Re: [PATCH v1] Add TLS variable debug support for AIX [ 64-bit XCOFF files only]

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

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

>+   The thread pointer in AIX (64-bit) is located at R13.
>+   The value of the XCOFF symbol is a signed offset from the thread
>+   pointer for the Local Exec TLS model, which is the only model that
>+   the AIX toolchain presently emits for executables.  The TLS
variable's
>+   per-thread address is as follows since GDB provides that raw
symbol value as OFFSET:
>+
>+   address = (int64_t) offset + thread_pointer  */

This doesn't look right to me.  The issue is not so much the
"TLS model", which is mostly a matter of optimization.  The
issue is more fundamental: thread-local variables defined in
libraries loaded with dlopen *cannot* be accessed directly
relative to the thread pointer, because the storage for
those is only allocated at dlopen time and not statically
known at compile or link time.

It is of course true that thread-local variables defined in
the main executable (or in libraries pulled in at startup)
can be accessed this way.  But your code doesn't even attempt
to verify that the variable in question *is* one of those,
so it will simply access random memory if used on a variable
in a dlopen'ed library.

Ideally, you should be able to implement full support (by
accessing the AIX libc equivalent of the DTV).  If that is
not possible for some reason, then you should at least
restrict the implementation to variables guaranteed to
work (e.g. by checking whether the objfile passsed to
fetch_tls_load_module_address corresponds to the main
executable or an initially loaded library).

Bye,
Ulrich