On Mon, May 23, 2016 at 07:49:37AM -0400, Brian Gerst wrote: > On Mon, May 23, 2016 at 7:40 AM, Josh Poimboeuf <[email protected]> wrote: > > On Sun, May 22, 2016 at 09:47:22PM -0700, Andy Lutomirski wrote: > >> On Sun, May 22, 2016 at 7:34 PM, Josh Poimboeuf <[email protected]> > >> 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. > > Where is this code? I don't see it in the mainline kernel.
Yeah, it hasn't been merged. Here's the last version: https://lkml.kernel.org/r/4d34d452bf8f85c7d6d5f93db1d3eeb4cba335c7.1461875890.git.jpoim...@redhat.com -- Josh
