Debugging ld.so in gdb

[Jacob Kroon]

On 2/7/22 14:53, Adhemerval Zanella wrote:
> 
> 
> On 07/02/2022 10:45, Jacob Kroon wrote:
>> On 2/7/22 14:39, Jacob Kroon wrote:
>>> On 2/7/22 13:32, Jacob Kroon wrote:
>>>> On 2/7/22 13:27, Florian Weimer wrote:
>>>>> * Jacob Kroon:
>>>>>
>>>>>>> What I find really confusing is that this is not the result of a dlopen
>>>>>>> call.  I definitely would expect that the maps array contains *all*
>>>>>>> objects that are being loaded.  Clearly this is not the case here.
>>>>>>> Somehow certain objects are missing, and then they get written into the
>>>>>>> rpo array.
>>>>>>>
>>>>>>> Please try to find libjvm.so among the l_initfini arrays of the objects.
>>>>>>
>>>>>> I do find libjvm.so in a couple of the maps[]->l_initfini[]->l_name
>>>>>> arrays, yes.
>>>>>
>>>>> Okay, and of course there is an assumption that those make it to the
>>>>> maps.  No wonder we run off the array.
>>>>>
>>>>>>> It must be present somewhere.  I assume it's also on the main list,
>>>>>>> which starts off at _rtld_global._dl_ns[0]._ns_loaded.
>>>>>>
>>>>>> Hmm how do I iterate over that data structure ?
>>>>>>
>>>>>> See below:
>>>>>>> (gdb) print _rtld_global._dl_ns[0]._ns_loaded[0]->l_name
>>>>>>> $186 = 0x7ffff7ff1d97 ""
>>>>>>> (gdb) print _rtld_global._dl_ns[0]._ns_loaded[1]->l_name
>>>>>>> $187 = 0x0
>>>>>>> (gdb) print _rtld_global._dl_ns[0]._ns_loaded[2]->l_name
>>>>>>> $188 = 0x0
>>>>>>> (gdb) print _rtld_global._dl_ns[0]._ns_loaded->l_name
>>>>>>> $189 = 0x7ffff7ff1d97 ""
>>>>>>> (gdb) print _rtld_global._dl_ns[1]._ns_loaded->l_name
>>>>>>> Cannot access memory at address 0x8
>>>>>>> (gdb) print _rtld_global._dl_ns[2]._ns_loaded->l_name
>>>>>>> Cannot access memory at address 0x8
>>>>>
>>>>> It's a list chained by l_prev/l_next.
>>>>>
>>>>
>>>> Ok, yes "libjvm.so" is there, in multiple entries.
>>>>
>>>
>>> I managed to build glibc master, and yes it also crashes. Reverting the
>>> suspicious commit:
>>>
>>> commit 15a0c5730d1d5aeb95f50c9ec7470640084feae8
>>> Author: Chung-Lin Tang <cltang@codesourcery.com>
>>> Date:   Thu Oct 21 21:41:22 2021 +0800
>>>
>>>     elf: Fix slow DSO sorting behavior in dynamic loader (BZ #17645)
>>>
>>> fixes the crash. Adding a couple of more people.
>>>
>>
>> And yes, using master (or host) glibc and running:
>>
>> $ GLIBC_TUNABLES="glibc.rtld.dynamic_sort=1" ldd mylib.so
>>
>> also works without crashing.
> 
> It is really hard to understand the issue you are seeing without the
> context, since I was cced with a cropped thread. Could you send the
> the full thread or the original issue?


The whole thread is archived here:
https://sourceware.org/pipermail/gdb/2022-February/049884.html

Jacob