[RFA] Improved linker-debugger interface

[Gary Benson <gbenson at redhat dot com>]

Hi all,

Back in June or so last year I spent some time working on an improved
debug interface with the runtime linker to help address the following
bugs:

  https://bugzilla.redhat.com/show_bug.cgi?id=658851
  aka http://sources.redhat.com/bugzilla/show_bug.cgi?id=2328
  "_dl_debug_state() RT_CONSISTENT called too early"

  https://bugzilla.redhat.com/show_bug.cgi?id=698001
  "improve GDB performance on an application performing
  a lot of object loading."

  http://sourceware.org/bugzilla/show_bug.cgi?id=11839
  "gdb does not detect calls to dlmopen"

The current linker-debugger interface has a structure (r_debug)
containing a list of loaded libraries, and an empty function
(_dl_debug_state) for debuggers to set breakpoints on and which
the linker calls both before and after modifying this list.
The problems with the current interface are as follows:

  - There is one place where glibc calls _dl_debug_state earlier than
    Solaris libc.  This is #658851.  It is unlikely that glibc will
    ever be changed to make it compatible with Solaris libc, which
    means GDB reports libraries as loaded and ready before they
    really are.

  - This interface was presumably invented before dlmopen() was, so
    there's only provision in it for one namespace.  In glibc each
    namespace has it's own r_debug structure, but there is no way for
    the linker to communicate the addresses of the others to the
    debugger.  This is PR 11839.

  - In normal use GDB only needs to stop _after_ the list is modified.
    Because _dl_debug_state is called both before and after, GDB stops
    twice as often as it needs to.  This is #698001, the gist of it at
    any rate.  
    
  - When stop-on-solib-events is set, however, it is useful to stop
    both before and after library loads.

My proposed solution is to insert a number of SystemTap probes into
glibc.  My current setup has a probe everywhere _dl_debug_state is
called, and an extra pair to surround relocation events, but new
probes could be added as and when necessary.

This approach solves the various problems like so:

  - Debuggers can pick and choose which probes to set breakpoints
    on.  By using the "relocation completed" probe instead of the
    one mirroring _dl_debug_state debuggers can stop after relocations
    have occurred, matching the behaviour of Solaris libc.

  - All probes have namespace id and r_debug address arguments,
    allowing debuggers to see changes in namespaces other than
    the default.

  - When stop-on-solib-events is unset, GDB does not have to stop
    before changes are made, only after.  By disabling the "before"
    breakpoints the number of stops made can be halved.

The attached patch modifies GDB to search for the SystemTap probes
in the runtime linker, and to use them instead of _dl_debug_state
if found.  If the probes are not found then GDB will fall back to
its previous behaviour.  When probes are used, GDB stops after
relocation, fixing #658851, and stops before changes are made are
inhibited.  I've not done anything on the GDB side to deal with the
dlmopen() issue, but it's now possible to fix it using the data
supplied by the new interface.

Does this look ok?

Cheers,
Gary

-- 
http://gbenson.net/

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