Re: [PATCH 1/3] rust: add static_call support

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 06, 2024 at 03:05:24PM +0000, Alice Ryhl wrote:
> Add static_call support by mirroring how C does. When the platform does
> not support static calls (right now, that means that it is not x86),
> then the function pointer is loaded from a global and called. Otherwise,
> we generate a call to a trampoline function, and objtool is used to make
> these calls patchable at runtime.

This is absolutely unreadable gibberish -- how am I supposed to keep
this in sync with the rest of the static_call infrastructure?

> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/lib.rs         |  1 +
>  rust/kernel/static_call.rs | 92 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 93 insertions(+)
> 
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index fbd91a48ff8b..d534b1178955 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -38,6 +38,7 @@
>  pub mod prelude;
>  pub mod print;
>  mod static_assert;
> +pub mod static_call;
>  #[doc(hidden)]
>  pub mod std_vendor;
>  pub mod str;
> diff --git a/rust/kernel/static_call.rs b/rust/kernel/static_call.rs
> new file mode 100644
> index 000000000000..f7b8ba7bf1fb
> --- /dev/null
> +++ b/rust/kernel/static_call.rs
> @@ -0,0 +1,92 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +// Copyright (C) 2024 Google LLC.
> +
> +//! Logic for static calls.
> +
> +#[macro_export]
> +#[doc(hidden)]
> +macro_rules! ty_underscore_for {
> +    ($arg:expr) => {
> +        _
> +    };
> +}
> +
> +#[doc(hidden)]
> +#[repr(transparent)]
> +pub struct AddressableStaticCallKey {
> +    _ptr: *const bindings::static_call_key,
> +}
> +unsafe impl Sync for AddressableStaticCallKey {}
> +impl AddressableStaticCallKey {
> +    pub const fn new(ptr: *const bindings::static_call_key) -> Self {
> +        Self { _ptr: ptr }
> +    }
> +}
> +
> +#[cfg(CONFIG_HAVE_STATIC_CALL)]
> +#[doc(hidden)]
> +#[macro_export]
> +macro_rules! _static_call {
> +    ($name:ident($($args:expr),* $(,)?)) => {{
> +        // Symbol mangling will give this symbol a unique name.
> +        #[cfg(CONFIG_HAVE_STATIC_CALL_INLINE)]
> +        #[link_section = ".discard.addressable"]
> +        #[used]
> +        static __ADDRESSABLE: $crate::static_call::AddressableStaticCallKey = unsafe {
> +            $crate::static_call::AddressableStaticCallKey::new(::core::ptr::addr_of!(
> +                $crate::macros::paste! { $crate::bindings:: [<__SCK__ $name >]; }
> +            ))
> +        };
> +
> +        let fn_ptr: unsafe extern "C" fn($($crate::static_call::ty_underscore_for!($args)),*) -> _ =
> +            $crate::macros::paste! { $crate::bindings:: [<__SCT__ $name >]; };
> +        (fn_ptr)($($args),*)
> +    }};
> +}
> +
> +#[cfg(not(CONFIG_HAVE_STATIC_CALL))]
> +#[doc(hidden)]
> +#[macro_export]
> +macro_rules! _static_call {
> +    ($name:ident($($args:expr),* $(,)?)) => {{
> +        let void_ptr_fn: *mut ::core::ffi::c_void =
> +            $crate::macros::paste! { $crate::bindings:: [<__SCK__ $name >]; }.func;
> +
> +        let fn_ptr: unsafe extern "C" fn($($crate::static_call::ty_underscore_for!($args)),*) -> _ =
> +            if true {
> +                ::core::mem::transmute(void_ptr_fn)
> +            } else {
> +                // This is dead code, but it influences type inference on `fn_ptr` so that we
> +                // transmute the function pointer to the right type.
> +                $crate::macros::paste! { $crate::bindings:: [<__SCT__ $name >]; }
> +            };
> +
> +        (fn_ptr)($($args),*)
> +    }};
> +}
> +
> +/// Statically call a global function.
> +///
> +/// # Safety
> +///
> +/// This macro will call the provided function. It is up to the caller to uphold the safety
> +/// guarantees of the function.
> +///
> +/// # Examples
> +///
> +/// ```ignore
> +/// fn call_static() {
> +///     unsafe {
> +///         static_call! { your_static_call() };
> +///     }
> +/// }
> +/// ```
> +#[macro_export]
> +macro_rules! static_call {
> +    // Forward to the real implementation. Separated like this so that we don't have to duplicate
> +    // the documentation.
> +    ($($args:tt)*) => { $crate::static_call::_static_call! { $($args)* } };
> +}
> +
> +pub use {_static_call, static_call, ty_underscore_for};
> 
> -- 
> 2.45.2.505.gda0bf45e8d-goog
>