[PATCH] rust: kernel: drop repetition in offset_of macro

[PATCH] rust: kernel: drop repetition in offset_of macro

[Wei Liu]

It doesn't make sense to allow multiple fields to be specified in
offset_of.

No functional change.

Signed-off-by: Wei Liu <wei.liu@kernel.org>
---
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Alex Gaynor <alex.gaynor@gmail.com>
Cc: Wedson Almeida Filho <wedsonaf@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Gary Guo <gary@garyguo.net>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.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

Re: [PATCH] rust: kernel: drop repetition in offset_of macro

[Wedson Almeida Filho]

On Fri, 16 Dec 2022 at 17:49, Wei Liu <wei.liu@kernel.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)
```

Which is something very plausible.

> No functional change.
>
> Signed-off-by: Wei Liu <wei.liu@kernel.org>
> ---
> Cc: Miguel Ojeda <ojeda@kernel.org>
> Cc: Alex Gaynor <alex.gaynor@gmail.com>
> Cc: Wedson Almeida Filho <wedsonaf@gmail.com>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Gary Guo <gary@garyguo.net>
> Cc: Björn Roy Baron <bjorn3_gh@protonmail.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
>

Re: [PATCH] rust: kernel: drop repetition in offset_of macro

[Boqun Feng]

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@kernel.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@kernel.org>
> > ---
> > Cc: Miguel Ojeda <ojeda@kernel.org>
> > Cc: Alex Gaynor <alex.gaynor@gmail.com>
> > Cc: Wedson Almeida Filho <wedsonaf@gmail.com>
> > Cc: Boqun Feng <boqun.feng@gmail.com>
> > Cc: Gary Guo <gary@garyguo.net>
> > Cc: Björn Roy Baron <bjorn3_gh@protonmail.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
> >

Re: [PATCH] rust: kernel: drop repetition in offset_of macro

[Wei Liu]

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@kernel.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)
> ```
> 
> Which is something very plausible.

You're right. I didn't consider that use case. This patch can be
ignored.

Thanks,
Wei.

Re: [PATCH] rust: kernel: drop repetition in offset_of macro

[Wei Liu]

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@kernel.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)
> ```
> 
> Which is something very plausible.
> 
> > No functional change.
> >
> > Signed-off-by: Wei Liu <wei.liu@kernel.org>
> > ---
> > Cc: Miguel Ojeda <ojeda@kernel.org>
> > Cc: Alex Gaynor <alex.gaynor@gmail.com>
> > Cc: Wedson Almeida Filho <wedsonaf@gmail.com>
> > Cc: Boqun Feng <boqun.feng@gmail.com>
> > Cc: Gary Guo <gary@garyguo.net>
> > Cc: Björn Roy Baron <bjorn3_gh@protonmail.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)*) => {{

Shouldn't this be + instead of *?

offset_of!(X,) is valid according to this pattern.

Thanks,
Wei.

Re: [PATCH] rust: kernel: drop repetition in offset_of macro

[Wei Liu]

On Fri, Dec 16, 2022 at 02:15:16PM -0800, Boqun Feng wrote:
> 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@kernel.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
> ;-)
> 

What will be  even better is someone please contribute such a macro to
libcore so that I don't have to replicate the code snippet everywhere.
:-)

I have a version somewhere, the second argument matches against ident,
which was definitely not as flexible as tt.

Thanks,
Wei.

Re: [PATCH] rust: kernel: drop repetition in offset_of macro

[Miguel Ojeda]

On Sat, Dec 17, 2022 at 12:30 AM Wei Liu <wei.liu@kernel.org> wrote:
>
> What will be  even better is someone please contribute such a macro to
> libcore so that I don't have to replicate the code snippet everywhere.
> :-)

It is happening! :-) See https://github.com/rust-lang/rfcs/pull/3308,
currently at the end of the FCP ("final comment period"), i.e. the RFC
is likely getting accepted soon.

We track it at https://github.com/Rust-for-Linux/linux/issues/514 (one
of the sub-lists in issue #2).

Cheers,
Miguel