rust: helpers: explain file with a top-level comment

Also copy the same `EXPORT_SYMBOL_GPL` explanation to `exports.c`.

This also sums up the discussion at #637 about exporting or not the helpers that are not technically required at the moment (thanks @bjorn3 for the note about `rustc` not guaranteeing codegen for non-inline functions).

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

rust/exports.c

@@ -6,6 +6,9 @@
 // This requires the Rust's new/future `v0` mangling scheme because the default
 // one ("legacy") uses invalid characters for C identifiers (thus we cannot use
 // the `EXPORT_SYMBOL_*` macros).
+//
+// All symbols are exported as GPL-only to guarantee no GPL-only feature is
+// accidentally exposed.
 
 #include <linux/module.h>
 

rust/helpers.c

@@ -1,4 +1,21 @@
 // SPDX-License-Identifier: GPL-2.0
+//
+// Non-trivial C macros cannot be used in Rust. Similarly, inlined C functions
+// cannot be called either. This file explicitly creates functions ("helpers")
+// that wrap those so that they can be called from Rust.
+//
+// Even though Rust kernel modules should never use directly the bindings, some
+// of these helpers need to be exported because Rust generics and inlined
+// functions may not get their code generated in the crate where they are
+// defined. Other helpers, called from non-inline functions, may not be
+// exported, in principle. However, in general, the Rust compiler does not
+// guarantee codegen will be performed for a non-inline function either.
+// Therefore, this file exports all the helpers. In the future, this may be
+// revisited to reduce the number of exports after the compiler is informed
+// about the places codegen is required.
+//
+// All symbols are exported as GPL-only to guarantee no GPL-only feature is
+// accidentally exposed.
 
 #include <linux/bug.h>
 #include <linux/build_bug.h>