linux-kernel - Re: [PATCH] rust: kernel: drop repetition in offset

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Date:   Fri, 16 Dec 2022 14:15:16 -0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Wedson Almeida Filho <wedsonaf@...il.com>
Cc:     Wei Liu <wei.liu@...nel.org>, rust-for-linux@...r.kernel.org,
        Linux Kernel List <linux-kernel@...r.kernel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>
Subject: Re: [PATCH] rust: kernel: drop repetition in offset_of macro

On Fri, Dec 16, 2022 at 06:26:57PM +0000, Wedson Almeida Filho wrote:
> On Fri, 16 Dec 2022 at 17:49, Wei Liu <wei.liu@...nel.org> wrote:
> >
> > It doesn't make sense to allow multiple fields to be specified in
> > offset_of.
> 
> Why do you say it doesn't make sense?
> 
> Here's what I had in mind:
> ```
>     struct Y {
>         z: u32
>     }
>     struct X {
>         y: Y
>     }
>     offset_of!(X, y.z)

For me, it's not very obvious that "y.z" is multiples of token trees
rather a single token tree ;-)

Maybe some examples of the match pattern of macros can help people catch
up faster? Like

	"y.z" => tt [y], tt [.], tt [z]

I will defer to Gary or Bjorn for a better quick guide of Rust macros
;-)

Regards,
Boqun

> ```
> 
> Which is something very plausible.
> 
> > No functional change.
> >
> > Signed-off-by: Wei Liu <wei.liu@...nel.org>
> > ---
> > Cc: Miguel Ojeda <ojeda@...nel.org>
> > Cc: Alex Gaynor <alex.gaynor@...il.com>
> > Cc: Wedson Almeida Filho <wedsonaf@...il.com>
> > Cc: Boqun Feng <boqun.feng@...il.com>
> > Cc: Gary Guo <gary@...yguo.net>
> > Cc: Björn Roy Baron <bjorn3_gh@...tonmail.com>
> > ---
> >  rust/kernel/lib.rs | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> > index 6a322effa60c..2f3601e4e27e 100644
> > --- a/rust/kernel/lib.rs
> > +++ b/rust/kernel/lib.rs
> > @@ -208,7 +208,7 @@ impl<'a> Drop for KParamGuard<'a> {
> >  /// ```
> >  #[macro_export]
> >  macro_rules! offset_of {
> > -    ($type:ty, $($f:tt)*) => {{
> > +    ($type:ty, $f:tt) => {{
> >          let tmp = core::mem::MaybeUninit::<$type>::uninit();
> >          let outer = tmp.as_ptr();
> >          // To avoid warnings when nesting `unsafe` blocks.
> > @@ -216,12 +216,14 @@ macro_rules! offset_of {
> >          // SAFETY: The pointer is valid and aligned, just not initialised; `addr_of` ensures that
> >          // we don't actually read from `outer` (which would be UB) nor create an intermediate
> >          // reference.
> > -        let inner = unsafe { core::ptr::addr_of!((*outer).$($f)*) } as *const u8;
> > +        let inner = unsafe { core::ptr::addr_of!((*outer).$f) } as *const u8;
> >          // To avoid warnings when nesting `unsafe` blocks.
> >          #[allow(unused_unsafe)]
> >          // SAFETY: The two pointers are within the same allocation block.
> > -        unsafe { inner.offset_from(outer as *const u8) }
> > -    }}
> > +        unsafe {
> > +            inner.offset_from(outer as *const u8)
> > +        }
> > +    }};
> >  }
> >
> >  /// Produces a pointer to an object from a pointer to one of its fields.
> > --
> > 2.35.1
> >