GDB TLS testing [actually running some tests finally: a success with -pthread used]

[Mark Millard]

On 2019-Jan-22, at 22:53, Mark Millard <marklmi at yahoo.com> wrote:



> On 2019-Jan-22, at 19:19, Mark Millard <marklmi at yahoo.com> wrote:
> 
> 
> 
>> On 2019-Jan-22, at 18:32, Mark Millard <marklm at yahoo.com> wrote:
>> 
>> 
>> 
>>> On 2019-Jan-22, at 17:06, Mark Millard <marklmi at yahoo.com> wrote:
>>> 
>>> 
>>> 
>>> . . .
>>> So I'm trying:
>>> 
>>> # git clean -f
>>> # rm */config.cache */*/config.cache
>>> # env CPATH=/usr/local/include ./configure
>>> . . .
>>> # env CPATH=/usr/local/include gmake
>>> . . .
>>> 
>>> in order to try to add paths after the command line -I paths.
>>> 
>>> . . . This looks like it built. I've not used the build yet.
>>> 
>> 
>> Looking at a *.core did not go well for my context:
>> 
>> # cc -g -O2 tls_gdb_test.c 
>> tls_gdb_test.c:16:2: warning: indirection of non-volatile null pointer will be deleted, not trap [-Wnull-dereference]
>>       *(char *)NULL = 1;
>>       ^~~~~~~~~~~~~
>> tls_gdb_test.c:16:2: note: consider using __builtin_trap() or qualifying pointer with 'volatile'
>> 1 warning generated.
>> . . .
> 
> 
> So far all tried-combinations of using gcc versions for build
> the test program and/or building the gdb used do not work for
> "p id" and "p &id" in doing the test. clang is not essential
> to the behavior observed.

Using "-g -O2 -pthread" to build the test program via system
clang or gcc8 (for example) did lead to the likes of:

(gdb) run 
Starting program: /root/c_tests/a.out 
main: PID 15350
id = 15350 (0x810055020)

^C
Program received signal SIGINT, Interrupt.
main (ac=<optimized out>, av=<optimized out>) at tls_gdb_test.c:16
16		*(char *)NULL = 1;
(gdb) bt
#0  main (ac=<optimized out>, av=<optimized out>) at tls_gdb_test.c:16
(gdb) info threads
  Id   Target Id                   Frame 
* 1    LWP 100324 of process 15350 main (ac=<optimized out>, av=<optimized out>) at tls_gdb_test.c:16
(gdb) p id
$1 = 15350
(gdb) p &id
$2 = (int *) 0x810055020

So it appears one branch of:

static void
fbsd_fetch_rtld_offsets (struct gdbarch *gdbarch, struct fbsd_pspace_data *data)
{
  TRY
    {
      /* Fetch offsets from debug symbols in rtld.  */
      data->off_linkmap = parse_and_eval_long ("&((Obj_Entry *)0)->linkmap");
      data->off_tlsindex = parse_and_eval_long ("&((Obj_Entry *)0)->tlsindex");
      data->rtld_offsets_valid = true;
      return;
    }
  CATCH (e, RETURN_MASK_ERROR)
    {
      data->off_linkmap = -1;
    }
  END_CATCH

  TRY
    {
      /* Fetch offsets from global variables in libthr.  Note that
         this does not work for single-threaded processes that are not
         linked against libthr.  */
      data->off_linkmap = fbsd_read_integer_by_name(gdbarch,
                                                    "_thread_off_linkmap");
      data->off_tlsindex = fbsd_read_integer_by_name(gdbarch,
                                                     "_thread_off_tlsindex");
      data->rtld_offsets_valid = true;
      return;
    }
  CATCH (e, RETURN_MASK_ERROR)
    {
      data->off_linkmap = -1;
    }
  END_CATCH
}

is working when -pthread is used.

===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)