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SubjectRe: [PATCH v1 bitops] bitops: Fix UBSAN undefined behavior warning for rotation right
Hi!

> (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?

Compiler is free to assume that shift !=0 after running ror64()... and
use that fact in optimalizations. so... if it is not problem today it
may easily become problem tommorow.
Pavel

--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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