DDB scripting, output capture, and textdumps

Dear all:

I've been hacking on-and-off for a while on a side project to improve our 
kernel debugging facilities.  Primarily, my concern has been to address 
three problems:

- The complications of employing kernel core dumps for debugging,
   including the large size of dumps making them unwieldy to distribute or
   store for any extended period (even with minidumps), the requirement to
   have relatively synchronized kernel source in order to use the dumps,
   the need to have a kernel with debugging symbols, and the problems with
   fsck causing sufficient swap use to invalidate dumps before they can be
   extracted.

- The decreasing likelihood that notebooks will ship with serial ports
   that can be used for interactive debugging using DDB.  Making end-users
   type in stack traces is cruel, photos are a pain, and X11 rules out
   both.

- The fact that a great many problems are most easily diagnosed using
   utility routines present in DDB, but not as easily using kgdb for
   offline analysis.  I find that for many bugs I analyze, simply looking
   at the DDB output is sufficient to identify the source of the problem.

An idea I punted around a bit at BSDCan earlier this year (or perhaps it 
was at EuroBSDCon the previous year) was an idea of a "textdump" -- that 
is, a new type of kernel dump based on capturing automatically extracted 
debugging information generated by DDB.  The result would be an ASCII text 
file that could be filed as a bug report, perhaps even automatically.

To this end, I have implemented three new facilities for use with DDB:

(1) DDB output capture.  The output of DDB is stored in a memory buffer,
     and can be extracted using a sysctl or textdumps (see below).  This
     can be turned on and off, both for use manually ("I'll want this
     later, but not that") and as part of scripts (see below).

(2) DDB scripting.  A limited number of named scripts can be defined to
     run a series of DDB commands.  No loops, etc, just simple command
     lists.  These can be caused to run automatically on entering DDB for
     various scenarios, including WITNESS violations and kernel panics.
     They can also be run by hand in order to save a bit of typing if you
     use DDB in a repetitive way (as I do).

(3) Textdumps.  A new dump type that stores a series of data files
     containing various pieces of information, including the DDB capture
     buffer, kernel message buffer, kernel configuration (if compiled into
     the kernel), panic message, and kernel version string.  These are
     stored in the ustar format inside the dump partition (aligned to the
     end) so can be easily extended, and savecore(8) requires almost no new
     logic to deal with them (it just drops numbered tar files in
     /var/crash).  This makes it straight forward to extend the textdump format
     to include new types of information and avoids the issue of how to safely
     simultaneously represent information in many different formats in the same
     file.

These are pretty flexible tools, and you can imagine doing the following 
sorts of things:

- Setting the kdb.enter.panic script to automatically turn on output
   capture, do full backtraces of all threads, show open file information,
   dump UMA stats, and save it all to a textdump and then reboot.

- Setting the kdb.enter.witness script to show lock information, generate
   a coredump, and reboot.  Or, just to automatically do "show allocks" and
   drop to the DDB prompt.

- Adding a flag to rc.conf to automatically submit textdumps via e-mail to
   a specific address, perhaps including GNATS or an automated bug system.
   These could be unpacked and automatically analyzed, and do to the compact
   size, kept for long-term trend analysis or to identify when a problem
   started occuring.

I've produced an initial snapshot of the above, which can be found here:

   http://www.watson.org/~robert/freebsd/20071218-ddb.tgz

This adds three files to DDB, patches quite a few kernel files (to pass 
more information into KDB about why it's being entered, in order to 
trigger the right script), enhancements to savecore(8) to know how to 
extract textdumps, adds a ddb(8) command line tool so that userspace can 
manage DDB scripts from outside the debugger, extensions to the ddb(4) man
page, and a new textdump(4) man page.

There are a number of known limitations; I've tried to document them at 
the top of the pertinent files where I am aware of them.  I also regret to 
say that to date I've been able to test only on i386, and not other 
platforms.  I'd welcome any feedback -- I'd like to get these changes into CVS 
in the next week or two.

Robert N M Watson
Computer Laboratory
University of Cambridge