From: Linus Torvalds <torvalds@linux-foundation.org>
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
Subject: Re: [RFC] Mitigating unexpected arithmetic overflow
Date: Thu, 9 May 2024 11:08:12 -0700 [thread overview]
Message-ID: <CAHk-=wgXm15gZHvt8waSFhXS9yZTfvMV95xyvNtPc6FSEA_rvA@mail.gmail.com> (raw)
In-Reply-To: <20240509175417.GR2118490@ZenIV>
On Thu, 9 May 2024 at 10:54, Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Thu, May 09, 2024 at 08:38:28AM -0700, Linus Torvalds wrote:
>
> > Going the other way is similar:
> >
> > all_bits = low_bits + ((u64) high_bits << 16) << 16);
> >
> > and again, the compiler will recognize this idiom and do the right
> > thing (and if 'all_bits' is only 32-bit, the compiler will optimize
> > the high bit noise away).
>
> Umm... That would make sense if it was
> all_bits = low_bits + ((T) high_bits << 16) << 16);
> with possibly 32bit T. But the way you wrote that (with u64) it's
> pointless - u64 _can_ be shifted by 32 just fine.
Casting to 'T' is probably a clearer option but doesn't work very well
in a helper functions that may not know what the final type is.
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.
Linus
next prev parent reply other threads:[~2024-05-09 18:08 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-07 23:27 [RFC] Mitigating unexpected arithmetic overflow Kees Cook
2024-05-08 12:22 ` David Laight
2024-05-08 23:43 ` Kees Cook
2024-05-08 17:52 ` Linus Torvalds
2024-05-08 19:44 ` Kees Cook
2024-05-08 20:07 ` Linus Torvalds
2024-05-08 22:54 ` Kees Cook
2024-05-08 23:47 ` Linus Torvalds
2024-05-09 0:06 ` Linus Torvalds
2024-05-09 0:23 ` Linus Torvalds
2024-05-09 6:11 ` Kees Cook
2024-05-09 14:08 ` Theodore Ts'o
2024-05-09 15:38 ` Linus Torvalds
2024-05-09 17:54 ` Al Viro
2024-05-09 18:08 ` Linus Torvalds [this message]
2024-05-09 18:39 ` Linus Torvalds
2024-05-09 18:48 ` Al Viro
2024-05-09 19:15 ` Linus Torvalds
2024-05-09 19:28 ` Al Viro
2024-05-09 21:06 ` David Laight
2024-05-18 5:11 ` Matthew Wilcox
2024-05-09 21:23 ` David Laight
2024-05-12 8:03 ` Martin Uecker
2024-05-12 16:09 ` Linus Torvalds
2024-05-12 19:29 ` Martin Uecker
2024-05-13 18:34 ` Kees Cook
2024-05-15 7:36 ` Peter Zijlstra
2024-05-15 17:12 ` Justin Stitt
2024-05-16 7:45 ` Peter Zijlstra
2024-05-16 13:30 ` Kees Cook
2024-05-16 14:09 ` Peter Zijlstra
2024-05-16 19:48 ` Justin Stitt
2024-05-16 20:07 ` Kees Cook
2024-05-16 20:51 ` Theodore Ts'o
2024-05-17 21:15 ` Kees Cook
2024-05-18 2:51 ` Theodore Ts'o
2024-05-17 22:04 ` Fangrui Song
2024-05-18 13:08 ` David Laight
2024-05-15 7:57 ` Peter Zijlstra
2024-05-17 7:45 ` Jonas Oberhauser
2024-05-11 16:19 ` Dan Carpenter
2024-05-13 19:43 ` Kees Cook
2024-05-14 8:45 ` Dan Carpenter
2024-05-18 15:39 ` David Laight
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