Re: [PATCH v1 2/2] Add TLS support for shared libraries in AIX

[Ulrich Weigand <Ulrich.Weigand@de.ibm.com>]

Aditya Vidyadhar Kamath <akamath996@gmail.com> wrote:

>For initial-exec (R_TLS_IE): read the runtime TP-relative offset from
>the .loader section TOC slot written by the AIX loader, returning
>it with bit 0 set as a sentinel so get_thread_local_address() uses
>it directly without re-adding the static symbol value.

>For global-dynamic/local-dynamic (R_TLSM): scan the .loader section
>for the R_TLSM relocation, read the 8-byte mod_id from the inferior's
>TOC, and dereference it as a TP-relative pointer to the per-thread
>block, then add the intra-module variable offset.

This is an interesting approach, which is quite different from what
Linux is doing.   The one question/concern I have this: are these
relocations present in the module where the TLS symbol is *defined*,
or rather in the module where the TLS symbol is being *used*.  On
Linux (ELF), it would be the latter - but I don't know all the AIX
(XCOFF) details here.

If it is the latter, then this approach has the drawback that the
debugger can only access symbols that are also accessed somewhere
within the program being debugged (in fact, with the current patch,
only symbols that are used in the same module they are defined in).
With the approach used on Linux, it is in principle possible to
access TLS symbols whether they are used or not.  (If there's no
better way to implement this, that of course may still a limitation
that can be accepted.)

Either way, it would be good to add test cases for the various
scenarios.

Bye,
Ulrich