On Tue, May 07, 2019 at 12:57:15PM +0000, David Laight wrote: > > Only the INT3 thing needs 'the gap', but the far bigger change here is > > that kernel frames now have a complete pt_regs set and all sorts of > > horrible crap can go away. > > I'm not doubting that generating the 'five register' interrupt stack frame > for faults in kernel space makes life simpler just suggesting that the > 'emulated call' can be done by emulating the 'iret' rather than generating > a gap in the stack. The thing you suggested doesn't actually work, INT3 can nest. > > For 32bit 'the gap' happens naturally when building a 5 entry frame. Yes > > it is possible to build a 5 entry frame on top of the old 3 entry one, > > but why bother... > > Presumably there is 'horrid' code to generate the gap in 64bit mode? > (less horrid than 32bit, but still horrid?) > Or does it copy the entire pt_regs into a local stack frame and use > that for the iret? It's in the patch you replied to; it is so small you might have overlooked it. It simply pushes another copy on top of what was already there. > I've just tried to parse the pseudo code for IRET in the intel docs. > Does anyone find that readable? No; it's abysmal. > I wonder if you can force 32bit mode to do a stack switch 'iret' > by doing something like a far jump to a different %cs ? I don't think that'll work.
