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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Z6zT6mZuxonewQ9z@pollux>
Date: Wed, 12 Feb 2025 18:01:30 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Gary Guo <gary@...yguo.net>
Cc: Tamir Duberstein <tamird@...il.com>, 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 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?

> 
> 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;  
>