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There is > no sane way to deliver a signal to the process which managed to run into > this. Appropriate info to parent or ptracer will still be delivered. Hmm yeah.  Something has already gone seriously wrong to end up here, so terminating it is probably best. > I really wish that we could disable syscall32 reliably on AMD and make > it raise #UD as it does on Intal. You know that's changing in FRED, and will follow the AMD model? The SYSCALL instruction is lower latency if it doesn't need to check %cs to conditionally #UD. >> Furthermore, despite CET-SS explicitly trying to account for and protect >> against accidental mismatches, there are errata in some parts which let >> userspace forge legal return addresses on the shadow stack by dropping >> into 32bit mode because, there's a #GP check missing in a microflow. > Didn't we assume that there are no CPU bugs? :) Tell me, is such a CPU delivered with or without a complimentary unicorn? :) >> For usecases where there ought not to be any 32bit code at all (and >> there absolutely are), it would be lovely if this could be enforced, >> rather than relying on blind hope that it doesn't happen. > I think it's rather clear what needs to be done here to achieve that, > but that's completely orthogonal to the intent of the patch series in > question which aims to make CONFIG_IA32_EMULATION a boot time decision. Fully inhibiting 32bit mode is stronger, because it impacts code which would otherwise operate in CONFIG_IA32_EMULATION=n configuration. Which is fine, but I agree that it doesn't want to be confused with being a "runtime CONFIG_IA32_EMULATION" knob. If the runtime form could also come with an equivalent Kconfig form, that would be awesome too. ~Andrew