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Standard debugger interface
===========================

The run-time linker exposes a rendezvous structure to allow debuggers
to interface with it.  This structure, r_debug, is defined in link.h.
If the executable's dynamic section has a DT_DEBUG element, the
run-time linker sets that element's value to the address where this
structure can be found.

The r_debug structure contains (amongst others) the following fields:

  struct link_map *r_map:
    A linked list of loaded objects.

  enum { RT_CONSISTENT, RT_ADD, RT_DELETE } r_state:
    The current state of the r_map list.  RT_CONSISTENT means that r_map
    is not currently being modified and may safely be inspected.  RT_ADD
    means that an object is being added to r_map, and that the list is
    not guaranteed to be consistent.  Likewise RT_DELETE means that an
    object is being removed from the list.

  ElfW(Addr) r_brk:
    The address of a function internal to the run-time linker which is
    called whenever r_state is changed.  The debugger should set a
    breakpoint at this address if it wants to notice mapping changes.

This protocol is widely supported, but somewhat limited in that it
has no provision to provide access to multiple namespaces, and that
the notifications (via r_brk) only refer to changes to r_map--the
debugger is notified that a new object has been added, for instance,
but there is no way for the debugger to discover whether any of the
objects in the link-map have been relocated or not.


Probe-based debugger interface
==============================

Systemtap is a dynamic tracing/instrumenting tool available on Linux.
Probes that are not fired at run time have close to zero overhead.
glibc contains a number of probes that debuggers can set breakpoints
on in order to notice certain events.

All rtld probes have the following arguments:

  arg1: Lmid_t lmid:
    The link-map ID of the link-map list that the object was loaded
    into.  This will be LM_ID_BASE for the application's main link-map
    list, or some other value for different namespaces.

  arg2: struct r_debug *r_debug:
    A pointer to the r_debug structure containing the link-map list
    that the object was loaded into.  This will be the value stored in
    DT_DEBUG for the application's main link-map list, or some other
    value for different namespaces.

map_complete and reloc_complete may have the following additional
argument:

  arg3: struct link_map *new:
    A pointer which, if not NULL, points to the entry in the specified
    r_debug structure's link-map list corresponding to the first new
    object to have been mapped or relocated, with new->l_next pointing
    to the link-map of the next new object to have been mapped or
    relocated, and so on.  Note that because `new' is an entry in a
    larger list, new->l_prev (if not NULL) will point to what was the
    last link-map in the link-map list prior to the new objects being
    mapped or relocated.

The following probes are available:

  init_start:
    This is called once, when the linker is about to fill in the main
    r_debug structure at application startup.  init_start always has
    lmid set to LM_ID_BASE and r_debug set to the value stored in
    DT_DEBUG.  r_debug is not guaranteed to be consistent until
    init_complete is fired.

  init_complete:
    This is called once, when the linker has filled in the main
    r_debug structure at application startup. init_complete always
    has lmid set to LM_ID_BASE and r_debug set to the value stored
    in DT_DEBUG.  The r_debug structure is consistent and may be
    inspected, and all objects in the link-map are guaranteed to
    have been relocated.

  map_start:
    The linker is about to map new objects into the specified
    namespace.  The namespace's r_debug structure is not guaranteed
    to be consistent until a corresponding map_complete is fired.

  map_complete:
    The linker has finished mapping new objects into the specified
    namespace.  The namespace's r_debug structure is consistent and
    may be inspected, although objects in the namespace's link-map
    are not guaranteed to have been relocated.

  map_failed:
    The linker failed while attempting to map new objects into
    the specified namespace.  The namespace's r_debug structure
    is consistent and may be inspected.

  reloc_start:
    The linker is about to relocate all unrelocated objects in the
    specified namespace.  The namespace's r_debug structure is not
    guaranteed to be consistent until a corresponding reloc_complete
    is fired.

  reloc_complete:
    The linker has relocated all objects in the specified namespace.
    The namespace's r_debug structure is consistent and may be
    inspected, and all objects in the namespace's link-map are
    guaranteed to have been relocated.

  unmap_start:
    The linker is about to remove objects from the specified
    namespace.  The namespace's r_debug structure is not guaranteed to
    be consistent until a corresponding unmap_complete is fired.

  unmap_complete:
    The linker has finished removing objects into the specified
    namespace.  The namespace's r_debug structure is consistent and
    may be inspected.