Re: [PATCH v3 2/2] rust: add firmware abstractions

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Jun 17, 2024 at 10:29:41PM +0200, Danilo Krummrich wrote:
> Add an abstraction around the kernels firmware API to request firmware
> images. The abstraction provides functions to access the firmware's size
> and backing buffer.
> 
> The firmware is released once the abstraction instance is dropped.
> 
> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
> ---
>  drivers/base/firmware_loader/Kconfig |  7 ++
>  rust/bindings/bindings_helper.h      |  1 +
>  rust/kernel/firmware.rs              | 98 ++++++++++++++++++++++++++++
>  rust/kernel/lib.rs                   |  2 +
>  4 files changed, 108 insertions(+)
>  create mode 100644 rust/kernel/firmware.rs
> 
> diff --git a/drivers/base/firmware_loader/Kconfig b/drivers/base/firmware_loader/Kconfig
> index 5ca00e02fe82..a03701674265 100644
> --- a/drivers/base/firmware_loader/Kconfig
> +++ b/drivers/base/firmware_loader/Kconfig
> @@ -37,6 +37,13 @@ config FW_LOADER_DEBUG
>  	  SHA256 checksums to the kernel log for each firmware file that is
>  	  loaded.
>  
> +config RUST_FW_LOADER_ABSTRACTIONS
> +	bool "Rust Firmware Loader abstractions"
> +	depends on RUST
> +	depends on FW_LOADER=y
> +	help
> +	  This enables the Rust abstractions for the firmware loader API.
> +
>  if FW_LOADER
>  
>  config FW_LOADER_PAGED_BUF
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index ddb5644d4fd9..18a3f05115cb 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -9,6 +9,7 @@
>  #include <kunit/test.h>
>  #include <linux/errname.h>
>  #include <linux/ethtool.h>
> +#include <linux/firmware.h>
>  #include <linux/jiffies.h>
>  #include <linux/mdio.h>
>  #include <linux/phy.h>
> diff --git a/rust/kernel/firmware.rs b/rust/kernel/firmware.rs
> new file mode 100644
> index 000000000000..05a4f84cfd42
> --- /dev/null
> +++ b/rust/kernel/firmware.rs
> @@ -0,0 +1,98 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Firmware abstraction
> +//!
> +//! C header: [`include/linux/firmware.h`](srctree/include/linux/firmware.h")
> +
> +use crate::{bindings, device::Device, error::Error, error::Result, str::CStr};
> +use core::ptr::NonNull;
> +
> +// One of the following: `bindings::request_firmware`, `bindings::firmware_request_nowarn`,
> +// `firmware_request_platform`, `bindings::request_firmware_direct`
> +type FwFunc =
> +    unsafe extern "C" fn(*mut *const bindings::firmware, *const i8, *mut bindings::device) -> i32;
> +
> +/// Abstraction around a C `struct firmware`.
> +///
> +/// This is a simple abstraction around the C firmware API. Just like with the C API, firmware can
> +/// be requested. Once requested the abstraction provides direct access to the firmware buffer as
> +/// `&[u8]`. The firmware is released once [`Firmware`] is dropped.
> +///
> +/// # Invariants
> +///
> +/// The pointer is valid, and has ownership over the instance of `struct firmware`.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// # use kernel::{c_str, device::Device, firmware::Firmware};
> +///
> +/// # // SAFETY: *NOT* safe, just for the example to get an `ARef<Device>` instance
> +/// # let dev = unsafe { Device::from_raw(core::ptr::null_mut()) };
> +///
> +/// let fw = Firmware::request(c_str!("path/to/firmware.bin"), &dev).unwrap();
> +/// let blob = fw.data();
> +/// ```
> +pub struct Firmware(NonNull<bindings::firmware>);
> +

I feel like eventually we need a very simple smart pointer type for
these case, for example:

    /// A smart pointer owns the underlying data.
    pub struct Owned<T: Ownable> {
        ptr: NonNull<T>,
    }

    impl<T: Ownable> Owned<T> {
        /// # Safety
	/// `ptr` needs to be a valid pointer, and it should be the
	/// unique owner to the object, in other words, no one can touch
	/// or free the underlying data.
        pub unsafe to_owned(ptr: *mut T) -> Self {
	    // SAFETY: Per function safety requirement.
	    Self { ptr: unsafe { NonNull::new_unchecked(ptr) } }
	}

	/// other safe constructors are available if a initializer (impl
	/// Init) is provided
    }

    /// A Ownable type is a type that can be put into `Owned<T>`, and
    /// when `Owned<T>` drops, `ptr_drop` will be called.
    pub unsafe trait Ownable {
        /// # Safety
	/// This could only be called in the `Owned::drop` function.
        unsafe fn ptr_drop(ptr: *mut Self);
    }

    impl<T: Ownable> Drop for Owned<T> {
        fn drop(&mut self) {
	    /// SAFETY: In Owned<T>::drop.
	    unsafe {
	        <T as Ownable>::ptr_drop(self.as_mut_ptr());
	    }
	}
    }

we can implement Deref and DerefMut easily on `Owned<T>`. And then we
could define Firmware as

