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

Richard Mitton richard at codersnotes.com
Sat Feb 1 11:57:45 PST 2014

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 
> <http://dev.gentoo.org/%7Edberkholz/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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