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RFC GDB Linux Awareness analysis


Hi GDB community,

Overview
========

The purpose of this email is to describe a useful feature which has been developed
by STMicroelectronics inside of GDB for debugging Linux kernels. I will cover at
a high level some of the implementation details and what I see as the advantages
/ disadvantages of the implementation. I will also cover some other alternative 
approaches that I'm aware of.

The purpose is to facilitate discussion with the GDB experts on this
mailing list as to what the "correct" way to implement this functionality would
be.

The end goal is to have an upstream implementation of this functionality.

Introduction
============

STMicroelectronics has a patchset on top of vanilla GDB which adds much better
Linux kernel awareness into GDB. They have called this GDB extension LKD (Linux
Kernel Debugger). This GDB extension is primarily used in conjunction with ST's
JTAG debugger for debugging ARM / SH4 SoCs, and is implemented as an internal
GDB extension, written in C.

The LKD extension is nicely abstracted from the underlying JTAG interface,
and I have used it to debug a ARMv7 kernel running in QEMU via gdbremote.

ST would like to contribute these patches back to GDB, as we think it could
be useful not only for ARM Linux debugging, but also other CPU/OS combinations
in the future.

LKD can be broadly split into the following parts: -

1) Linux task awareness
2) Loadable module support
3) Various Linux helper commands

The next section looks at each of the three parts, and any other implementations
I'm aware of that currently exist.


1) Task Awareness
=================

1.1) LKD Linux Task awareness
=============================

When using mainline GDB for debugging a Linux kernel via JTAG GDB will typically
only show the actual hardware threads.

The LKD task awareness extension (lkd-process.c) adds the ability for GDB to
parse some kernel data structures, so it can build a thread list of all kernel
threads in the running Linux kernel.

When halting the processor (via a breakpoint, ctrl+c etc) the thread list is
re-enumerated, so new tasks are visible to GDB and tasks which have exited
are removed from the thread list.

To achieve this GDB has to know about various Linux kernel data structures,
and also various fields within these structures (mainly task_struct,
thread_info, mm_struct). Code has been implemented to parse these structs,
and also do virtual to physical address translations and cache handling.
Various frame unwinders are also implemented to stop backtracing on various
exceptions, and entry points to the kernel (these would be a useful addition
regardless of which task awareness approach is taken).

Advantages
 - Adds Linux kernel task awareness to GDB
 - Supports symbolic debugging of user processes
 - Contextual information (structs / field offsets) are readily available inside of GDB
 - Has been used and well tested inside ST for some time

Disadvantages
 - Being implemented in C within GDB creates a dependency between GDB and the Linux kernel
 - Mainly tested on 3.10 and 2.6.30 with ARM and SH4 kernels, being upstreamed would
   expose it to many differing kernel versions and architectures.

 - I can't see any other "OS awareness" support currently in the GDB code base


1.2) OpenOCD / GDB Linux task awareness
=======================================

Whilst looking for any prior art in this area I found that OpenOCD already
implements some basic Linux task awareness. See here: -
 http://openocd.org/doc/doxygen/html/linux_8c_source.html

I've this working to the extent where I can connect via JTAG to a ARMv7 U8500
Snowball board and enumerate a thread list in GDB. It required some debugging
& hacking to get this far.

This implementation passes the thread list to GDB via the gdbremote protocol
and as such changes required in GDB are minimal.

Advantages
 - OpenOCD already supports many target types (ARM / MIPS), and has support
for virt to phys translations / cache handling etc.

 - OpenOCD also implements task awareness for other RTOSâs (ThreadX / FreeRTOS / eCos)

 - Using gdbremote means GDB changes are (so far) minimal.

 - OpenOCD / Linux Kernel dependency already exists


Disadvantages
 - Creates a dependency between Linux kernel data structures and OpenOCD.

 - I believe finding field offsets within structs is currently not possible via
   gdbremote protocol.

   Currently OpenOCD generates these offsets at compile time which is ugly and needs
   fixing. See here http://openocd.org/doc-release/doxygen/linux__header_8h_source.html.

   Being able to find field offsets would in my opinion be a useful addition to the
   gdbremote protocol which would allow OpenOCD task awareness to work much better
   at runtime.
   
