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    SubjectRe: [PATCH v10 12/27] rust: add `kernel` crate
    On Tue, Sep 27, 2022 at 10:22 AM Greg Kroah-Hartman
    <gregkh@linuxfoundation.org> 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@linuxfoundation.org>

    Cheers,
    Alex

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

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