    #[repr(transparent)]
    pub struct Firmware(Opaque<bindings::firmware>);

and

    unsafe impl Ownable for Firmware {
        unsafe fn ptr_drop(ptr: *mut Self) {
	    // SAFETY: Per function safety, this is called in
	    // Owned::drop(), so `ptr` is a unique pointer to object,
	    // it's safe to release the firmware.
            unsafe { bindings::release_firmware(ptr.cast()); }
        }
    }

and the request_*() will return a `Result<Owned<Self>>`. 

Alice mentioned the need of this in page as well:

	https://lore.kernel.org/rust-for-linux/CAH5fLgjrt0Ohj1qBv=GrqZumBTMQ1jbsKakChmxmG2JYDJEM8w@mail.gmail.com		

Just bring it up while we are (maybe not? ;-)) at it. Also I would like
to hear whether this would work for Firmware in the longer-term ;-) But
yes, I'm not that worried about merging it as it is if others are all
OK.

> +impl Firmware {
> +    fn request_internal(name: &CStr, dev: &Device, func: FwFunc) -> Result<Self> {
> +        let mut fw: *mut bindings::firmware = core::ptr::null_mut();
> +        let pfw: *mut *mut bindings::firmware = &mut fw;
> +
> +        // SAFETY: `pfw` is a valid pointer to a NULL initialized `bindings::firmware` pointer.
> +        // `name` and `dev` are valid as by their type invariants.
> +        let ret = unsafe { func(pfw as _, name.as_char_ptr(), dev.as_raw()) };
> +        if ret != 0 {
> +            return Err(Error::from_errno(ret));
> +        }
> +
> +        // SAFETY: `func` not bailing out with a non-zero error code, guarantees that `fw` is a
> +        // valid pointer to `bindings::firmware`.
> +        Ok(Firmware(unsafe { NonNull::new_unchecked(fw) }))
> +    }
> +
> +    /// Send a firmware request and wait for it. See also `bindings::request_firmware`.
> +    pub fn request(name: &CStr, dev: &Device) -> Result<Self> {
> +        Self::request_internal(name, dev, bindings::request_firmware)
> +    }
> +
> +    /// Send a request for an optional firmware module. See also
> +    /// `bindings::firmware_request_nowarn`.
> +    pub fn request_nowarn(name: &CStr, dev: &Device) -> Result<Self> {
> +        Self::request_internal(name, dev, bindings::firmware_request_nowarn)
> +    }
> +
> +    fn as_raw(&self) -> *mut bindings::firmware {
> +        self.0.as_ptr()
> +    }
> +
> +    /// Returns the size of the requested firmware in bytes.
> +    pub fn size(&self) -> usize {
> +        // SAFETY: Safe by the type invariant.
> +        unsafe { (*self.as_raw()).size }
> +    }
> +
> +    /// Returns the requested firmware as `&[u8]`.
> +    pub fn data(&self) -> &[u8] {
> +        // SAFETY: Safe by the type invariant. Additionally, `bindings::firmware` guarantees, if

Does this "Safe by the type invariant" also covers the following safe
requirement of `from_raw_parts`?

	The memory referenced by the returned slice must not be mutated for the duration of lifetime 'a, except inside an UnsafeCell.

in that `&[u8]` has the same lifetime as `&self`, and as long as
`&self` exists, no function can touch the inner `data`? If so, I
probably want to call this out.

Regards,
Boqun

> +        // successfully requested, that `bindings::firmware::data` has a size of
> +        // `bindings::firmware::size` bytes.
> +        unsafe { core::slice::from_raw_parts((*self.as_raw()).data, self.size()) }
> +    }
> +}
> +
> +impl Drop for Firmware {
> +    fn drop(&mut self) {
> +        // SAFETY: Safe by the type invariant.
> +        unsafe { bindings::release_firmware(self.as_raw()) };
> +    }
> +}
> +
> +// SAFETY: `Firmware` only holds a pointer to a C `struct firmware`, which is safe to be used from
> +// any thread.
> +unsafe impl Send for Firmware {}
> +
> +// SAFETY: `Firmware` only holds a pointer to a C `struct firmware`, references to which are safe to
> +// be used from any thread.
> +unsafe impl Sync for Firmware {}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index dd1207f1a873..7707cb013ce9 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -30,6 +30,8 @@
>  mod build_assert;
>  pub mod device;
>  pub mod error;
> +#[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)]
> +pub mod firmware;
>  pub mod init;
>  pub mod ioctl;
>  #[cfg(CONFIG_KUNIT)]
> -- 
> 2.45.1
>