[GSOC] Implementation of debugging support for dynamic loaded files in rtl

[Peng Fan]

Hi,

This is a new task, but is an intersting and challenging task.

A *simple* implementation of debugging support is finished, but it needs
improvement.
Now I only test it on sparc/sis with rap format file. In the following
steps, I'll test it on other archs, improve it and *may* refactor the
current implementation.

The following is the details of the work. Because I am new to gdb
internals, there maybe errors and hope you can correct them.

1. The work with the gdb part.
    added files. solib-rtems.c, spart-rtems-tdep.c, solib-rtems.h to gdb.
    I used solib-target.c and solib-svr4.c as the templates to code
solib-rtems.c.

    1.1 how gdb dynamically loaded files on the target?
         There is an interface to let gdb get the information of the loaded
files on the target. In solib-rtems.c, a solib event breakpoint is set at
'_rtld_debug_state'. _rtld_debug_state is an empty function in RTL, and
dlopen should invoke _rtld_debug_state. When the target stops at
_rtld_debug_state, sparc-rtems4.11-gdb will  execute
update_solib_list(solib.c) and then call rtems_current_sos(solib-rtems.c)
to collect the details of loaded files on the target.

   1.2 how gdb collect the details?
       Target side:
        Each loaded elf object file has a corresponding link_map variable.
all the link_map variables will be linked into a pointer named
'_rtld_debug'. The link_map structrue contains all the section details of a
elf object file.
      Host side:
      sparc-rtems4.11-gdb will locate the base address of _rtld_debug, then
collect the elf object file details and section details of each elf object
file.

  1.3 How the symbols are correctly figured out?
       Because the symbols loaded by sparc-rtems4.11-gdb, but not(should
not) transfered from target. Thus relocate_section_addresses should be
implemented in gdb. It will assigned section addresses collected from
_rtld_debug to target_section which can be handled by sparc-rtems4.11-gdb.

And now, sparc-rtems4.11-gdb can catch the breakpoints.

2. A sample debug session.
./sparc-rtems4.11-gdb ../../../../rtl/build/sparc-rtems4.11-sis/rtld
(gdb) tar sim
Connected to the simulator.
(gdb) load
(gdb) *set solib-search-path
../../../../rtl/build/sparc-rtems4.11-sis/testcase/ (This step will be
removed, using --rpath in rtems-ld)*
(gdb) b hello
*Function "hello" not defined. (hello is a function in the rap file)*
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (hello) pending.
(gdb) tar sim
Connected to the simulator.
(gdb) load
(gdb) r
......................
[/] # *rap ld ./test.rap (load the rap file)*
In rtems() local = 1

*Breakpoint 1, hello (arg=arg at entry=1) at ../../testcase/1.c:16 (It stops
here)*
16  switch (arg) {
(gdb) p arg
$1 = 1
(gdb) c
Continuing.
Inner-module call hello()
In test() global = 1

*Breakpoint 1, hello (arg=arg at entry=2) at ../../testcase/1.c:16 (stops
again)*
16  switch (arg) {


Hope this explains cleary. If not, contact me or read the code[1][2][3].
Any adivces or comments.

Thanks,
Peng.

[1]https://github.com/MrVan/gsoc2013-rtl
[2]https://github.com/MrVan/gsoc2013-rtl-host
[3]https://github.com/MrVan/gdb
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