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 <ido...@mellanox.com> > Cc: Vadim Pasternak <vad...@mellanox.com> > Signed-off-by: Rasmus Villemoes <li...@rasmusvillemoes.dk>
Tested-by: Ido Schimmel <ido...@mellanox.com> Thanks!