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

[Benno Lossin]

On Sat Sep 20, 2025 at 6:19 PM CEST, 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.
>
> The issue can be reproduced with the following simple snippet:
>   let offset = 0;
>   pr_err!("{}", offset);
>   io.write32(base, offset);

I took a long time to understand this and I think I got it now, but
maybe I'm still wrong: passing `offset` to `printk` via a reference
results in the compiler thinking that the value of `offset` might be
changed (even though its a shared reference I assume). For this reason
the `build_assert!` used by `io.write32` cannot be optimized away.

> 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.

I don't think this is the fix we want to have. In fact it already
doesn't compile all of the existing code:

    error[E0277]: the `?` operator can only be used in a closure that returns `Result` or `Option` (or another type that implements `FromResidual`)
        --> rust/doctests_kernel_generated.rs:3446:70
         |
    3446 |     pr_info!("The frequency at index 0 is: {:?}\n", table.freq(index)?);
         |     -----------------------------------------------------------------^-
         |     |                                                                |
         |     |                                                                cannot use the `?` operator in a closure that returns `()`
         |     this function should return `Result` or `Option` to accept `?`

(originating from `rust/kernel/cpufreq.rs:217`)

Can't we just mark the pointer properly as read-only?

---
Cheers,
Benno

> 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>