lkml.org 
[lkml]   [2019]   [Apr]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH v1 bitops] bitops: Fix UBSAN undefined behavior warning for rotation right
(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?

\
 
 \ /
  Last update: 2019-04-09 00:52    [W:0.072 / U:0.964 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site