ __rust_do_trace_##name(args); \
+ }
+
/*
* Make sure the alignment of the structure in the __tracepoints section will
* not add unwanted padding between the beginning of the section and the
@@ -252,6 +264,13 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
extern int __traceiter_##name(data_proto); \
DECLARE_STATIC_CALL(tp_func_##name, __traceiter_##name); \
extern struct tracepoint __tracepoint_##name; \
+ extern void rust_do_trace_##name(proto); \
+ static inline void __rust_do_trace_##name(proto) \
+ { \
+ __DO_TRACE(name, \
+ TP_ARGS(args), \
+ TP_CONDITION(cond), 0); \
+ } \
static inline void trace_##name(proto) \
{ \
if (static_key_false(&__tracepoint_##name.key)) \
@@ -336,7 +355,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
void __probestub_##_name(void *__data, proto) \
{ \
} \
- DEFINE_STATIC_CALL(tp_func_##_name, __traceiter_##_name);
+ DEFINE_STATIC_CALL(tp_func_##_name, __traceiter_##_name); \
+ DEFINE_RUST_DO_TRACE(_name, TP_PROTO(proto), TP_ARGS(args))

#define DEFINE_TRACE(name, proto, args) \
DEFINE_TRACE_FN(name, NULL, NULL, PARAMS(proto), PARAMS(args));
diff --git a/include/trace/define_trace.h b/include/trace/define_trace.h
index 00723935dcc7..8159294c2041 100644
--- a/include/trace/define_trace.h
+++ b/include/trace/define_trace.h
@@ -72,6 +72,13 @@
#define DECLARE_TRACE(name, proto, args) \
DEFINE_TRACE(name, PARAMS(proto), PARAMS(args))

+/* If requested, create helpers for calling these tracepoints from Rust. */
+#ifdef CREATE_RUST_TRACE_POINTS
+#undef DEFINE_RUST_DO_TRACE
+#define DEFINE_RUST_DO_TRACE(name, proto, args) \
+ __DEFINE_RUST_DO_TRACE(name, PARAMS(proto), PARAMS(args))
+#endif
+
#undef TRACE_INCLUDE
#undef __TRACE_INCLUDE

@@ -129,6 +136,11 @@
# undef UNDEF_TRACE_INCLUDE_PATH
#endif

+#ifdef CREATE_RUST_TRACE_POINTS
+# undef DEFINE_RUST_DO_TRACE
+# define DEFINE_RUST_DO_TRACE(name, proto, args)
+#endif
+
/* We may be processing more files */
#define CREATE_TRACE_POINTS

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 8fd092e1b809..fc6f94729789 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -20,6 +20,7 @@
#include <linux/refcount.h>
#include <linux/sched.h>
#include <linux/slab.h>
+#include <linux/tracepoint.h>
#include <linux/wait.h>
#include <linux/workqueue.h>

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 91af9f75d121..d00a44b000b6 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -51,6 +51,7 @@
pub mod sync;
pub mod task;
pub mod time;
+pub mod tracepoint;
pub mod types;
pub mod uaccess;
pub mod workqueue;
diff --git a/rust/kernel/tracepoint.rs b/rust/kernel/tracepoint.rs
new file mode 100644
index 000000000000..cf2d9ad15912
--- /dev/null
+++ b/rust/kernel/tracepoint.rs
@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Logic for tracepoints.
+
+/// Declare the Rust entry point for a tracepoint.
+///
+/// This macro generates an unsafe function that calls into C, and its safety requirements will be
+/// whatever the relevant C code requires. To document these safety requirements, you may add
+/// doc-comments when invoking the macro.
+#[macro_export]
+macro_rules! declare_trace {
+ ($($(#[$attr:meta])* $pub:vis unsafe fn $name:ident($($argname:ident : $argtyp:ty),* $(,)?);)*) => {$(
+ $( #[$attr] )*
+ #[inline(always)]
+ $pub unsafe fn $name($($argname : $argtyp),*) {
+ #[cfg(CONFIG_TRACEPOINTS)]
+ {
+ // SAFETY: It's always okay to query the static key for a tracepoint.
+ let should_trace = unsafe {
+ $crate::macros::paste! {
+ $crate::jump_label::static_key_false!(
+ $crate::bindings::[< __tracepoint_ $name >],
+ $crate::bindings::tracepoint,
+ key
+ )
+ }
+ };
+
+ if should_trace {
+ $crate::macros::paste! {
+ // SAFETY: The caller guarantees that it is okay to call this tracepoint.
+ unsafe { $crate::bindings::[< rust_do_trace_ $name >]($($argname),*) };
+ }
+ }
+ }
+
+ #[cfg(not(CONFIG_TRACEPOINTS))]
+ {
+ // If tracepoints are disabled, insert a trivial use of each argument
+ // to avoid unused argument warnings.
+ $( let _unused = $argname; )*
+ }
+ }
+ )*}
+}
+
+pub use declare_trace;

--
2.46.0.184.g6999bdac58-goog