Re: [PATCH 1/2] rust: Add initial cpumask abstractions

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Feb 25, 2025 at 6:12 PM Alice Ryhl <aliceryhl@google.com> wrote:
>
> On Tue, Feb 25, 2025 at 6:02 PM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> >
> > On Tue, 25 Feb 2025 at 17:23, Alice Ryhl <aliceryhl@google.com> wrote:
> >
> > > Is it a problem if a value of type struct cpumask is moved? It looks
> > > like it is just an array of longs?
> >
> > With the current code, if I replace the Box with an on-stack variable,
> > the kernel crashes.
> >
> > In my usecase, the pointer to the cpumask array is sent to the OPP
> > core, which may update the content, though it doesn't save the pointer.
> >
> > But for another usecase, the C code may end up saving the pointer.
>
> I don't think that case applies here. Variables that could be stashed
> like that need a destructor that removes the pointer from the relevant
> C structures. This is because pinning only prevents the owner from
> moving the variable - it doesn't prevent the owner from destroying it.

I believe you can implement the offstack stuff like this. The idea is to have
one type that corresponds to `struct cpumask` and another type that corresponds
to `cpumask_var_t` on the C side. Only the latter type would need to do
anything wrt. the CPUMASK_OFFSTACK setting.


diff --git a/rust/kernel/cpumask.rs b/rust/kernel/cpumask.rs
index 13864424420b..f82dafbc4e61 100644
--- a/rust/kernel/cpumask.rs
+++ b/rust/kernel/cpumask.rs
@@ -4,111 +4,45 @@
 //!
 //! C header: [`include/linux/cpumask.h`](srctree/include/linux/cpumask.h)
 
-use crate::{bindings, error::Result, prelude::ENOMEM};
-
-#[cfg(not(CONFIG_CPUMASK_OFFSTACK))]
-use crate::prelude::{KBox, GFP_KERNEL};
-
+use crate::{
+    alloc::{AllocError, Flags},
+    bindings,
+    prelude::*,
+    types::Opaque,
+};
 #[cfg(CONFIG_CPUMASK_OFFSTACK)]
-use core::ptr;
+use core::ptr::{self, NonNull};
 
-/// A simple implementation of `struct cpumask` from the C code.
+/// This corresponds to the C type `struct cpumask`.
+#[repr(transparent)]
 pub struct Cpumask {
-    ptr: *mut bindings::cpumask,
-    owned: bool,
+    mask: Opaque<bindings::cpumask>,
 }
 
 impl Cpumask {
-    /// Creates cpumask.
-    #[cfg(CONFIG_CPUMASK_OFFSTACK)]
-    fn new_inner(empty: bool) -> Result<Self> {
-        let mut ptr: *mut bindings::cpumask = ptr::null_mut();
-
-        // SAFETY: Depending on the value of `gfp_flags`, this call may sleep. Other than that, it
-        // is always safe to call this method.
-        if !unsafe {
-            if empty {
-                bindings::zalloc_cpumask_var(&mut ptr, bindings::GFP_KERNEL)
-            } else {
-                bindings::alloc_cpumask_var(&mut ptr, bindings::GFP_KERNEL)
-            }
-        } {
-            return Err(ENOMEM);
-        }
-
-        Ok(Self { ptr, owned: true })
-    }
-
-    /// Creates cpumask.
-    #[cfg(not(CONFIG_CPUMASK_OFFSTACK))]
-    fn new_inner(empty: bool) -> Result<Self> {
-        let ptr = KBox::into_raw(KBox::new([bindings::cpumask::default(); 1], GFP_KERNEL)?);
-
-        // SAFETY: Depending on the value of `gfp_flags`, this call may sleep. Other than that, it
-        // is always safe to call this method.
-        if !unsafe {
-            if empty {
-                bindings::zalloc_cpumask_var(ptr, bindings::GFP_KERNEL)
-            } else {
-                bindings::alloc_cpumask_var(ptr, bindings::GFP_KERNEL)
-            }
-        } {
-            return Err(ENOMEM);
-        }
-
-        Ok(Self {
-            ptr: ptr as *mut _,
-            owned: true,
-        })
-    }
-
-    /// Creates empty cpumask.
-    pub fn new() -> Result<Self> {
-        Self::new_inner(true)
-    }
-
-    /// Creates uninitialized cpumask.
-    fn new_uninit() -> Result<Self> {
-        Self::new_inner(false)
-    }
-
-    /// Clones cpumask.
-    pub fn try_clone(&self) -> Result<Self> {
-        let mut cpumask = Self::new_uninit()?;
-
-        self.copy(&mut cpumask);
-        Ok(cpumask)
-    }
-
-    /// Creates a new abstraction instance of an existing `struct cpumask` pointer.
+    /// Creates a reference to an existing `struct cpumask` pointer.
     ///
     /// # Safety
     ///
-    /// Callers must ensure that `ptr` is valid, and non-null.
-    #[cfg(CONFIG_CPUMASK_OFFSTACK)]
-    pub unsafe fn get_cpumask(ptr: &mut *mut bindings::cpumask) -> Self {
-        Self {
-            ptr: *ptr,
-            owned: false,
-        }
+    /// The caller must ensure that `ptr` is valid for writing and remains valid for the lifetime
+    /// of the returned reference.
+    pub unsafe fn from_raw_mut<'a>(ptr: *mut bindings::cpumask) -> &'a mut Self {
+        unsafe { &mut *ptr.cast() }
     }
 
