Re: [PATCH 3/4] x86: Rewrite switch_to() code

[Brian Gerst]

On Sun, May 22, 2016 at 10:34 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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.

Ok, how about if it pushed RBP first, then we teach get_wchan() to add
the fixed offset from thread.sp to get bp?  that way it don't have to
push it twice.

--
Brian Gerst