Re: [PATCH] include/linux/bitops.h: sanitize rotate primitives

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Jun 03, 2019 at 11:39:46AM -0700, Matthias Kaehlcke wrote:
> From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> 
> commit ef4d6f6b275c498f8e5626c99dbeefdc5027f843 upstream.
> 
> 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.
> 
> Link: http://lkml.kernel.org/r/20190410211906.2190-1-linux@rasmusvillemoes.dk
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Reported-by: Ido Schimmel <idosch@mellanox.com>
> Tested-by: Ido Schimmel <idosch@mellanox.com>
> Reviewed-by: Will Deacon <will.deacon@arm.com>
> Cc: Vadim Pasternak <vadimp@mellanox.com>
> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Cc: Jacek Anaszewski <jacek.anaszewski@gmail.com>
> Cc: Pavel Machek <pavel@ucw.cz>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> ---
> Hi Greg and Sasha,
> 
> Please pick this patch for 4.19. It fixes (at least) crashes due
> to undefined instructions in BPF code on arm32 when building with
> clang:

What about for the 5.1 kernel?  You don't want anyone updating from 4.19
to the latest stable and having a regression, right?

thanks,

greg k-h