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

Steve Pucci spucci at google.com
Sat Feb 1 16:57:43 PST 2014

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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