Re: [PATCH WIP 5/8] rust: alloc: implement AllocatorWithFlags for KernelAllocator

[Danilo Krummrich <dakr@redhat.com>]

On Wed, May 01, 2024 at 08:44:46AM +0000, Benno Lossin wrote:
> On 29.04.24 22:11, Danilo Krummrich wrote:
> > Signed-off-by: Danilo Krummrich <dakr@redhat.com>
> > ---
> >  rust/kernel/alloc.rs           |  2 +-
> >  rust/kernel/alloc/allocator.rs | 38 ++++++++++++++++++++++++++++++++--
> >  2 files changed, 37 insertions(+), 3 deletions(-)
> > 
> > diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> > index ec45ba8f77ce..12c6c4e423f5 100644
> > --- a/rust/kernel/alloc.rs
> > +++ b/rust/kernel/alloc.rs
> > @@ -4,7 +4,7 @@
> > 
> >  #[cfg(not(test))]
> >  #[cfg(not(testlib))]
> > -mod allocator;
> > +pub mod allocator;
> >  pub mod box_ext;
> >  pub mod vec_ext;
> > 
> > diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
> > index 236100649ae1..173347ff018a 100644
> > --- a/rust/kernel/alloc/allocator.rs
> > +++ b/rust/kernel/alloc/allocator.rs
> > @@ -3,12 +3,14 @@
> >  //! Allocator support.
> > 
> >  use super::{flags::*, Flags};
> > -use core::alloc::{GlobalAlloc, Layout};
> > +use core::alloc::{GlobalAlloc, Layout, Allocator, AllocError};
> >  use core::ptr;
> > +use core::ptr::NonNull;
> > 
> >  use crate::bindings;
> > +use crate::alloc::AllocatorWithFlags;
> > 
> > -struct KernelAllocator;
> > +pub struct KernelAllocator;
> > 
> >  pub(crate) fn aligned_size(new_layout: Layout) -> usize {
> >      // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
> > @@ -89,6 +91,38 @@ unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
> >      }
> >  }
> > 
> > +unsafe impl Allocator for KernelAllocator {
> > +    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
> > +        self.default_allocate(layout)
> > +    }
> > +
> > +    fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
> > +        self.default_allocate_zeroed(layout)
> > +    }
> > +
> > +    unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
> > +        unsafe { self.dealloc(ptr.as_ptr(), layout) }
> > +    }
> > +}
> > +
> > +unsafe impl AllocatorWithFlags for KernelAllocator {
> > +    unsafe fn alloc_flags(&self, layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError>
> > +    {
> > +        unsafe { self.realloc_flags(ptr::null_mut(), 0, layout, flags) }
> > +    }
> > +
> > +    unsafe fn realloc_flags(&self, ptr: *mut u8, _old_size: usize, layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError>
> > +    {
> > +        let size = aligned_size(layout);
> > +
> > +        unsafe {
> > +            let raw_ptr = krealloc(ptr, size, flags);
> 
> I looked at the safety requirements of `krealloc`, it needs that
> `size` is non-zero. But who guarantees that? I could call
> `KernelAllocator::allocate()` with a zero-sized layout, since it is a
> safe function.

Indeed, as mentioned in the other thread, realloc_flags() shouldn't be unsafe
IMHO and instead should fulfill the safety requirement of krealloc().

Gonna fix that.

> 
> -- 
> Cheers,
> Benno
> 
> > +            let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
> > +            Ok(NonNull::slice_from_raw_parts(ptr, size))
> > +        }
> > +    }
> > +}
> > +
> >  #[global_allocator]
> >  static ALLOCATOR: KernelAllocator = KernelAllocator;
> > 
> > --
> > 2.44.0
> > 
>