Re: [PATCH v4 2/2] rust: add firmware abstractions

[Danilo Krummrich <dakr@redhat.com>]

On 6/20/24 15:36, Gary Guo wrote:
> On Tue, 18 Jun 2024 17:48:35 +0200
> Danilo Krummrich <dakr@redhat.com> 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              | 101 +++++++++++++++++++++++++++
>>   rust/kernel/lib.rs                   |   2 +
>>   4 files changed, 111 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..b55ea1b45368
>> --- /dev/null
>> +++ b/rust/kernel/firmware.rs
>> @@ -0,0 +1,101 @@
>> +// 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`.
>> +///
>> +/// Once requested, the `Firmware` backing buffer is not modified until it is freed when `Firmware`
>> +/// is dropped.
>> +///
>> +/// # 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>);
>> +
>> +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()) };
> 
> This is line is unsound if this function is called with an arbitrary
> FwFunc, therefore the safety comment should also mention that `func`
> cannot be an arbitrary function and it must be one of
> `request_firmware`, `firmware_request_nowarn`,
> `firmware_request_platform`, `request_firmware_direct`, and this is

This is documented in the type definition of `FwFunc`. We can link to this invariant though
and explicitly mark it as such. Does that make sense?

- Danilo

> true because the function is not public and all users in the file
> satisfy this safety precondition.
> 
> 
>> +        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
>> +        // 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)]
>