Debugging ld.so in gdb

[Jacob Kroon]

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.

Jacob