Messages in this thread | | | Date | Sun, 22 May 2016 21:34:14 -0500 | From | Josh Poimboeuf <> | Subject | Re: [PATCH 3/4] x86: Rewrite switch_to() code |
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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.
-- Josh
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