Received-SPF: pass (google.com: domain of linux-kernel+bounces-174805-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Precedence: bulk
MIME-Version: 1.0
References: <202405081144.D5FCC44A@keescook> <CAHk-=wjeiGb1UxCy6Q8aif50C=wWDX9Pgp+WbZYrO72+B1f_QA@mail.gmail.com>
 <202405081354.B0A8194B3C@keescook> <CAHk-=wgoE5EkH+sQwi4KhRhCZizUxwZAnC=+9RbZcw7g6016LQ@mail.gmail.com>
 <202405081949.0565810E46@keescook> <20240509140854.GF3620298@mit.edu>
 <CAHk-=wgKyP2ffZPa6aKYtytzzFibiNVN5MS=D2cn7_UGCECKdw@mail.gmail.com>
 <20240509175417.GR2118490@ZenIV> <CAHk-=wgXm15gZHvt8waSFhXS9yZTfvMV95xyvNtPc6FSEA_rvA@mail.gmail.com>
 <CAHk-=wgBprh=8Us-MtwH9sVNELZK2hdOkFn3EoauwecYgtXOCQ@mail.gmail.com> <20240509184806.GS2118490@ZenIV>
In-Reply-To: <20240509184806.GS2118490@ZenIV>
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 9 May 2024 12:15:40 -0700
Message-ID: <CAHk-=wjHA8Di-cpT0pKcScwcWNVYRFvmhBwMrug=Mj5WUwa2rw@mail.gmail.com>
Subject: Re: [RFC] Mitigating unexpected arithmetic overflow
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Theodore Ts'o" <tytso@mit.edu>, Kees Cook <keescook@chromium.org>, 
	Justin Stitt <justinstitt@google.com>, Peter Zijlstra <peterz@infradead.org>, 
	Mark Rutland <mark.rutland@arm.com>, linux-hardening@vger.kernel.org, 
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev
Content-Type: text/plain; charset="UTF-8"

On Thu, 9 May 2024 at 11:48, Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> FWIW, the thing that somewhat worries me about having a helper along
> the lines of combine_to_u64(low, high) is that
>         foo->splat = combine_to_u64(something, something_else);
> would be inviting hard-to-catch brainos -
>         foo->splat = combine_to_u64(something_else, something);

Yeah, we'd have to be very clear about naming and ordering. So it
would probably have to be something like

        result = combine_to_u64_hi_lo(high, low);

to be easy to use.

The good news is that if you *do* get it wrong despite clear naming,
the resulting value will be so obviously wrong that it's generally a
"Duh!" thing if you do any testing what-so-ever.

Of course, I say that as somebody who always points out that I haven't
tested my own patches at all, and they are "something like this,
perhaps?".

But having "hi_lo" kind of naming would hopefully make it really
obvious even when just looking at the source code.

              Linus