From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH v1 bitops] bitops: Fix UBSAN undefined behavior warning for rotation right Date: Mon, 8 Apr 2019 15:52:17 -0700 Message-ID: <20190408155217.3f723554ae7fbcb34eeacb30@linux-foundation.org> References: <20190407125325.24440-1-vadimp@mellanox.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20190407125325.24440-1-vadimp@mellanox.com> Sender: linux-kernel-owner@vger.kernel.org To: Vadim Pasternak Cc: jacek.anaszewski@gmail.com, pavel@ucw.cz, linux-kernel@vger.kernel.org, linux-leds@vger.kernel.org, idosch@mellanox.com, Andrey Ryabinin List-Id: linux-leds@vger.kernel.org (resend, cc Andrey) On Sun, 7 Apr 2019 12:53:25 +0000 Vadim Pasternak 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?