From: Andrew Morton <akpm@linux-foundation.org>
To: Vadim Pasternak <vadimp@mellanox.com>
Cc: jacek.anaszewski@gmail.com, pavel@ucw.cz,
linux-kernel@vger.kernel.org, linux-leds@vger.kernel.org,
idosch@mellanox.com, Andrey Ryabinin <aryabinin@virtuozzo.com>
Subject: Re: [PATCH v1 bitops] bitops: Fix UBSAN undefined behavior warning for rotation right
Date: Mon, 8 Apr 2019 15:52:17 -0700 [thread overview]
Message-ID: <20190408155217.3f723554ae7fbcb34eeacb30@linux-foundation.org> (raw)
In-Reply-To: <20190407125325.24440-1-vadimp@mellanox.com>
(resend, cc Andrey)
On Sun, 7 Apr 2019 12:53:25 +0000 Vadim Pasternak <vadimp@mellanox.com> wrote:
> The warning is caused by call to rorXX(), if the second parameters of
> this function "shift" is zero. In such case UBSAN reports the warning
> for the next expression: (word << (XX - shift), where XX is
> 64, 32, 16, 8 for respectively ror64, ror32, ror16, ror8.
> Fix adds validation of this parameter - in case it's equal zero, no
> need to rotate, just original "word" is to be returned to caller.
>
> The UBSAN undefined behavior warning has been reported for call to
> ror32():
> [ 11.426543] UBSAN: Undefined behaviour in ./include/linux/bitops.h:93:33
> [ 11.434045] shift exponent 32 is too large for 32-bit type 'unsigned int'
hm, do we care?
> ...
>
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -70,6 +70,9 @@ static inline __u64 rol64(__u64 word, unsigned int shift)
> */
> static inline __u64 ror64(__u64 word, unsigned int shift)
> {
> + if (!shift)
> + return word;
> +
> return (word >> shift) | (word << (64 - shift));
> }
Is there any known architecture or compiler for which UL<<64 doesn't
reliably produce zero? Is there any prospect that this will become a
problem in the future?
next prev parent reply other threads:[~2019-04-08 22:52 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-07 12:53 [PATCH v1 bitops] bitops: Fix UBSAN undefined behavior warning for rotation right Vadim Pasternak
2019-04-08 22:51 ` Andrew Morton
2019-04-08 22:52 ` Andrew Morton [this message]
2019-04-09 7:36 ` Vadim Pasternak
2019-04-09 8:08 ` Rasmus Villemoes
2019-04-09 8:57 ` Rasmus Villemoes
2019-04-09 9:50 ` Pavel Machek
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