On May 4, 2016 2:42:53 PM PDT, John Denker <j...@av8n.com> wrote: >On 05/04/2016 12:07 PM, ty...@thunk.org wrote: > >> it doesn't hit the >> UB case which Jeffrey was concerned about. > >That should be good enough for present purposes.... > >However, in the interests of long-term maintainability, I >would suggest sticking in a comment or assertion saying >that ror32(,shift) is never called with shift=0. This >can be removed if/when bitops.h is upgraded. > >There is a track record of compilers doing Bad Things in >response to UB code, including some very counterintuitive >Bad Things. > >On Wed, May 04, 2016 at 11:29:57AM -0700, H. Peter Anvin wrote: >>> >>> If bitops.h doesn't do the right thing, we need to >>> fix bitops.h. > >Most of the ror and rol functions in linux/bitops.h >should be considered unsafe, as currently implemented. >http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/include/linux/bitops.h?id=04974df8049fc4240d22759a91e035082ccd18b4#n103 > >I don't see anything in the file that suggests any limits >on the range of the second argument. So at best it is an >undocumented trap for the unwary. This has demonstrably >been a problem in the past. The explanation in the attached >fix-rol32.diff makes amusing reading. > >Of the eight functions > ror64, rol64, ror32, rol32, ror16, rol16, ror8, and rol8, >only one of them can handle shifting by zero, namely rol32. >It was upgraded on Thu Dec 3 22:04:01 2015; see the attached >fix-rol32.diff. > >I find it very odd that the other seven functions were not >upgraded. I suggest the attached fix-others.diff would make >things more consistent. > >Beware that shifting by an amount >= the number of bits in the >word remains Undefined Behavior. This should be either documented >or fixed. It could be fixed easily enough.
This construct has been supported as a rotate since at least gcc2. -- Sent from my Android device with K-9 Mail. Please excuse brevity and formatting. -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
