linux-kernel - Re: [PATCH] bitops.h: sanitize rotate primitives

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Date:   Tue, 16 Apr 2019 16:21:11 +0100
From:   Will Deacon <will.deacon@....com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vineet Gupta <vineet.gupta1@...opsys.com>,
        Anthony Yznaga <anthony.yznaga@...cle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Pavel Machek <pavel@....cz>,
        Ido Schimmel <idosch@...lanox.com>,
        Vadim Pasternak <vadimp@...lanox.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bitops.h: sanitize rotate primitives

On Wed, Apr 10, 2019 at 11:19:06PM +0200, Rasmus Villemoes wrote:
> The ror32 implementation (word >> shift) | (word << (32 - shift) has
> undefined behaviour if shift is outside the [1, 31] range. Similarly
> for the 64 bit variants. Most callers pass a compile-time
> constant (naturally in that range), but there's an UBSAN report that
> these may actually be called with a shift count of 0.
> 
> Instead of special-casing that, we can make them DTRT for all values
> of shift while also avoiding UB. For some reason, this was already
> partly done for rol32 (which was well-defined for [0, 31]). gcc 8
> recognizes these patterns as rotates, so for example
> 
> __u32 rol32(__u32 word, unsigned int shift)
> {
> 	return (word << (shift & 31)) | (word >> ((-shift) & 31));
> }
> 
> compiles to
> 
> 0000000000000020 <rol32>:
>   20:   89 f8                   mov    %edi,%eax
>   22:   89 f1                   mov    %esi,%ecx
>   24:   d3 c0                   rol    %cl,%eax
>   26:   c3                      retq
> 
> Older compilers unfortunately do not do as well, but this only affects
> the small minority of users that don't pass constants.
> 
> Due to integer promotions, ro[lr]8 were already well-defined for
> shifts in [0, 8], and ro[lr]16 were mostly well-defined for shifts in
> [0, 16] (only mostly - u16 gets promoted to _signed_ int, so if bit 15
> is set, word << 16 is undefined). For consistency, update those as
> well.
> 
> Reported-by: Ido Schimmel <idosch@...lanox.com>
> Cc: Vadim Pasternak <vadimp@...lanox.com>
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
>  include/linux/bitops.h | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)

Reviewed-by: Will Deacon <will.deacon@....com>

I guess it would be possible to roll some of this up into macros using
sizeof, but perhaps that would make things even more difficult for the
compiler.

Will