Re: [PATCH v5] rust: alloc: satisfy POSIX alignment requirement

[Danilo Krummrich <dakr@kernel.org>]

On Wed, Feb 12, 2025 at 04:38:48PM +0000, Gary Guo wrote:
> On Wed, 12 Feb 2025 16:40:37 +0100
> Danilo Krummrich <dakr@kernel.org> wrote:
> 
> > On Wed, Feb 12, 2025 at 09:43:02AM -0500, Tamir Duberstein wrote:
> > > diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
> > > index e3240d16040b..17a475380253 100644
> > > --- a/rust/kernel/alloc/allocator_test.rs
> > > +++ b/rust/kernel/alloc/allocator_test.rs
> > > @@ -62,6 +62,26 @@ unsafe fn realloc(
> > >              ));
> > >          }
> > >  
> > > +        // ISO C (ISO/IEC 9899:2011) defines `aligned_alloc`:
> > > +        //
> > > +        // > The value of alignment shall be a valid alignment supported by the implementation
> > > +        // [...].
> > > +        //
> > > +        // As an example of the "supported by the implementation" requirement, POSIX.1-2001 (IEEE
> > > +        // 1003.1-2001) defines `posix_memalign`:
> > > +        //
> > > +        // > The value of alignment shall be a power of two multiple of sizeof (void *).
> > > +        //
> > > +        // and POSIX-based implementations of `aligned_alloc` inherit this requirement. At the time
> > > +        // of writing, this is known to be the case on macOS (but not in glibc).
> > > +        //
> > > +        // Satisfy the stricter requirement to avoid spurious test failures on some platforms.
> > > +        let min_align = core::mem::size_of::<*const crate::ffi::c_void>();
> > > +        let layout = layout.align_to(min_align).unwrap_or_else(|_err| {
> > > +            crate::build_error!("invalid alignment")  
> > 
> > That's not what I thought this patch will look like. I thought you'll directly
> > follow Gary's proposal, which is why I said you can keep the ACK.
> > 
> > build_error!() doesn't work here, there is no guarantee that this can be
> > evaluated at compile time.
> 
> `align_to` will only fail if `min_align` is not a valid alignment (i.e.
> not power of two), which the compiler should be easy to notice that the
> size of pointer is indeed power of 2.

From the documentation of align_to():

"Returns an error if the combination of self.size() and the given align violates
the conditions listed in Layout::from_size_align."

Formally self.size() may still be unknown at compile time.

Do I miss anything?

> 
> I think both `build_error!` and `map_err` version below is fine to me. 
> 
> Best,
> Gary
> 
> > 
> > I think this should just be:
> > 
> > let layout = layout.align_to(min_align).map_err(|_| AllocError)?.pad_to_align();
> > 
> > - Danilo
> > 
> > > +        });
> > > +        let layout = layout.pad_to_align();
> > > +
> > >          // SAFETY: Returns either NULL or a pointer to a memory allocation that satisfies or
> > >          // exceeds the given size and alignment requirements.
> > >          let dst = unsafe { libc_aligned_alloc(layout.align(), layout.size()) } as *mut u8;  
>