On Thu, May 02, 2019 at 11:02:40AM -0700, Linus Torvalds wrote: > On Thu, May 2, 2019 at 9:21 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > TL;DR, on x86_32 kernel->kernel IRET frames are only 3 entries and do > > not include ESP/SS, so not only wasn't regs->sp setup, if you changed it > > it wouldn't be effective and corrupt random stack state. > > Indeed, the 32-bit case for same-RPL exceptions/iret is entirely > different, and I'd forgotten about that. > > And honestly, this makes the 32-bit case much worse. Now the entry > stack modifications of int3 suddenly affect not just the entry, but > every exit too. > > This is _exactly_ the kind of subtle kernel entry/exit code I wanted > us to avoid. I actually love this patch (absent the bugs). This is already something that has been sorely needed for years. The "struct pt_regs is incomplete on x86-32" thing is a monstrosity which has long been a source of confusion and bugs. Sure, this patch adds some complexity to the entry code, but on the other hand it actually makes it possible to use pt_regs sanely: regs->sp is no longer uninitialized. So a class of (very non-obvious) bugs is eliminated. I don't think it would make sense to make this change for int3 only, because the benefits are global. -- Josh
