Messages in this thread | | | From | Andy Lutomirski <> | Date | Sun, 22 May 2016 21:47:22 -0700 | Subject | Re: [PATCH 3/4] x86: Rewrite switch_to() code |
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On Sun, May 22, 2016 at 7:34 PM, Josh Poimboeuf <jpoimboe@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?
In any event, whatever we settle on for general pt_regs unwinding should work for this, too.
> > -- > Josh
-- Andy Lutomirski AMA Capital Management, LLC
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