Segfault of executables with shared libraries.

[Carmelo Amoroso]

Colin Whittaker wrote:
> Carmelo Amoroso wrote:
>> Colin Whittaker wrote:
>>  
>>> Carmelo Amoroso wrote:
>>>    
>>>> Try enabling debug early support into dynamic linker and run again.
>>>> Just to see if it is failing in the dynamic linker or in the
>>>> application.
>>>> I suggest to build uclibc with full debug symbols too and start a
>>>> real debuggging session
>>>> with gdb.
>>>> I'd suggest to set bp in __uClibc_main (unless you are failing
>>>> before entering in this
>>>> function), and then go step by step.
>>>>
>>>> I successfully debugged the ld.so in the past, so it is not impossible.
>>>>
>>>> Carmelo
>>>>         
>>> Thanks Carmelo,
>>> I enabled debug early support into dynamic linker and have the same
>>> results.
>>> I tried to break at __uClibc_main, and it segfaults before that.
>>>
>>> Any hints on how to debug ld.so ?
>>> Where does it get invoked ?
>>>
>>> Colin..
>>>
>>>     
>> Hi,
>> may you post the output, and readelf -a ? it may helps.
>> (if you prefer send me privately)
>>
>> For debugging the ld.so
>> 1) hack ld.so Makefile file to add -g to  (it was removed in trunk)
>> 2) install uclibc in a glibc/uclibc working system into a non system path
>>   (let's say /broken/lib/)
>> 3) compile your hello world by overriding the dynamic linker with
>>    -Wl,--dynamic-linker,/broken/lib/ld-uClibc.so.0
>> 4) install helloworl on the mips target
>> 5) on the target launch gdbserver localhost:xxx ./helloworld
>> 6) on the host, from within the source dir, launch xxx-gdb
>> ./lib/ld-uClibc.so.0
>> 7) connect from gdb console to the remote target as usual
>> 8) now you can set bp in _dl_start, _dl_get_ready_to_run and so on
>> (you cannot step in libc.so.0, but
>>    if you are ure something wrong happens in ld.so, that's the way)
>>
>>
>> Not a standard way, but really an interesting debugging session for
>> what are
>> involved into the ld.so internals ;)
>>
>> Cheers,
>> Carmelo
>>   
> Thanks for the debugging help. That is a great trick.
I was sure you liked it ;)

> So, I did this and found the segfault happens when we try to execute:
> DL_BOOT_COMPUTE_GOT(got);  in _dl_start() on line 195 dl-startup.c
> 
> The assembly code for that line is:
>    lw      v0,-32736(gp)
>    addiu  t9,v0,9476
>    jalr      t9
>    nop
> 
> The data at -32736+gp = 0, so t9 = 0x2504, causing the segfault.
>
so it seems that the got is not correctly set.
continue with gdb to track down why it is NULL.

> Looks like pretty fundamental stuff. Any thoughts ?
> 
> Colin..
Carmelo
> 
>