Implement array and string parameters

Author: adamrk

drivers/char/rust_example.rs

@@ -101,7 +101,7 @@ impl KernelModule for RustExample {
         // stack probing on the supported archs.
         // This will verify that stack probing does not lead to
         // any errors if we need to link `__rust_probestack`.
-        let x: [u64; 1028] = core::hint::black_box([5; 1028]);
+        let x: [u64; 514] = core::hint::black_box([5; 514]);
         println!("Large array has length: {}", x.len());
 
         let mut chrdev_reg =

rust/kernel/lib.rs

@@ -12,7 +12,7 @@
 //! do so first instead of bypassing this crate.
 
 #![no_std]
-#![feature(allocator_api, alloc_error_handler)]
+#![feature(allocator_api, alloc_error_handler, const_fn, const_mut_refs)]
 #![deny(clippy::complexity)]
 #![deny(clippy::correctness)]
 #![deny(clippy::perf)]

rust/kernel/module_param.rs

@@ -15,6 +15,13 @@ use core::fmt::Write;
 ///
 /// [`PAGE_SIZE`]: `crate::PAGE_SIZE`
 pub trait ModuleParam: core::fmt::Display + core::marker::Sized {
+    /// The `ModuleParam` will be used by the kernel module through this type.
+    ///
+    /// This may differ from `Self` if, for example, `Self` needs to track
+    /// ownership without exposing it or allocate extra space for other possible
+    /// parameter values. See [`StringParam`] or [`ArrayParam`] for examples.
+    type Value: ?Sized;
+
     /// Whether the parameter is allowed to be set without an argument.
     ///
     /// Setting this to `true` allows the parameter to be passed without an
@@ -27,7 +34,13 @@ pub trait ModuleParam: core::fmt::Display + core::marker::Sized {
     /// `arg == None` indicates that the parameter was passed without an
     /// argument. If `NOARG_ALLOWED` is set to `false` then `arg` is guaranteed
     /// to always be `Some(_)`.
-    fn try_from_param_arg(arg: Option<&[u8]>) -> Option<Self>;
+    fn try_from_param_arg(arg: Option<&'static [u8]>) -> Option<Self>;
+
+    /// Get the current value of the parameter for use in the kernel module.
+    ///
+    /// This function should not be used directly. Instead use the wrapper
+    /// `read` which will be generated by [`module::module`].
+    fn value(&self) -> &Self::Value;
 
     /// Set the module parameter from a string.
     ///
@@ -161,13 +174,18 @@ impl_parse_int!(usize);
 macro_rules! impl_module_param {
     ($ty:ident) => {
         impl ModuleParam for $ty {
+            type Value = $ty;
             const NOARG_ALLOWED: bool = false;
 
-            fn try_from_param_arg(arg: Option<&[u8]>) -> Option<Self> {
+            fn try_from_param_arg(arg: Option<&'static [u8]>) -> Option<Self> {
                 let bytes = arg?;
                 let utf8 = core::str::from_utf8(bytes).ok()?;
                 <$ty as crate::module_param::ParseInt>::from_str(utf8)
             }
+
+            fn value(&self) -> &Self::Value {
+                self
+            }
         }
     };
 }
@@ -184,27 +202,27 @@ macro_rules! impl_module_param {
 /// );
 /// ```
 macro_rules! make_param_ops {
-    ($ops:ident, $ty:ident) => {
-        make_param_ops!(
+    ($ops:ident, $ty:ty) => {
+        $crate::make_param_ops!(
             #[doc=""]
             $ops,
             $ty
         );
     };
-    ($(#[$meta:meta])* $ops:ident, $ty:ident) => {
+    ($(#[$meta:meta])* $ops:ident, $ty:ty) => {
         $(#[$meta])*
         ///
         /// Static [`kernel_param_ops`](../../../include/linux/moduleparam.h)
         /// struct generated by [`make_param_ops`].
-        pub static $ops: crate::bindings::kernel_param_ops = crate::bindings::kernel_param_ops {
-            flags: if <$ty as crate::module_param::ModuleParam>::NOARG_ALLOWED {
-                crate::bindings::KERNEL_PARAM_OPS_FL_NOARG
+        pub static $ops: $crate::bindings::kernel_param_ops = $crate::bindings::kernel_param_ops {
+            flags: if <$ty as $crate::module_param::ModuleParam>::NOARG_ALLOWED {
+                $crate::bindings::KERNEL_PARAM_OPS_FL_NOARG
             } else {
                 0
             },
-            set: Some(<$ty as crate::module_param::ModuleParam>::set_param),
-            get: Some(<$ty as crate::module_param::ModuleParam>::get_param),
-            free: Some(<$ty as crate::module_param::ModuleParam>::free),
+            set: Some(<$ty as $crate::module_param::ModuleParam>::set_param),
+            get: Some(<$ty as $crate::module_param::ModuleParam>::get_param),
+            free: Some(<$ty as $crate::module_param::ModuleParam>::free),
         };
     };
 }
@@ -282,16 +300,21 @@ make_param_ops!(
 );
 
 impl ModuleParam for bool {
+    type Value = bool;
     const NOARG_ALLOWED: bool = true;
 
