Date:   Sat, 25 Feb 2023 22:14:05 +0000
From:   Gary Guo <gary@garyguo.net>
To:     Asahi Lina <lina@asahilina.net>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        Sven Van Asbroeck <thesven73@gmail.com>,
        Fox Chen <foxhlchen@gmail.com>, rust-for-linux@vger.kernel.org,
        linux-kernel@vger.kernel.org, asahi@lists.linux.dev
Subject: Re: [PATCH 1/5] rust: error: Add Error::to_ptr()
Message-ID: <20230225221405.62e989c8.gary@garyguo.net>
In-Reply-To: <20230224-rust-error-v1-1-f8f9a9a87303@asahilina.net>
References: <20230224-rust-error-v1-0-f8f9a9a87303@asahilina.net>
        <20230224-rust-error-v1-1-f8f9a9a87303@asahilina.net>
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On Fri, 24 Feb 2023 17:50:19 +0900
Asahi Lina <lina@asahilina.net> wrote:

> This is the Rust equivalent to ERR_PTR(), for use in C callbacks.
> Marked as #[allow(dead_code)] for now, since it does not have any
> consumers yet.
> 
> Signed-off-by: Asahi Lina <lina@asahilina.net>
> ---
>  rust/helpers.c       | 7 +++++++
>  rust/kernel/error.rs | 7 +++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/rust/helpers.c b/rust/helpers.c
> index 09a4d93f9d62..89f4cd1e0df3 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -20,6 +20,7 @@
>  
>  #include <linux/bug.h>
>  #include <linux/build_bug.h>
> +#include <linux/err.h>
>  #include <linux/refcount.h>
>  
>  __noreturn void rust_helper_BUG(void)
> @@ -46,6 +47,12 @@ bool rust_helper_refcount_dec_and_test(refcount_t *r)
>  }
>  EXPORT_SYMBOL_GPL(rust_helper_refcount_dec_and_test);
>  
> +__force void *rust_helper_ERR_PTR(long err)
> +{
> +	return ERR_PTR(err);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_ERR_PTR);
> +

I know that we already have `IS_ERR` in helpers.c, but having to go
through FFI and helper functions for something as simple as a cast
feels awkward to me.

Given that `ERR_PTR`'s C definition is very unlike to change, would it
be problematic if we just reimplement it in Rust as

```rust
fn ERR_PTR(error: core::ffi::c_long) -> *mut core::ffi::c_void {
    error as _
    // Or `core::ptr::invalid(error as _)` with strict provenance
}
```
?

I personally think it should be fine, but I'll leave the decision to
Miguel.

Best,
Gary