musl - Re: Accessing Thread-Local-Storage in GDB

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Message-ID: <87h797zs6a.fsf@oldenburg.str.redhat.com>
Date: Wed, 09 Feb 2022 21:44:13 +0100
From: Florian Weimer <fweimer@...hat.com>
To: Rich Felker <dalias@...c.org>
Cc: Yehuda Yitchak <yehuda80@...il.com>,  musl@...ts.openwall.com
Subject: Re: Accessing Thread-Local-Storage in GDB

* Rich Felker:

> On Wed, Feb 09, 2022 at 09:32:10AM +0200, Yehuda Yitchak wrote:
>> Hello
>> 
>> one of the applications i am debugging is compiled using MUSL libc
>> (aarch64) Version 1.1.19
>> when i inspect a core-dump and try to view TLS variables with "print
>> __percpu_dev" gdb (v8.3) complains:
>> 
>> Cannot find thread-local storage for LWP 22873, executable file
>> <core-dump-file>
>> Cannot find thread-local variables on this target
>> 
>> Any idea how I can overcome this?
>> Appreciate if you could CC me since i'm not subscribed.
>
> I think this is a consequence of gdb wanting to have a libthread_db (a
> GNU- and Solaris-ism) to tell it how to interact with the threads
> implementation of the process being debugged rather than just doing
> things in a libpthread-agnostic way like musl wants it to do. I think
> it should be fixable on the gdb side without too much headache, but I
> don't know how.

GDB would have to poke at libc internals and basically reimplement
__tls_get_addr.  Maybe something can be done by calling libc ABI
functions for TLS access instead, but that doesn't work for core files.

> As a workaround, if the program is dynamic-linked, you can do
> something like print *(int *)dlsym(0, "foo") from the relevant thread
> context and it should work.

This only works if foo is a dynamic symbol (and if you aren't examining
a coredump).

My interest in initial-exec TLS (despite its impact on dlopen) is
somewhat motivated by a desire to make things easier for debuggers.

Thanks,
Florian

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