On Mon, Jun 03, 2019 at 11:39:46AM -0700, Matthias Kaehlcke wrote:
> From: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxxxxx
> Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> Reported-by: Ido Schimmel <idosch@xxxxxxxxxxxx>
> Tested-by: Ido Schimmel <idosch@xxxxxxxxxxxx>
> Reviewed-by: Will Deacon <will.deacon@xxxxxxx>
> Cc: Vadim Pasternak <vadimp@xxxxxxxxxxxx>
> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx>
> Cc: Pavel Machek <pavel@xxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
> ---
> 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
