On Wed, 2018-07-11 at 16:16 -0700, Dave Hansen wrote: > On 07/11/2018 04:00 PM, Yu-cheng Yu wrote: > > > > On Wed, 2018-07-11 at 15:40 -0700, Dave Hansen wrote: > > > > > > On 07/11/2018 03:10 PM, Yu-cheng Yu wrote: > > > > > > > > > > > > On Tue, 2018-07-10 at 17:11 -0700, Dave Hansen wrote: > > > > > > > > > > > > > > > Is this feature *integral* to shadow stacks? Or, should it just > > > > > be > > > > > in a > > > > > different series? > > > > The whole CET series is mostly about SHSTK and only a minority for > > > > IBT. > > > > IBT changes cannot be applied by itself without first applying > > > > SHSTK > > > > changes. Would the titles help, e.g. x86/cet/ibt, x86/cet/shstk, > > > > etc.? > > > That doesn't really answer what I asked, though. > > > > > > Do shadow stacks *require* IBT? Or, should we concentrate on merging > > > shadow stacks themselves first and then do IBT at a later time, in a > > > different patch series? > > > > > > But, yes, better patch titles would help, although I'm not sure > > > that's > > > quite the format that Ingo and Thomas prefer. > > Shadow stack does not require IBT, but they complement each other. If > > we can resolve the legacy bitmap, both features can be merged at the > > same time. > As large as this patch set is, I'd really prefer to see you get shadow > stacks merged and then move on to IBT. I say separate them. Ok, separate them. > > > > > GLIBC does the bitmap setup. It sets bits in there. > > I thought you wanted a smaller bitmap? One way is forcing legacy libs > > to low address, or not having the bitmap at all, i.e. turn IBT off. > I'm concerned with two things: > 1. the virtual address space consumption, especially the *default* case > which will be apps using 4-level address space amounts, but having > 5-level-sized tables. > 2. the driving a truck-sized hole in the address space limits > > You can force legacy libs to low addresses, but you can't stop anyone > from putting code into a high address *later*, at least with the code we > have today. So we will always reserve a big space for all CET tasks? Currently if an application does dlopen() a legacy lib, it will have only partial IBT protection and no SHSTK. Do we want to consider simply turning off IBT in that case? Yu-cheng
