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. Richard.