On May 5, 2016 3:18:09 PM PDT, tytso@xxxxxxx wrote: >On Thu, May 05, 2016 at 05:34:50PM -0400, Sandy Harris wrote: >> >> I completely fail to see why tests or compiler versions should be >> part of the discussion. The C standard says the behaviour in >> certain cases is undefined, so a standard-compliant compiler >> can generate more-or-less any code there. >> > >> As long as any of portability, reliability or security are among our >> goals, any code that can give undefined behaviour should be >> considered problematic. > >Because compilers have been known not necessarily to obey the specs, >and/or interpret the specs in way that not everyone agrees with. It's >also the case that we are *already* disabling certain C optimizations >which are technically allowed by the spec, but which kernel >programmers consider insane (e.g., strict aliasing). > >And of course, memzero_explicit() which crypto people understand is >necessary, is something that technically compilers are allowed to >optimize according to the spec. So trying to write secure kernel code >which will work on arbitrary compilers may well be impossible. > >And which is also why people have said (mostly in jest), "A >sufficiently advanced compiler is indistinguishable from an >adversary." (I assume people will agree that optimizing away a memset >needed to clear secrets from memory would be considered adversarial, >at the very least!) > >So this is why I tend to take a much more pragmatic viewpoint on >things. Sure, it makes sense to pay attention to what the C standard >writers are trying to do to us; but if we need to suppress certain >optimizations to write sane kernel code --- I'm ok with that. And >this is why using a trust-but-verify on a specific set of compilers >and ranges of compiler versions is a really good idea.... > > - Ted I have filed a gcc bug to have the preexisting rotate idiom officially documented as a GNU C extension. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70967 -- 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 majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
