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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 9 May 2024 11:39:04 -0700
Message-ID: <CAHk-=wgBprh=8Us-MtwH9sVNELZK2hdOkFn3EoauwecYgtXOCQ@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 11:08, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
>  Any half-way decent compiler will end up optimizing away the shifts
> and adds for the high bits because they see the assignment to
> 'all_bits'. There's no point in generating high bits that just get
> thrown away.

. it might also actually be a good idea *IF* we were to have some
kind of "implicit cast drops bits" warning, in that the compiler for
that case wouldn't remove the upper bits calculation, but would
trigger a warning if they are non-zero.

So there are actually potential advantages to just always apparently
doing the full 64-bit arithmetic.

Without debug warnings, it's a no-op that the compiler will just skip.
And with some hypothetical debug flag, it would be a "you are now
losing the high bits of the time value when assigning the result to a
limited 32-bit time_t" warning.

             Linus