On Mon, May 23, 2016 at 7:40 AM, Josh Poimboeuf <jpoim...@redhat.com> 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 <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.
Where is this code? I don't see it in the mainline kernel. -- Brian Gerst
