From: David Laight <David.Laight@ACULAB.COM>
To: 'Linus Torvalds' <torvalds@linux-foundation.org>,
Theodore Ts'o <tytso@mit.edu>
Cc: 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-hardening@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"llvm@lists.linux.dev" <llvm@lists.linux.dev>
Subject: RE: [RFC] Mitigating unexpected arithmetic overflow
Date: Thu, 9 May 2024 21:06:54 +0000 [thread overview]
Message-ID: <5b37e8b1da534ebb8abc9b7b0f7022d2@AcuMS.aculab.com> (raw)
In-Reply-To: <CAHk-=wgKyP2ffZPa6aKYtytzzFibiNVN5MS=D2cn7_UGCECKdw@mail.gmail.com>
...
> 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).
On a 32bit system the compiler might not do the expected thing.
I had terrible trouble with the 32bit div_u64_u32() code I was
playing with getting the compiler to do 'something sensible' for
that.
I just couldn't get it to stop generating two 64bit values
(in two register pairs) and then oring them together.
I didn't try using a union - that might work.
On x64 the asm "A" (edx:eax) and "a" and "d" constraints will DTRT
but force the values into edx:eax which is ok if you are doing divides.
David
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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2024-05-09 21:07 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
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 [this message]
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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