On Thu, Jul 11, 2019 at 07:45:56AM -0700, Andy Lutomirski wrote: > On Thu, Jul 11, 2019 at 4:51 AM Peter Zijlstra <[email protected]> wrote: > > > > Since INT3/#BP no longer runs on an IST, this workaround is no longer > > required. > > > > Tested by running lockdep+ftrace as described in the initial commit: > > > > 5963e317b1e9 ("ftrace/x86: Do not change stacks in DEBUG when calling > > lockdep") > > > > Reviewed-by: Steven Rostedt (VMware) <[email protected]> > > Signed-off-by: Peter Zijlstra (Intel) <[email protected]> > > I would definitely like to see this happen, but there are all kinds of > possibly nasties here. Ideally we'd like get rid of IST for #DB, but > we can't due to the MOV SS mess. There are a few relevant cases we > care about: > > #DB from user mode -> anything that hits in C code: irrelevant -- > we've exited the IST stack already. > > #DB from user mode -> NMI/MCE in the asm -> #DB: The NMI code tries to > get this right. The MCE code does not. > > #DB from kernel mode -> NMI/MCE -> #DB: same as above. > > MOV SS -> #DB from entry -> #DB again: ugh. We get some protection > from shift_ist. > > IMO we would ideally just clear DR7 in sensitive contexts. Or extend > the debug_stack_set_zero(), etc hack. > > All that being said, the actual _DEBUG macros shouldn't matter here, I > think. But I'd like to sleep on it. So not-yet-acked-by me.
How about something lovely like: #DB from kernel space; in say lockdep. the #DB entry calls back into lockdep through trace_irq which then hits the same #DB and we get recursive #DB. Now, I don't think we can actually make that happen, because most/all the relevant functions have NOKPROBE_SYMBOL() on. Even the idtentry generates _ASM_NOKPROBE(). Still, it might make sense to have #DB itself clear/restore DR7 if it doesn't already. Also, the comment on do_debug() seems wrong; we can set watchpoints on kernel text just fine these days.
