On Sat, Jul 18, 2020 at 7:16 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > Andy Lutomirski <luto@xxxxxxxxxx> writes: > > On Fri, Jul 17, 2020 at 12:29 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > >> The alternative is to play nasty games with TIF_IA32, TIF_ADDR32 and > >> TIF_X32 to free up bits for 32bit and make the flags field 64 bit on 64 > >> bit kernels, but I prefer to do the above seperation. > > > > I'm all for cleaning it up, but I don't think any nasty games would be > > needed regardless. IMO at least the following flags are nonsense and > > don't belong in TIF_anything at all: > > > > TIF_IA32, TIF_X32: can probably be deleted. Someone would just need > > to finish the work. > > TIF_ADDR32: also probably removable, but I'm less confident. > > TIF_FORCED_TF: This is purely a ptrace artifact and could easily go > > somewhere else entirely. > > > > So getting those five bits back would be straightforward. > > > > FWIW, TIF_USER_RETURN_NOTIFY is a bit of an odd duck: it's an > > entry/exit word *and* a context switch word. The latter is because > > it's logically a per-cpu flag, not a per-task flag, and the context > > switch code moves it around so it's always set on the running task. > > Gah, I missed the context switch thing of that. That stuff is hideous. It's also delightful because anything that screws up that dance (such as failure to do the exit-to-usermode path exactly right) likely results in an insta-root-hole. If we fail to run user return notifiers, we can run user code with incorrect syscall MSRs, etc.
