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Date:   Wed, 29 Mar 2023 21:34:21 +0100
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>,
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        Sven Van Asbroeck <thesven73@gmail.com>,
        Fox Chen <foxhlchen@gmail.com>,
        Martin Rodriguez Reboredo <yakoyoku@gmail.com>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        asahi@lists.linux.dev
Subject: Re: [PATCH v2 2/6] rust: error: Add Error::to_ptr()
Message-ID: <20230329213421.7d132f78.gary@garyguo.net>
In-Reply-To: <20230224-rust-error-v2-2-3900319812da@asahilina.net>
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On Wed, 29 Mar 2023 21:04:34 +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);
> +
>  /*
>   * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
>   * as the Rust `usize` type, so we can use it in contexts where Rust
> diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
> index 35894fa35efe..e97e652a1aec 100644
> --- a/rust/kernel/error.rs
> +++ b/rust/kernel/error.rs
> @@ -76,6 +76,13 @@ impl Error {
>      pub fn to_errno(self) -> core::ffi::c_int {
>          self.0
>      }
> +
> +    /// Returns the error encoded as a pointer.
> +    #[allow(dead_code)]
> +    pub(crate) fn to_ptr<T>(self) -> *mut T {
> +        // SAFETY: Valid as long as self.0 is a valid error

This should be something like "SAFETY: self.0 is a valid error due to
its invariant."?

Best,
Gary

> +        unsafe { bindings::ERR_PTR(self.0.into()) as *mut _ }
> +    }
>  }
>  
>  impl From<AllocError> for Error {
>