Re: [PATCH v3] rust: check type of `$ptr` in `container_of!`

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Apr 28, 2025 at 12:54:19PM -0700, John Hubbard wrote:
> On 4/28/25 2:40 AM, Alice Ryhl wrote:
> > On Sun, Apr 27, 2025 at 03:59:48PM -0700, John Hubbard wrote:
> >> On 4/23/25 10:40 AM, Tamir Duberstein wrote:
> >> ...
> >>> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> >>> index 1df11156302a..d14ed86efb68 100644
> >>> --- a/rust/kernel/lib.rs
> >>> +++ b/rust/kernel/lib.rs
> >>> @@ -198,9 +198,15 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
> >>>   /// ```
> >>>   #[macro_export]
> >>>   macro_rules! container_of {
> >>> -    ($ptr:expr, $type:ty, $($f:tt)*) => {{
> >>> -        let offset: usize = ::core::mem::offset_of!($type, $($f)*);
> >>> -        $ptr.byte_sub(offset).cast::<$type>()
> >>> +    ($field_ptr:expr, $Container:ty, $($fields:tt)*) => {{
> >>> +        let offset: usize = ::core::mem::offset_of!($Container, $($fields)*);
> >>> +        let field_ptr = $field_ptr;
> >>> +        let container_ptr = field_ptr.byte_sub(offset).cast::<$Container>();
> >>> +        if false {
> >>
> >> This jumped out at me. It's something that I'd like to recommend NOT
> >> doing, here or anywhere else, because:
> >>
> >>     a) Anything of the form "if false" will get removed by any compiler
> >>        worthy of the name, especially in kernel builds.
> > 
> > The `if false` branch is used to trigger a compilation failure when the
> > macro is used incorrectly. The intent is that the compiler should
> > optimize it out. I don't think there's anything wrong with that pattern.
> 
> OK...probably best to either encapsulate that, or at least comment
> it. I'm accustomed to seeing that pattern in cases where people
> expected the code to *not* get optimized out, so it triggers me. :)

Okay ... why exactly would people do that? I can't imagine what purpose
that would serve.

Alice