-    /// Creates a new abstraction instance of an existing `struct cpumask` pointer.
+    /// Creates a reference to an existing `struct cpumask` pointer.
     ///
     /// # Safety
     ///
-    /// Callers must ensure that `ptr` is valid, and non-null.
-    #[cfg(not(CONFIG_CPUMASK_OFFSTACK))]
-    pub unsafe fn get_cpumask(ptr: &mut bindings::cpumask_var_t) -> Self {
-        Self {
-            ptr: ptr as *mut _,
-            owned: false,
-        }
+    /// The caller must ensure that `ptr` is valid for reading and remains valid for the lifetime
+    /// of the returned reference.
+    pub unsafe fn from_raw<'a>(ptr: *const bindings::cpumask) -> &'a Self {
+        unsafe { &*ptr.cast() }
     }
 
     /// Obtain the raw `struct cpumask *`.
-    pub fn as_raw(&mut self) -> *mut bindings::cpumask {
-        self.ptr
+    pub fn as_raw(&self) -> *mut bindings::cpumask {
+        self as *const Cpumask as *mut bindings::cpumask
     }
 
     /// Sets CPU in the cpumask.
@@ -117,7 +51,7 @@ pub fn as_raw(&mut self) -> *mut bindings::cpumask {
     pub fn set(&mut self, cpu: u32) {
         // SAFETY: `ptr` is guaranteed to be valid for the lifetime of `self`. And it is safe to
         // call `cpumask_set_cpus()` for any CPU.
-        unsafe { bindings::cpumask_set_cpu(cpu, self.ptr) };
+        unsafe { bindings::cpumask_set_cpu(cpu, self.as_raw()) };
     }
 
     /// Clears CPU in the cpumask.
@@ -126,43 +60,74 @@ pub fn set(&mut self, cpu: u32) {
     pub fn clear(&mut self, cpu: i32) {
         // SAFETY: `ptr` is guaranteed to be valid for the lifetime of `self`. And it is safe to
         // call `cpumask_clear_cpu()` for any CPU.
-        unsafe { bindings::cpumask_clear_cpu(cpu, self.ptr) };
+        unsafe { bindings::cpumask_clear_cpu(cpu, self.as_raw()) };
     }
 
     /// Sets all CPUs in the cpumask.
     pub fn set_all(&mut self) {
         // SAFETY: `ptr` is guaranteed to be valid for the lifetime of `self`. And it is safe to
         // call `cpumask_setall()`.
-        unsafe { bindings::cpumask_setall(self.ptr) };
+        unsafe { bindings::cpumask_setall(self.as_raw()) };
     }
 
     /// Gets weight of a cpumask.
     pub fn weight(&self) -> u32 {
         // SAFETY: `ptr` is guaranteed to be valid for the lifetime of `self`. And it is safe to
         // call `cpumask_weight()`.
-        unsafe { bindings::cpumask_weight(self.ptr) }
+        unsafe { bindings::cpumask_weight(self.as_raw()) }
     }
 
     /// Copies cpumask.
     pub fn copy(&self, dstp: &mut Self) {
         // SAFETY: `ptr` is guaranteed to be valid for the lifetime of `self`. And it is safe to
         // call `cpumask_copy()`.
-        unsafe { bindings::cpumask_copy(dstp.as_raw(), self.ptr) };
+        unsafe { bindings::cpumask_copy(dstp.as_raw(), self.as_raw()) };
     }
 }
 
-impl Drop for Cpumask {
-    fn drop(&mut self) {
-        if self.owned {
-            // SAFETY: `ptr` is guaranteed to be valid for the lifetime of `self`. And it is safe
-            // to call `free_cpumask_var()`.
-            unsafe { bindings::free_cpumask_var(self.ptr) }
+/// This corresponds to the C type alias `cpumask_var_t`.
+pub struct CpumaskBox {
+    #[cfg(CONFIG_CPUMASK_OFFSTACK)]
+    ptr: NonNull<Cpumask>,
+    #[cfg(not(CONFIG_CPUMASK_OFFSTACK))]
+    mask: Cpumask,
+}
 
+impl CpumaskBox {
+    pub fn new(_flags: Flags) -> Result<Self, AllocError> {
+        Ok(Self {
+            #[cfg(CONFIG_CPUMASK_OFFSTACK)]
+            ptr: {
+                let mut ptr: *mut bindings::cpumask = ptr::null_mut();
+                unsafe { bindings::zalloc_cpumask_var(&mut ptr, _flags.as_raw()) };
+                NonNull::new(ptr.cast()).ok_or(AllocError)?
+            },
             #[cfg(not(CONFIG_CPUMASK_OFFSTACK))]
-            // SAFETY: The pointer was earlier initialized from the result of `KBox::into_raw()`.
-            unsafe {
-                drop(KBox::from_raw(self.ptr))
-            };
-        }
+            mask: unsafe { core::mem::zeroed() },
+        })
+    }
+}
+
+// Make CpumaskBox behave like a pointer to Cpumask.
+impl core::ops::Deref for CpumaskBox {
+    type Target = Cpumask;
+
+    #[cfg(CONFIG_CPUMASK_OFFSTACK)]
+    fn deref(&self) -> &Cpumask {
+        unsafe { &*self.ptr.as_ptr() }
+    }
+
+    #[cfg(not(CONFIG_CPUMASK_OFFSTACK))]
+    fn deref(&self) -> &Cpumask {
+        &self.mask
+    }
+}
+
+impl Drop for CpumaskBox {
+    fn drop(&mut self) {
+        #[cfg(CONFIG_CPUMASK_OFFSTACK)]
+        unsafe {
+            bindings::free_cpumask_var(self.as_raw())
+        };
     }
 }