Re: [PATCH v2 36/44] x86/entry/unwind: encode pt_regs pointer in frame pointer

[Josh Poimboeuf]

On Thu, Aug 04, 2016 at 05:22:32PM -0500, Josh Poimboeuf wrote:
> With frame pointers, when a task is interrupted, its stack is no longer
> completely reliable because the function could have been interrupted
> before it had a chance to save the previous frame pointer on the stack.
> So the caller of the interrupted function could get skipped by a stack
> trace.
> 
> This is problematic for live patching, which needs to know whether a
> stack trace of a sleeping task can be relied upon.  There's currently no
> way to detect if a sleeping task was interrupted by a page fault
> exception or preemption before it went to sleep.
> 
> Another issue is that when dumping the stack of an interrupted task, the
> unwinder has no way of knowing where the saved pt_regs registers are, so
> it can't print them.
> 
> This solves those issues by encoding the pt_regs pointer in the frame
> pointer on entry from an interrupt or an exception.  The frame pointer
> unwinder is also updated to decode it.
> 
> Suggested-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

When doing some testing on x86_32, I realized there's a flaw here in the
unwinder's pt_regs detection, when an interrupt hits when we're already
in the entry code.  For example, a page fault can be interrupted by an
irq, after the page fault entry code encoded the frame pointer, but
before it had a chance to call into the C handler.

In that case, regs->bp itself is encoded, and the current "pt_regs
aren't real frames" design falls apart because there can actually be
more than one set of pt_regs per "frame".  So the unwinder gets confused
and stops walking the stack too early.

So really the unwinder needs to consider each pt_regs as a frame in and
of itself.  Which of course Andy already suggested before, but I
stupidly shot it down.

Working on that for v3.

-- 
Josh