-    fn try_from_param_arg(arg: Option<&[u8]>) -> Option<Self> {
+    fn try_from_param_arg(arg: Option<&'static [u8]>) -> Option<Self> {
         match arg {
             None => Some(true),
             Some(b"y") | Some(b"Y") | Some(b"1") | Some(b"true") => Some(true),
             Some(b"n") | Some(b"N") | Some(b"0") | Some(b"false") => Some(false),
             _ => None,
         }
     }
+
+    fn value(&self) -> &Self::Value {
+        self
+    }
 }
 
 make_param_ops!(
@@ -300,3 +323,157 @@ make_param_ops!(
     PARAM_OPS_BOOL,
     bool
 );
+
+/// An array of at __most__ `N` values.
+///
+/// # Invariant
+///
+/// The first `self.used` elements of `self.values` are initialized.
+pub struct ArrayParam<T, const N: usize> {
+    values: [core::mem::MaybeUninit<T>; N],
+    used: usize,
+}
+
+impl<T, const N: usize> ArrayParam<T, { N }> {
+    fn values(&self) -> &[T] {
+        // SAFETY: The invariant maintained by `ArrayParam` allows us to cast
+        // the first `self.used` elements to `T`.
+        unsafe {
+            &*(&self.values[0..self.used] as *const [core::mem::MaybeUninit<T>] as *const [T])
+        }
+    }
+}
+
+impl<T: Copy, const N: usize> ArrayParam<T, { N }> {
+    const fn new() -> Self {
+        // INVARIANT: The first `self.used` elements of `self.values` are
+        // initialized.
+        ArrayParam {
+            values: [core::mem::MaybeUninit::uninit(); N],
+            used: 0,
+        }
+    }
+
+    const fn push(&mut self, val: T) {
+        if self.used < N {
+            // INVARIANT: The first `self.used` elements of `self.values` are
+            // initialized.
+            self.values[self.used] = core::mem::MaybeUninit::new(val);
+            self.used += 1;
+        }
+    }
+
+    /// Create an instance of `ArrayParam` initialized with `vals`.
+    ///
+    /// This function is only meant to be used in the [`module::module`] macro.
+    #[doc(hidden)]
+    pub const fn create(vals: &[T]) -> Self {
+        let mut result = ArrayParam::new();
+        let mut i = 0;
+        while i < vals.len() {
+            result.push(vals[i]);
+            i += 1;
+        }
+        result
+    }
+}
+
+impl<T: core::fmt::Display, const N: usize> core::fmt::Display for ArrayParam<T, { N }> {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        for val in self.values() {
+            write!(f, "{},", val)?;
+        }
+        Ok(())
+    }
+}
+
+impl<T: Copy + core::fmt::Display + ModuleParam, const N: usize> ModuleParam
+    for ArrayParam<T, { N }>
+{
+    type Value = [T];
+    const NOARG_ALLOWED: bool = false;
+
+    fn try_from_param_arg(arg: Option<&'static [u8]>) -> Option<Self> {
+        arg.and_then(|args| {
+            let mut result = Self::new();
+            for arg in args.split(|b| *b == b',') {
+                result.push(T::try_from_param_arg(Some(arg))?);
+            }
+            Some(result)
+        })
+    }
+
+    fn value(&self) -> &Self::Value {
+        self.values()
+    }
+}
+
+/// A C-style string parameter.
+///
+/// The Rust version of the [`charp`] parameter. This type is meant to be
+/// used by the [`module::module`] macro, not handled directly. Instead use the
+/// `read` method generated by that macro.
+///
+/// [`charp`]: ../../../include/linux/moduleparam.h
+pub enum StringParam {
+    /// A borrowed parameter value.
+    ///
+    /// Either the default value (which is static in the module) or borrowed
+    /// from the original argument buffer used to set the value.
+    Ref(&'static [u8]),
+
+    /// A value that was allocated when the parameter was set.
+    ///
+    /// The value needs to be freed when the parameter is reset or the module is
+    /// unloaded.
+    Owned(alloc::vec::Vec<u8>),
+}
+
+impl StringParam {
+    fn bytes(&self) -> &[u8] {
+        match self {
+            StringParam::Ref(bytes) => *bytes,
+            StringParam::Owned(vec) => &vec[..],
+        }
+    }
+}
+
+impl core::fmt::Display for StringParam {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        let bytes = self.bytes();
+        match core::str::from_utf8(bytes) {
+            Ok(utf8) => write!(f, "{}", utf8),
+            Err(_) => write!(f, "{:?}", bytes),
+        }
+    }
+}
+
+impl ModuleParam for StringParam {
+    type Value = [u8];
+    const NOARG_ALLOWED: bool = false;
+
+    fn try_from_param_arg(arg: Option<&'static [u8]>) -> Option<Self> {
+        // SAFETY: It is always safe to call [`slab_is_available`](../../../include/linux/slab.h).
+        let slab_available = unsafe { crate::bindings::slab_is_available() };
+        arg.map(|arg| {
+            if slab_available {
+                let mut vec = alloc::vec::Vec::new();
+                vec.extend_from_slice(arg);
+                StringParam::Owned(vec)
+            } else {
+                StringParam::Ref(arg)
+            }
+        })
+    }
+
+    fn value(&self) -> &Self::Value {
+        self.bytes()
+    }
+}
+
+make_param_ops!(
+    /// Rust implementation of [`kernel_param_ops`](../../../include/linux/moduleparam.h)
+    /// for [`StringParam`].
+    PARAM_OPS_STR,
+    StringParam
+);