gdb / thread_db / multiple ABI question

Steve Ellcey
2017-07-07T18:20:37Z

I have a question about debugging, the thread_db library, and multiple ABI's.
I am trying to support an ILP32 ABI to the aarch64 architecture. Now I am
still building gdb as a LP64 object. In most cases this seems fine, I can use
an LP64 gdb to debug an ILP32 object. But when I have threaded objects and
need to use libthread_db.so, things don't seem to work. When I try this
and set libthread_db_debug to 1, I get:

Trying host libthread_db library: /home/sellcey/gdb-ilp32/install/libilp32/libth
read_db.so.1.
dlopen failed: /home/sellcey/gdb-ilp32/install/libilp32/libthread_db.so.1: wrong
ELF class: ELFCLASS32.
thread_db_load_search returning 0
warning: Unable to find libthread_db matching inferior's thread library, thread
debugging will not be available.

It makes sense that the LP64 gdb cannot dlopen the ILP32 thread_db that
corresponds to the ILP32 thread library that the program being debugged
uses but I don't know what to do about it.

I looked at x86 and when I compile a 32 bit program there and then debug
it with a 64 bit gdb I see:

[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

So I am guessing that aarch64 needs to use the 64 bit libthread_db.so.1 even
when debugging 32 bit programs, is that right? What do I need to do to tell
gdb to do that? I tried adding /home/sellcey/gdb-ilp32/install/lib64 to
solib-search-path but that did not help. I am not sure what triggers the
difference in behaviour between aarch64 and x86.

Steve Ellcey
sellcey@cavium.com