linux-kernel - Re: [WIP 0/3] Memory model and atomic API in Rust

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: <ZgSNvzTkR4CY7kQC@boqun-archlinux>
Date: Wed, 27 Mar 2024 14:21:03 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,	comex <comexk@...il.com>,
	"Dr. David Alan Gilbert" <dave@...blig.org>,
	Philipp Stanner <pstanner@...hat.com>,
	rust-for-linux <rust-for-linux@...r.kernel.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	llvm@...ts.linux.dev, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrea Parri <parri.andrea@...il.com>,	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nicholas Piggin <npiggin@...il.com>,	David Howells <dhowells@...hat.com>,
	Jade Alglave <j.alglave@....ac.uk>,	Luc Maranget <luc.maranget@...ia.fr>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Akira Yokosawa <akiyks@...il.com>,	Daniel Lustig <dlustig@...dia.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,	kent.overstreet@...il.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Marco Elver <elver@...gle.com>, Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,	Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-fsdevel@...r.kernel.org
Subject: Re: [WIP 0/3] Memory model and atomic API in Rust

On Wed, Mar 27, 2024 at 03:41:16PM -0400, Kent Overstreet wrote:
> On Wed, Mar 27, 2024 at 12:07:26PM -0700, Linus Torvalds wrote:
> > On Wed, 27 Mar 2024 at 11:51, Kent Overstreet <kent.overstreet@...ux.dev> wrote:
> > >
> > > On Wed, Mar 27, 2024 at 09:16:09AM -0700, comex wrote:
> > > > Meanwhile, Rust intentionally lacks strict aliasing.
> > >
> > > I wasn't aware of this. Given that unrestricted pointers are a real
> > > impediment to compiler optimization, I thought that with Rust we were
> > > finally starting to nail down a concrete enough memory model to tackle
> > > this safely. But I guess not?
> > 
> > Strict aliasing is a *horrible* mistake.
> > 
> > It's not even *remotely* "tackle this safely". It's the exact
> > opposite. It's completely broken.
> > 
> > Anybody who thinks strict aliasing is a good idea either
> > 
> >  (a) doesn't understand what it means
> > 
> >  (b) has been brainwashed by incompetent compiler people.
> > 
> > it's a horrendous crock that was introduced by people who thought it
> > was too complicated to write out "restrict" keywords, and that thought
> > that "let's break old working programs and make it harder to write new
> > programs" was a good idea.
> 
> Strict aliasing is crap in C and C++ because we started out with
> unrestricetd pointers, and it just doesn't work in C and C++ with the
> realities of the kind of code we have to write, and we never got any
> kind of a model that would have made it workable. Never mind trying to
> graft that onto existing codebases...
> 
> (Restrict was crap too... no scoping, nothing but a single f*cking
> keyword? Who ever thought _that_ was going to work?)
> 
> _But_: the lack of any aliasing guarantees means that writing through
> any pointer can invalidate practically anything, and this is a real

I don't know whether I'm 100% correct on this, but Rust has references,
so things like "you have a unique reference to a part of memory, no one
would touch it in the meanwhile" are represented by `&mut`, to get a
`&mut` from a raw pointer, you need unsafe, where programmers can
provide the reasoning of the safety of the accesses. More like "pointers
can alias anyone but references cannot" to me.

Regards,
Boqun

> problem. A lot of C programmers have stockholm syndrome when it comes to
> this, we end up writing a lot of code in weirdly baroque and artificial
> styles to partially work around this when we care about performance -
> saving things into locals because at least the _stack_ generally can't
> alias to avoid forced reloads, or passing and returning things by
> reference instead of by value when that's _not the semantics we want_
> because otherwise the compiler is going to do an unnecessary copy -
> again, that's fundamentally because of aliasing.