[Crash-utility] [RFC v2] gcore: process core dump feature for crash utility

[HATAYAMA Daisuke]

Hello.

This is the RFC version 2 of gcore sub-command that provides a process
core dump feature for crash utility.

During the period from RFC v1, I had investigated how to restore
user-mode register values. The patch reflects the investigation.

Any comments or suggestions are welcome.


Changes in short
================

The changes include:

  1) implement a collection of user-space register values more
     appropriately, but not ideally.

  2) re-design gcore sub-command as an extension module


By (1), GDB's bt command displays backtrace normally.

diffstat ouput

 Makefile                                  |    6 +-
 defs.h                                    |    2 +
 extensions/gcore.c                        |   21 +
 extensions/gcore.mk                       |   48 +
 extensions/libgcore/2.6.34/x86_64/gcore.c | 2033 +++++++++++++++++++++++++++++
 extensions/libgcore/2.6.34/x86_64/gcore.h |  651 +++++++++
 netdump.c                                 |   27 +
 tools.c                                   |    1 -
 8 files changed, 2787 insertions(+), 2 deletions(-)

Current Status
==============

  I've continued to develop gcore sub-command, but this version is
  still under development.

  Ultimately, I'm going to implement gcore as I described in RFC v1
  and as I will explain in ``Detailed Changes and Issues'' below.


How to build and use
====================

  I've attached the patchset to this mail.

    - crash-gcore-RFCv2.patch

  Please use crash version 5.0.5 on x86_64.

  Follow the next instructions:

    $ tar xf crash-5.0.5.tar.gz
    $ cd crash-5.0.5/
    $ patch -p 1 < crash-gcore-v2.patch
    $ make
    $ make extensions
    $ crash <debuginfo> <vmcore> .... (*)
    crash> extend gcore.so

  In (*), gcore.so is generated under the extensions/ directory.


Detailed Changes and Issues
===========================

1) implement collection of user-space register values more
   appropriately, but not ideally

  The previous version doesn't retrieve appropriate register values
  because it doesn't consider save/restore operations at interrupts on
  kernel at all.

  I've added restore operations according to which kinds of interrupts
  the target task entered kernel-mode. See fill_pr_reg() in gcore.c.

  But unfortunately, the current version is still not ideal, since it
  would take some time to do.

  More precisely, all part of user-mode registers are not always
  restored. The full part is saved only at exceptions, NMI and some
  kinds of system calls. At other kinds of interrupts, saved are
  register values except for 6 callee-saved registers: rbp, rbx, r12,
  r13, r14, r15.

  In theory, these can be restored using Call Frame Information
  generated by a compiler as part of debugging information, whose
  section name is .debug_frame, which tells us offsets of respective
  callee-saved registers.

  But currently, I don't do this yet, since I don't find any useful
  library to do this. Yes, I think I can implement it manually, but it
  would take some time. I've of course found unwind_x86_32_64.c
  providing related library but it looks to me unfinished.

  On the other hand, a frame pointer, rbp, can be restored by
  unwinding it repeatedly until its address value reaches any
  user-space address.


2) re-design gcore sub-command as an extension module

In respond to my previous post, Dave gave me a suggestion that gcore
subcommand should be provided as an extension module per kernel
versions and type of architecutes, since process core dump feature
inherently depends on kernel data structures.

I agreed the suggestion and have tried to redesign the patchset.

Although the current patchset merely moved gcore files into
./extensions directory, I've also considered better design. That is,

  (1) architecture- or kernel-version independent part is provided
      just under ./extensions

  (2) only architecture- or kernel-version specific part is provided as
      certain extension module.

The next directory structure depicts this shortly:

  crash-5.0.5/
    extensions/
      gcore.mk
      gcore.c  ... (1)
      libgcore/ ... (2)
        2.6.34/
          x86_64/
            gcore_note.h
            gcore_note.c

I think it relatively easily feasible by porting regset interface in
kernel, which is used to implement ptrace feature, hiding
implementation details on many architectures.

Also, it helps port kernel codes in gcore and maintain source codes
ranging over a variety of kernel versions on multiple architectures
uniformly.

I'm going to re-implement this way in the next version. From that
version, I won't change gcore source code dramatically, change only
when adding newly extension modules.


Thanks
--
HATAYAMA Daisuke
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