Debugging ld.so in gdb

[Adhemerval Zanella]

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?