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

[Josh Poimboeuf]

On Mon, May 23, 2016 at 07:14:14AM -0400, Brian Gerst wrote:
> 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.

In theory I like the idea, and it would work: the unwinder could just
use the inactive_task_frame struct (as Andy suggested) to find the frame
pointer.

But I suspect it would break all existing unwinders, both in-tree and
out-of-tree.  The only out-of-tree one I know of is crash, not sure if
there are more out there.

-- 
Josh