[RFC Patch 0/2] Improvements to stack traces

[Andrew Cooper <andrew.cooper3@citrix.com>]

This series is RFC for two reasons; firstly because I have not dev-tested it
yet, but mainly because of a specific question.

In the algorithm using frame pointers, the lower bound is adjusted by two
words from the provided stack pointer.

This appears to be the behaiour right from its introduction in:

    commit aa24d38a469b59abf1b95b732b6ea9ed86e511cf
    Author: kaf24@firebug.cl.cam.ac.uk <kaf24@firebug.cl.cam.ac.uk>
    Date:   Thu Sep 1 15:31:12 2005 +0000

What is the reason for the adjustment?  Tim and I couldn't think of a case
where a valid frame pointer could be outside the stack. Any well formed use of
frame pointers should require the callee to push the old frame pointer at
entry, and pop it on right before exit.

Am I missing something obvious?

The potential problem comes in the stack overflow case, where rsp points to
the boundary of the primary stack, and rbp points just below it, at which
point the bounday condition will pass but referencing rbp will cause a triple
fault.

This can be detected and worked around, but if the adjustment is erronious
then by far the easiest solution is to just discard the adjustment.

~Andrew

CC: Keir Fraser <keir@xen.org>
CC: Jan Beulich <JBeulich@suse.com>
CC: Tim Deegan <tim@xen.org>

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