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Re: [PATCH] Linux kernel thread runtime support


Hi Philipp,

On Wed, 15 Feb 2017, Philipp Rudo wrote:

> Hi Peter,
> 
> while looking into your patch in more detail, I found some problems
> with your code.  In particular I dislike your general approach using
> private_thread_info, since the remote-target already uses this field
> inside thread_info.  Thus "forcibly" adding your own version of
> private_thread_info, like you do, can lead to undefined behavior.

> For
> example remote_stopped_by_sw_breakpoint requires information from
> private_tread_info and won't work reliably.  Even worse, remote_resume
> you call by passing target_resume to the target beneath writes (via
> resume_clear_thread_private_info) to the private_thread_info thus
> causing a memory corruption.

Yes that is a very good point and spot!

I checked back through to see if this was something I have introduced
when re-working the patches from ST's initial plugin, but it looks like
this bug has always been present, although in GDB 7.6 the remote.c priv
structure only had two fields the corruption and forced update of ->priv
of threads from remote layer still happens.

> 
> With your and the remote-target being that interlaced, I don't think
> it's possible to separate them cleanly.  That's why I see no other way
> than allowing each target to manage its own thread list.

Certainly each target layer managing their own thread lists would be
a neat solution to the problem. At a very minimum we shouldn't ever mess
with a GDB thread object created by another layer.

regards,

Peter.

> 
> Philipp
> 
> On Thu, 22 Dec 2016 17:31:03 +0000
> Peter Griffin <peter.griffin@linaro.org> wrote:
> 
> > Hi GDB maintainers,
> > 
> > The following patch implements a Linux kernel thread runtime stratum
> > which can be used when using GDB to debug a Linux kernel. For example
> > when connecting to a QEMU GDB stub, or OpenOCD which then communicates
> > with the target via JTAG or SWD.
> > 
> > This patch is a refactored version based on the 'Linux kernel
> > debugger' GDB plugin written at STMicroelectronics which used to
> > be packaged with their JTAG debuggers. There has been some discussion
> > previously on the list by myself [1], Kieran [2] and one of the
> > original authors at ST Marc Titinger [3].
> > 
> > This patchset I'm hoping is a lot closer to something which can
> > be upstreamed to GDB project after some discussion about structure
> > with Yao Qi at Linaro Connect, the code has been refactored to be
> > structured much more like the existing upstream thread runtimes (such
> > as ravenscar-thread.c and sparc-ravenscar-thread etc).
> > 
> > Since the original email [1] various helper commands have also been
> > migrated into python and merged into the Linux kernel source tree.
> > The GDB python extensions, combined with the linux-kthread GDB
> > thread runtime implemented in this patchset provide a powerful
> > Linux kernel debug solution, and is a working implementation
> > of what Andreas talked about on slide 17 and 18 of his talk at GNU
> > Cauldron [4].
> > 
> > I have currently been testing this patchset using mainline GDB
> > debugging arm-linux kernels in Qemu and via OpenoCD to real hardware.
> > It is straight forward with the current strructure to add new
> > architecture support, and I'm looking at adding PowerPC so I can
> > easily validate big endian targets.
> > 
> > What I'm really hoping for now is some patch review on the GDB
> > mailing list from the GDB maintainers and community with a view to
> > getting this functionality which has been talked about for quite a
> > few years finally merged to the upstream GDB project.
> > 
> > All patch review feedback greatfully received :)
> > 
> > kind regards,
> > 
> > Peter.
> > 
> > [1] https://cygwin.com/ml/gdb/2015-09/msg00032.html
> > [2] https://www.sourceware.org/ml/gdb/2016-01/msg00028.html
> > [3]
> > https://lists.linaro.org/pipermail/linaro-toolchain/2011-November/001754.html
> > [4]
> > https://gcc.gnu.org/wiki/cauldron2015?action=AttachFile&do=view&target=Andreas+Arnez_+Debugging+Linux+kernel+dumps+with+GDB.pdf
> > 
> > 
> > 
> > Peter Griffin (1):
> >   Add Linux kernel thread runtime support.
> > 
> >  gdb/ChangeLog           |   12 +
> >  gdb/Makefile.in         |    8 +-
> >  gdb/arm-linux-kthread.c |  178 +++++
> >  gdb/arm-linux-kthread.h |   27 +
> >  gdb/arm-tdep.c          |    4 +
> >  gdb/configure.tgt       |    6 +-
> >  gdb/gdbarch.c           |   23 +
> >  gdb/gdbarch.h           |    5 +
> >  gdb/gdbarch.sh          |    3 +
> >  gdb/linux-kthread.c     | 1828
> > +++++++++++++++++++++++++++++++++++++++++++++++
> > gdb/linux-kthread.h     |  223 ++++++ 11 files changed, 2311
> > insertions(+), 6 deletions(-) create mode 100644
> > gdb/arm-linux-kthread.c create mode 100644 gdb/arm-linux-kthread.h
> >  create mode 100644 gdb/linux-kthread.c
> >  create mode 100644 gdb/linux-kthread.h
> > 
> 


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