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

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Message-ID: <CAJ-ks9kaZ=nwfNXexJxqGy+R6EKmn5xaft5rOpnUF94052n8MA@mail.gmail.com>
Date: Wed, 12 Feb 2025 10:42:52 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, DJ Delorie <dj@...hat.com>, Eric Blake <eblake@...hat.com>, 
	Paul Eggert <eggert@...ucla.edu>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, rust-for-linux@...r.kernel.org, 
	linux-man@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] rust: alloc: satisfy POSIX alignment requirement

On Wed, Feb 12, 2025 at 10:40 AM Danilo Krummrich <dakr@...nel.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.

It's not guaranteed, but it does work. I could use some clarification
on the appropriate use of `build_error`. Here I'm using it to mean "I
want the compiler to let me know if the guarantees change". When is
that inappropriate?

> 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;