Re: [PATCH] rust: print: Fix issue with rust_build_error

[Boqun Feng]

On Sat, Sep 20, 2025 at 12:19:58PM -0400, Joel Fernandes wrote:
> When printing just before calling io.write32(), modpost fails due to
> build_assert's missing rust_build_error symbol. The issue is that, the
> printk arguments are passed as reference in bindings code, thus Rust
> cannot trust its value and fails to optimize away the build_assert.
> 

I think "cannot trust" is a bit vague and misleading here, for this
kind of "workaround", we want the reason to be a bit clear. @Gary, could
you help explain it a bit more?

> The issue can be reproduced with the following simple snippet:
>   let offset = 0;
>   pr_err!("{}", offset);
>   io.write32(base, offset);
> 
> Fix it by just using a closure to call printk. Rust captures the
> arguments into the closure's arguments thus breaking the dependency.
> This can be fixed by simply creating a variable alias for each variable
> however the closure is a simple and concise fix.
> 

Similar here, "breaking the dependency" and "creating a variable alias"
can be described more accurately, e.g. the latter can be "creating a new
binding".

All in all, we need to establish a wide understanding of the issue and
agree on a reasonable fix. But anyway, thank Joel for doing this ;-)

Regards,
Boqun

> Another approach with using const-generics for the io.write32 API was
> investigated, but it cannot work with code that dynamically calculates
> the write offset.
> 
> Disassembly of users of pr_err!() with/without patch shows identical
> code generation, thus the fix has no difference in the final binary.
> 
> Signed-off-by: Joel Fernandes <joelagnelf@xxxxxxxxxx>
> ---
>  rust/kernel/print.rs | 44 ++++++++++++++++++++++++--------------------
>  1 file changed, 24 insertions(+), 20 deletions(-)
> 
> diff --git a/rust/kernel/print.rs b/rust/kernel/print.rs
> index 2d743d78d220..5847942195a7 100644
> --- a/rust/kernel/print.rs
> +++ b/rust/kernel/print.rs
> @@ -143,34 +143,38 @@ pub fn call_printk_cont(args: fmt::Arguments<'_>) {
>  #[expect(clippy::crate_in_macro_def)]
>  macro_rules! print_macro (
>      // The non-continuation cases (most of them, e.g. `INFO`).
> -    ($format_string:path, false, $($arg:tt)+) => (
> +    ($format_string:path, false, $($arg:tt)+) => ({
>          // To remain sound, `arg`s must be expanded outside the `unsafe` block.
>          // Typically one would use a `let` binding for that; however, `format_args!`
>          // takes borrows on the arguments, but does not extend the scope of temporaries.
>          // Therefore, a `match` expression is used to keep them around, since
>          // the scrutinee is kept until the end of the `match`.
> -        match $crate::prelude::fmt!($($arg)+) {
> -            // SAFETY: This hidden macro should only be called by the documented
> -            // printing macros which ensure the format string is one of the fixed
> -            // ones. All `__LOG_PREFIX`s are null-terminated as they are generated
> -            // by the `module!` proc macro or fixed values defined in a kernel
> -            // crate.
> -            args => unsafe {
> -                $crate::print::call_printk(
> -                    &$format_string,
> -                    crate::__LOG_PREFIX,
> -                    args,
> -                );
> +        (|| {
> +            match $crate::prelude::fmt!($($arg)+) {
> +                // SAFETY: This hidden macro should only be called by the documented
> +                // printing macros which ensure the format string is one of the fixed
> +                // ones. All `__LOG_PREFIX`s are null-terminated as they are generated
> +                // by the `module!` proc macro or fixed values defined in a kernel
> +                // crate.
> +                args => unsafe {
> +                    $crate::print::call_printk(
> +                        &$format_string,
> +                        crate::__LOG_PREFIX,
> +                        args,
> +                    );
> +                }
>              }
> -        }
> -    );
> +        })();
> +    });
>  
>      // The `CONT` case.
> -    ($format_string:path, true, $($arg:tt)+) => (
> -        $crate::print::call_printk_cont(
> -            $crate::prelude::fmt!($($arg)+),
> -        );
> -    );
> +    ($format_string:path, true, $($arg:tt)+) => ({
> +        (|| {
> +            $crate::print::call_printk_cont(
> +                $crate::prelude::fmt!($($arg)+),
> +            );
> +        })();
> +    });
>  );
>  
>  /// Stub for doctests
> -- 
> 2.34.1
>