[PATCH glibc] nptl_db: different libpthread/ld.so load orders (bug 27744)

[Pedro Alves]

On 16/04/21 17:43, Florian Weimer wrote:
> * Pedro Alves:
> 
>> On 16/04/21 17:28, Florian Weimer wrote:
>>> * Pedro Alves:
>>>
>>>> IIRC, the order which libraries are loaded by GDB hasn't changed.  The
>>>> issue is that until recently (before glibc 1daccf403b1b), the stacks
>>>> lists lived in libpthread (stack_used/__stack_user), so the fact that
>>>> GDB loaded libthread_db.so before ld.so's symbols were loaded didn't
>>>> make a difference.  Now they were moved to ld.so, so libthread_db.so
>>>> can't find them until GDB reads the ld.so symbols.  Is this assessment
>>>> correct?
>>>
>>> Yes, I believe this is what happens.
>>>
>>
>> OK, I believe what is confusing in your commit log was the reference to
>> two different kinds of "loaded":
>>
>>   "libthread_db is loaded once GDB encounters libpthread, and at this
>>   point, ld.so may not have been loaded yet. "
>>
>> The first loaded is about GDB dlopening libthread_db.so.  The second loaded
>> refers to reading symbols -- ld.so has been loaded by the inferior already
>> at that point.
>>
>> It would be clearer as:
>>
>>   "libthread_db is loaded once GDB encounters libpthread, and at this
>>   point, ld.so's symbols may not have been read by GDB yet. "
> 
> I'm going to go with:
> 
> “
> libthread_db is loaded once GDB encounters libpthread, and at this
> point, ld.so may not have been processed by GDB yet.
> ”

Sounds good.

> 
>> If I understood that correctly, then the following sentence is also a
>> bit confusing:
>>
>>   "As a result, _rtld_global cannot be accessed by regular means from
>>   libthread_db."
>>
>> Because that sounds to me like you were perhaps talking about some
>> magic means to reference globals, some magic relocations, or some
>> other magic voodoo only understood by glibc experts.
> 
> We use the magic that GDB provides to us (ps_pglobal_lookup, I think).
> I thought that this was understood by GDB experts only. 8-)

LOL

I skimmed the patch, and FWIW, it LGTM.  Just spotted a couple typos:

> +/* This test runs GDB against a forked copy of itself, to check
> +   whether libthreaddb can be loaded, and that access to thread-local

libthreaddb -> libthread_db

> +/* This function implements the subprocess un der test.  It creates a

"un der" -> "under"

Thanks,
Pedro Alves