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

[Josh Poimboeuf]

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