[PATCH v4 4/4] rust: Add warn_on macro

[Alice Ryhl]

On Wed, Mar 5, 2025 at 12:10 PM FUJITA Tomonori
<fujita.tomonori at gmail.com> wrote:
>
> Add warn_on macro, uses the BUG/WARN feature (lib/bug.c) via assembly
> for x86_64/arm64/riscv.
>
> The current Rust code simply wraps BUG() macro but doesn't provide the
> proper debug information. The BUG/WARN feature can only be used from
> assembly.
>
> This uses the assembly code exported by the C side via ARCH_WARN_ASM
> macro. To avoid duplicating the assembly code, this approach follows
> the same strategy as the static branch code: it generates the assembly
> code for Rust using the C preprocessor at compile time.
>
> Similarly, ARCH_WARN_REACHABLE is also used at compile time to
> generate the assembly code; objtool's reachable anotation code. It's
> used for only architectures that use objtool.
>
> For now, Loongarch just uses a wrapper for WARN macro.
>
> UML doesn't use the assembly BUG/WARN feature; just wrapping generic
> BUG/WARN functions implemented in C works.
>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori at gmail.com>

Overall looks reasonable. A few nits below:

> +#[macro_export]
> +#[doc(hidden)]
> +#[cfg(all(CONFIG_BUG, not(CONFIG_UML), not(CONFIG_LOONGARCH)))]
> +#[cfg(CONFIG_DEBUG_BUGVERBOSE)]
> +macro_rules! warn_flags {
> +    ($flags:expr) => {
> +        const FLAGS: u32 = $crate::bindings::BUGFLAG_WARNING | $flags;
> +        const _FILE: &[u8] = file!().as_bytes();
> +        // Plus one for null-terminator.
> +        static FILE: [u8; _FILE.len() + 1] = {
> +            let mut bytes = [0; _FILE.len() + 1];
> +            let mut i = 0;
> +            while i < _FILE.len() {
> +                bytes[i] = _FILE[i];
> +                i += 1;
> +            }
> +            bytes
> +        };
> +        // SAFETY: Just an FFI call.
> +        unsafe {
> +            $crate::asm!(concat!(
> +                "/* {size} */",
> +                include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_warn_asm.rs")),
> +                include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_reachable_asm.rs")));
> +            file = sym FILE,
> +            line = const line!(),
> +            flags = const FLAGS,
> +            size = const ::core::mem::size_of::<$crate::bindings::bug_entry>(),

The indentation here is odd. Shouldn't the arguments be indented one more?

> +            );
> +        }
> +    }
> +}
> +
> +#[macro_export]
> +#[doc(hidden)]
> +#[cfg(all(CONFIG_BUG, not(CONFIG_UML), not(CONFIG_LOONGARCH)))]
> +#[cfg(not(CONFIG_DEBUG_BUGVERBOSE))]
> +macro_rules! warn_flags {
> +    ($flags:expr) => {
> +        const FLAGS: u32 = $crate::bindings::BUGFLAG_WARNING | $flags;
> +        // SAFETY: Just an FFI call.
> +        unsafe {
> +            $crate::asm!(
> +            concat!(
> +                "/* {size} */",
> +                include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_warn_asm.rs")),
> +                include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_reachable_asm.rs")));
> +            flags = const FLAGS,
> +            size = const ::core::mem::size_of::<$crate::bindings::bug_entry>(),

Same thing here.

> +            );
> +        }
> +    }
> +}
> +
> +#[macro_export]
> +#[doc(hidden)]
> +#[cfg(all(CONFIG_BUG, CONFIG_UML))]
> +macro_rules! warn_flags {
> +    ($flags:expr) => {
> +        // SAFETY: Just an FFI call.
> +        unsafe {
> +            $crate::bindings::warn_slowpath_fmt(
> +                $crate::c_str!(::core::file!()).as_ptr() as *const ::core::ffi::c_uchar,
> +                line!() as i32,
> +                $flags as u32,
> +                ::core::ptr::null() as *const ::core::ffi::c_uchar,

Why unsigned char? I think this should be ::kernel::ffi::c_char to
utilize the custom integer type definitions in kernel rather than the
faulty ones in core.

(Maybe $crate rather than kernel)

Alice