linux-kernel - Re: [PATCH 3/3] rust: add WMI abstractions

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Message-ID: <284c09c9-c4ed-4838-a76b-dc90210b0be6@foxido.dev>
Date: Fri, 9 Jan 2026 14:31:08 +0300
From: Gladyshev Ilya <foxido@...ido.dev>
To: Kari Argillander <kari.argillander@...il.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Len Brown <lenb@...nel.org>,
 Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>, Benno Lossin <lossin@...nel.org>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>,
 Tamir Duberstein <tamird@...il.com>, Armin Wolf <W_Armin@....de>,
 platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org, linux-acpi@...r.kernel.org
Subject: Re: [PATCH 3/3] rust: add WMI abstractions

On 1/9/26 14:15, Kari Argillander wrote:
> pe 9.1.2026 klo 13.01 Gladyshev Ilya (foxido@...ido.dev) kirjoitti:
>>
>> On 1/8/26 23:48, Kari Argillander wrote:
>>> On Wed, 7 Jan 2026 at 22:56, Gladyshev Ilya <foxido@...ido.dev> wrote:
>>> <snip>
>>>
>>>> +impl DeviceId {
>>>> +    /// Constructs new DeviceId from GUID string.
>>>> +    pub const fn new(guid: &[u8; bindings::UUID_STRING_LEN as usize]) -> Self {
>>>> +        // SAFETY: FFI type is valid to be zero-initialized.
>>>> +        let mut inner: bindings::wmi_device_id = unsafe { MaybeUninit::zeroed().assume_init() };
>>>> +
>>>> +        build_assert!(inner.guid_string.len() == bindings::UUID_STRING_LEN as usize + 1);
>>>> +
>>>> +        // SAFETY: It's safe to copy UUID_STRING_LEN, because we validated lengths.
>>>> +        // Also we leave last byte zeroed, so guid_string is valid C string.
>>>> +        unsafe {
>>>> +            ::core::ptr::copy_nonoverlapping(
>>>> +                guid.as_ptr(),
>>>> +                &raw mut inner.guid_string[0],
>>>> +                bindings::UUID_STRING_LEN as usize,
>>>> +            );
>>>> +        }
>>>
>>> Just use while here so no unsafe is needed at all. Then probably patch
>>> 1/3 is not needed.
>>
>> Overall this operation is still unsafe because we are constructing C
>> string in FFI object. So for me avoiding `unsafe` via less readable
>> (imo) loop will just mask unsafe operation without any real benefits.
> 
> It is not unsafe if you also use pin_init::zeroed()
>          let mut inner: bindings::wmi_device_id = pin_init::zeroed();
> 
>          let mut i = 0usize;
>          while i < bindings::UUID_STRING_LEN as usize {
>              inner.guid_string[i] = guid[i];
>              i += 1;
>          }
> 
> you can then also remove 'core::mem::MaybeUninit'

I was talking more about "constructing C string is still 'unsafe' 
because you shouldn't miss the \0 byte". IMO unsafe but primitive memcpy 
is more readable and alerting than while loop, but that's not something 
I will insist on

I'll play around more with `guid: &CStr` API, probably I missed 
something simple)