On Sun, May 22, 2016 at 09:47:22PM -0700, Andy Lutomirski wrote: > On Sun, May 22, 2016 at 7:34 PM, Josh Poimboeuf <jpoim...@redhat.com> wrote: > > On Sun, May 22, 2016 at 10:59:38AM -0700, Andy Lutomirski wrote: > >> cc: Josh Poimboeuf: do you care about the exact stack layout of the > >> bottom of the stack of an inactive task? > > > > So there's one minor issue with this patch, relating to unwinding the > > stack of a newly forked task. For detecting reliable stacks, the > > unwinder needs to unwind all the way to the syscall pt_regs to make sure > > the stack is sane. But for newly forked tasks, that won't be possible > > here because the unwinding will stop at the fork_frame instead. > > > > So from an unwinder standpoint it might be nice for copy_thread_tls() to > > place a frame pointer on the stack next to the ret_from_fork return > > address, so that it would resemble an actual stack frame. The frame > > pointer could probably just be hard-coded to zero. And then the first > > bp in fork_frame would need to be a pointer to it instead of zero. That > > would make it nicely resemble the stack of any other task. > > > > Alternatively I could teach the unwinder that if the unwinding starts at > > the fork_frame offset from the end of the stack page, and the saved rbp > > is zero, it can assume that it's a newly forked task. But that seems a > > little more brittle to me, as it requires the unwinder to understand > > more of the internal workings of the fork code. > > > > But overall I think this patch is a really nice cleanup, and other than > > the above minor issue it should be fine with my reliable unwinder, since > > rbp is still at the top of the stack. > > Is this a regression or is there some reason that it works right > without the patch?
Without the patch, it uses TIF_FORK to determine the stack is empty. > In any event, whatever we settle on for general pt_regs unwinding > should work for this, too. Yeah, agreed. -- Josh
