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

[Gary Guo <gary@garyguo.net>]

On Wed, 12 Feb 2025 18:01:30 +0100
Danilo Krummrich <dakr@kernel.org> wrote:

> 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?

Ah, I think you're indeed correct. If I get a type that has the size of
`isize::MAX - 1` and alignment of 1, and then trying to align up to
pointer size will cause an error.

We should proceed with `map_err` then.

Best,
Gary

> 
> > 
> > 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