[rtems commit] libdl: Add support to import base image TLS symbols

[Sebastian Huber]

On 14.09.23 23:56, Chris Johns wrote:
> On 14/9/2023 7:33 pm, Sebastian Huber wrote:
>> On 14.09.23 10:51, Chris Johns wrote:
>>> On 14/9/2023 5:58 pm, Sebastian Huber wrote:
>>>> On 14.09.23 09:38, Chris Johns wrote:
>>>>>>> The issue I faced was no score interface to get the TLS base for a thread to
>>>>>>> determine a symbol's offset. If we had that and something to say if TLS is
>>>>>>> supported libdl would be easy to fix.
>>>>>> Why don't we add this interface if it simplifies things?
>>>>> Yes please, that would be good. I do not know the TSL support well enough to do
>>>>> a decent job of it.
>>>> Which interface do you need?
>>> A define that says TLS is support for an arch?
>>>
>>> A call that returns the base address for an arch? I have added some in libdl:
>>>
>>> https://git.rtems.org/rtems/tree/cpukit/libdl/rtl-tls.c
>>>
>>> however having this in score would mean a new arch does not break libdl.
>> Yes, this should be definitely added to the CPU port. What is the purpose of
>> rtems_rtl_tls_get_base()? It seems to be unused.
> It is used by the symbol table loader so is referenced in the symbol table
> generated code ...
> 
> https://git.rtems.org/rtems-tools/tree/linkers/rtems-syms.cpp#n66
> https://git.rtems.org/rtems-tools/tree/linkers/rtems-syms.cpp#n319
> 
> A TLS symbol is an offset into the TLS area. I encountered two problems. First I
> could not find a way to equate a TLS symbol offset to a label in asm or C and
> second the offsets move with each link phase and with the 2 stage linking the
> offsets move. As a result the offset needs to be computer at runtime. To do this
> the base image TLS symbols each get a small piece of code that determines the
> offset at runtime. The symbol table in the target contains the TLS area offset
> in an arch ABI specific format. The relocs know how to handle the offset the
> symbol holds.

It would be really nice to have a Doxygen comment for 
rtems_rtl_tls_get_base() which contains this information. There are also 
no test cases for this function. Without test cases it is easy to break 
something without knowing it.

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