Re: [PATCH] nptl: Move stack list variables into _rtld_global

[Florian Weimer via Gdb-patches <gdb-patches@sourceware.org>]

* Simon Marchi:

> On 2021-03-05 12:15 p.m., Florian Weimer wrote:> * Simon Marchi via Libc-alpha:
>> 
>>> On 2020-11-13 10:10 a.m., Florian Weimer wrote:
>>>> Now __thread_gscope_wait (the function behind THREAD_GSCOPE_WAIT,
>>>> formerly __wait_lookup_done) can be implemented directly in ld.so,
>>>> eliminating the unprotected GL (dl_wait_lookup_done) function
>>>> pointer.
>>>
>>> Hi Florian,
>>>
>>> Presumably starting with this commit (I don't really know how to build a
>>> glibc and test against it), GDB fails to attach to a threaded process
>>> because libthread_db fails to initialize.  See:
>>>
>>>   https://sourceware.org/bugzilla/show_bug.cgi?id=27526
>>>
>>> The difference in behavior as seen from GDB is that libthread_db now
>>> asks to look up a symbol "_dl_stack_user" in module NULL.  GDB can't
>>> find this symbol, which fails the initialization.
>>>
>>> Can you shed some light on this?  Is this request expected, and where is
>>> GDB expected to find this symbol?
>> 
>> It is not expected.  This is the fallback path if _rtld_global cannot be
>> located.  The actual failure is that __td_ta_rtld_global does not
>> succeed.
>
> [adding gdb-patches]
>
> Ok, thanks for that tip.  Indeed I see that GDB returns PS_NOSYM for
> _rtld_global.  If I now log what GDB returns:
>
>     LOOKUP nptl_version in libpthread.so.0
>     Found 0x7fdb02713037
>     LOOKUP _rtld_global in ld-linux-x86-64.so.2
>     Not found
>     LOOKUP _dl_stack_user in (null)
>     Not found
>
> So this lookup of _rtld_global is new too.  And I think I see the
> problem, it looks like an ordering issue: libthread_db is loaded when
> GDB notices the program has libpthread loaded in it.  When attaching,
> GDB walks the shared library list.  In that list, libpthread comes
> before ld-linux.  So at the time we try to load libthread_db, GDB hasn't
> yet noticed that the program has ld-linux loaded in it, hasn't ingested
> its symbols, so doesn't find _rtld_global.

Oh, that reads like a plausible explanation.  And I assume the
non-attaching case, where GDB starts the process, is very different, and
this ordering issue does not appear?

> If we have to deal with this, I guess that GDB should now do things in a
> different order: go through the whole library list and load their
> symbols.  And then if one of those libraries were libpthread, try to
> initialize libthread_db.

Initialization of libthread_db should be unconditional.  Programs use
TLS data without linking against libpthread.  And glibc 2.34 might not
have a separate libpthread at all.

Thanks,
Florian