On 19/12/2018 17:17, Richard Biener wrote: > On Wed, 19 Dec 2018, Florian Weimer wrote: > >> * Peter Bergner: >> >>> On 12/19/18 7:59 AM, Florian Weimer wrote: >>>> * Richard Biener: >>>> >>>>> Sure, if we'd ever deploy this in production placing this in the >>>>> TCB for glibc targets might be beneifical. But as said the >>>>> current implementation was just an experiment intended to be >>>>> maximum portable. I suppose the dynamic loader takes care >>>>> of initializing the TCB data? >>>> >>>> Yes, the dynamic linker will initialize it. If you need 100% reliable >>>> initialization with something that is not zero, it's going to be tricky >>>> though. Initial-exec TLS memory has this covered, but in the TCB, we >>>> only have zeroed-out reservations today. >>> >>> We have non-zero initialized TCB entries on powerpc*-linux which are used >>> for the GCC __builtin_cpu_is() and __builtin_cpu_supports() builtin >>> functions. Tulio would know the magic that was used to get them setup. >> >> Yes, there's a special symbol, __parse_hwcap_and_convert_at_platform, to >> verify that the dynamic linker sets up the TCB as required. This way, >> binaries which need the feature will fail to run on older loaders. This >> is why I said it's a bit tricky to implement this. It's even more >> complicated if you want to backport this into released glibcs, where we >> normally do not accept ABI changes (not even ABI additions). > > It's easy to change the mitigation scheme to use a zero for the > non-speculated path, you'd simply replace ands with zero by > ors with -1. For address parts that gets you some possible overflows > you do not want though.
And you have to invert the value before using it as a mask. R. > > Richard. >
