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

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Message-ID: <Z6z-FlEUk9OfeJCV@cassiopeiae>
Date: Wed, 12 Feb 2025 21:01:26 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Tamir Duberstein <tamird@...il.com>
Cc: Gary Guo <gary@...yguo.net>, 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>,
	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 01:44:45PM -0500, Tamir Duberstein wrote:
> On Wed, Feb 12, 2025 at 12:01 PM Danilo Krummrich <dakr@...nel.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@...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.
> > >
> > > `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?
> 
> Formally, I agree. I tried testing (in allocator_test.rs):
> 
> #[cfg(test)]
> mod tests {
>     use super::*;
> 
>     #[test]
>     fn test_allocate() {
>         #[inline(never)]
>         fn non_const_usize() -> usize {
>             let x = 0;
>             &x as *const _ as usize
>         }
> 
>         let layout = Layout::array::<bool>(non_const_usize()).unwrap();
>         let ptr = Cmalloc::alloc(layout, GFP_KERNEL).unwrap();
>         let ptr = ptr.cast();
>         // SAFETY:
>         // - `ptr` was previously allocated with `Cmalloc`.
>         // - `layout` is equal to the `Layout´ `ptr` was allocated with.
>         unsafe { Cmalloc::free(ptr, layout) };
>     }
> }
> 
> and it compiled (and passed).

I suggest to try the following.

Move non_const_usize() into allocator_test.rs and within realloc(), try [1];
then try [2].

Besides that, I still think build_error!() can't be used here correctly, since
layout.size() might not be known at compile time. Please change things to what I
did suggest previously.

--

[1]
```
if non_const_usize() < 0x42 {
   crate::build_error!();
}
```

[2]
```
if non_const_usize() >= 0x42 {
   crate::build_error!();
}
```