Re: [PATCH v10 12/27] rust: add `kernel` crate

[Alex Gaynor]

On Tue, Sep 27, 2022 at 10:22 AM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Sep 27, 2022 at 03:14:43PM +0200, Miguel Ojeda wrote:
> > +unsafe impl GlobalAlloc for KernelAllocator {
> > +    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
> > +        // `krealloc()` is used instead of `kmalloc()` because the latter is
> > +        // an inline function and cannot be bound to as a result.
> > +        unsafe { bindings::krealloc(ptr::null(), layout.size(), bindings::GFP_KERNEL) as *mut u8 }
>
> This feels "odd" to me.  Why not just use __kmalloc() instead of
> krealloc()?  I think that will get you the same kasan tracking, and
> should be a tiny bit faster (1-2 less function calls).
>

This may literally be the oldest code in the project :-). To the best
of my recollection, it's krealloc simply because that seemed like a
public API that worked. I don't think it even occurred to use to look
at __kmalloc.

> I guess it probably doesn't matter right now, just curious, and not a
> big deal at all.
>
> Other minor comments:
>
>
> > +/// Contains the C-compatible error codes.
> > +pub mod code {
> > +    /// Out of memory.
> > +    pub const ENOMEM: super::Error = super::Error(-(crate::bindings::ENOMEM as i32));
> > +}
>
> You'll be adding other error values here over time, right?
>

Yes -- this is the most minimal set that's needed for the initial
patch set. The full branch has every error constant bound.

>
> > +/// A [`Result`] with an [`Error`] error type.
> > +///
> > +/// To be used as the return type for functions that may fail.
> > +///
> > +/// # Error codes in C and Rust
> > +///
> > +/// In C, it is common that functions indicate success or failure through
> > +/// their return value; modifying or returning extra data through non-`const`
> > +/// pointer parameters. In particular, in the kernel, functions that may fail
> > +/// typically return an `int` that represents a generic error code. We model
> > +/// those as [`Error`].
> > +///
> > +/// In Rust, it is idiomatic to model functions that may fail as returning
> > +/// a [`Result`]. Since in the kernel many functions return an error code,
> > +/// [`Result`] is a type alias for a [`core::result::Result`] that uses
> > +/// [`Error`] as its error type.
> > +///
> > +/// Note that even if a function does not return anything when it succeeds,
> > +/// it should still be modeled as returning a `Result` rather than
> > +/// just an [`Error`].
> > +pub type Result<T = ()> = core::result::Result<T, Error>;
>
> What about functions that do have return functions of:
>         >= 0 number of bytes read/written/consumed/whatever
>         < 0  error code
>
> Would that use Result or Error as a type?  Or is it best just to not try
> to model that mess in Rust calls? :)
>

We'd model that as a `Result<usize>`. Negative values would become
`Err(EWHATEVER)` and non-negative values would be `Ok(n)`. Then at the
boundaries of Rust/C code we'd convert as appropriate.

> > +macro_rules! pr_info (
> > +    ($($arg:tt)*) => (
> > +        $crate::print_macro!($crate::print::format_strings::INFO, $($arg)*)
> > +    )
> > +);
>
> In the long run, using "raw" print macros like this is usually not the
> thing to do.  Drivers always have a device to reference the message to,
> and other things like filesystems and subsystems have a prefix to use as
> well.
>
> Hopefully not many will use these as-is and we can wrap them properly
> later on.
>
> Then there's the whole dynamic debug stuff, but that's a different
> topic.
>
> Anyway, all looks sane to me, sorry for the noise:
>
> Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Cheers,
Alex

-- 
All that is necessary for evil to succeed is for good people to do nothing.