[lldb-dev] question about link_map data structure in elf on 64-bit Linux systems

Steve Pucci spucci at google.com
Mon Feb 3 08:31:14 PST 2014

Hi Richard,

I'm looking at this now.  I'm having trouble finding the symbol name I need
to query to get the modid size in DYLDRendezvous::GetThreadInfo().  Is
there a document somewhere that describes those symbols (like
_thread_db_link_map_l_tls_modid)?  I'm not finding them with Google.

If it's not readily available, I guess I could just use the size of a
pointer on the target?

Thanks again for your help here,

On Sat, Feb 1, 2014 at 4:57 PM, Steve Pucci <spucci at google.com> wrote:

> Great, thanks a lot, Richard.  I'll make the change you suggest to the
> code I'm working on.
>  - Steve
> On Sat, Feb 1, 2014 at 11:57 AM, Richard Mitton <richard at codersnotes.com>wrote:
>>  The definitive document is here: http://www.akkadia.org/drepper/tls.pdf
>> I just had a look through and I guess it probably is a bug. Technically
>> the code in DLYDRendezvous::GetThreadInfo() should query not only the
>> location, but also the size of modid (using the same API), and then later
>> issue the appropriately-sized read.
>> It's not really "wrong" for little-endian systems right now, unless you
>> have 4 billion modules loaded.
>> Steve Pucci wrote:
>> Hi all,
>>  I'm trying to determine if some code in DynamicLoaderPOSIXDYLD is
>> correct and I could use some confirmation:
>>  The code in question reads the module id of a module as part of finding
>> the TLS block associated with that module, by determining its location and
>> then reading with a hardcoded 32-bit unsigned read.  (The code in question
>> is source/Plugins/DynamicLoader/POSIX-DYLD/DynamicLoaderPOSIXDYLD.cpp, in
>> DynamicLoaderPOSIXDYLD::GetThreadLocalData).
>>  I've read http://dev.gentoo.org/~dberkholz/articles/toolchain/tls.pdf ,
>> which describes a number of architectures and in all cases the value seems
>> to be either size_t or "unsigned long int".  Thus the code appears to be
>> wrong in at least some cases, in that it should be reading 64 bits when
>> debugging 64-bit runtimes.  The code presumably works today for 64-bit LSB
>> (little-endian) systems (which is most of the 64-bit Linuxes) because it's
>> reading the right 32-bits out of the 64-bit location.
>> But I'm not 100% sure, because
>> * I'm not sure that the referenced pdf is definitive
>> * I don't have an MSB 64-bit system to test on to verify that it's a bug.
>>  Can anyone with any familiarity with this issue comment on this?
>>  Thanks,
>>    Steve
>> ------------------------------
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