 - Doesn't support debugging user processes. I think this would still require some
   GDB changes, and also gdbremote protocol changes to get working correctly.

 - Needs to be made more generic


1.3) Python Task awareness
==========================

Jan Kiszka from Siemens has implemented some basic Linux kernel task awareness using
the GDB Python interface.

See here https://lwn.net/Articles/631167/ and here
https://github.com/torvalds/linux/blob/master/Documentation/gdb-kernel-debugging.txt

This support is currently limited to the following commands: -

lx_current -- Return current task
lx_per_cpu -- Return per-cpu variable
lx_task_by_pid -- Find Linux task by PID and return the task_struct variable
lx_thread_info -- Calculate Linux thread_info from task variable

However this could be extended to build up a thread list. The GDB Python interface
would I believe need to be extended to allow a thread list to be passed back to GDB
via Python.

Advantages
 - Code parsing Linux kernel structures lives in the kernel source tree
 - Contextual information is easily available
 - Works with all OCD implementations not just OpenOCD 

Disadvantages
 - Doesn't exist yet
 - Python / GDB interface would need to be extended to support threads

Questions
========

1) What do GDB community think about having Linux OS task awareness in the GDB
   codebase?

2) Is there a preferred way to implement this (C extension / gdbremote / Python
   / something else)?

3) Any other implementations / advantages / disadvantages that I'm not currently
   aware of?

2) Loadable module support
==========================

2.1) LKD Loadable module support
================================

lkd-modules.c adds better Linux kernel module symbolic symbol support to GDB using the
GDB shared libraries infrastructure (solib).

It has hooks to enable the debugging of module __init sections to reflect that these pages
get discarded post-load. It also implements the same section layout algorithm as
the kernel to speed up symbol resolution, only inspecting the targetâs memory if there is a
mismatch (inspecting target memory can be slow).

I think this part could be upstreamed separately to the task awareness support, although
Iâve not tried separating it yet from the other LKD patches.

Advantages
 - Allows full symbolic debugging of Linux loadable modules including init sections

 - Would be independent of underlying communication mechanism (JTAG / gdbremote etc)

Disadvantages
 - Some dependency between GDB and Linux kernel is still present

Questions:

Are GDB community happy to have a Linux specific solib functionality in the GDB code base?

2.2) Python Loadable module support
===================================

I've not managed to look to much at this, but some basic support exists, see here
https://github.com/torvalds/linux/blob/master/Documentation/gdb-kernel-debugging.txt

Having read the LKD modules implementation I donât think this can be anywhere near
as functionally complete.


3) Linux Helper commands
========================

3.1) LKD helper commands
========================

LKD implements various Linux helper commands inside GDB such as: -
 - dmesg - dump dmesg log buffer from kernel
 - process_info - prints various info about current process
 - pmap - prints memory map of current process
 - vm_translate - translates virtual to physical address
 - proc_interrupts - prints interrupt statistics
 - proc_iomem - prints I/O mem map
 - proc_cmdline - prints the contents of /proc/cmdline
 - proc_version - prints the contents of /proc/version
 - proc_mounts - print the contents of /proc/mounts
 - proc_meminfo - print the contents of /proc/meminfo.

Advantages
 - Can be used by all GDB based debug solutions
 - Some precedent of OS related commands in the GDB code base 
   https://sourceware.org/gdb/current/onlinedocs/gdb/OS-Information.html#OS-Information

Disadvantages
 - Creates a GDB dependency with the kernel

Python helper commands
======================

Janâs python scripts also implement some of the same LKD commands such as 'dmesg'.

See here https://lwn.net/Articles/631167/ and 
https://github.com/torvalds/linux/blob/master/Documentation/gdb-kernel-debugging.txt

Advantages
 - Code that is parsing Linux kernel structures lives in the kernel source tree
 - Can be used by all GDB based debug solutions


Questions
 - Do GDB community mind Linux specific custom commands being added to GDB code base? 

My current opinion is that helper commands which can be, should be migrated from C code
into Python, and merged into the kernel source tree (and then retired from the LKD patchset).

If you got here, thanks for reading this far! Like I said at the beginning, the purpose of
this email is to stimulate some discussion on what you folks consider the 'correct' way to
implement this OS awareness functionality is.

All feedback is welcome.

Kind regards,

Peter.


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