Using libthread_db.so with single-threaded programs, for TLS access (was: Re: [RFC PATCH 0/3] Pretty-printing for errno)

[Pedro Alves]

On 09/07/2017 12:34 PM, Pedro Alves wrote:
> On 09/06/2017 10:03 PM, Zack Weinberg wrote:
> 
>> So, changes to both gdb and libthread_db seem to be required here.  I
>> do think that _in principle_ it ought to be possible to use
>> libthread_db to retrieve the address of thread-local data even if the
>> inferior is not linked with libpthread; glibc has quite a few
>> thread-specific variables (errno most prominent, of course, but also
>> h_errno, _res, etc), and so might any library which can be used from
>> both single- and multithreaded programs.
>>
>> This is really not code I feel comfortable hacking up, though, and
>> it's probably more of a project than I have time for, in any case.
> 
> Sounds like a promising approach though.  I'd like to see this path
> explored a bit more.  I'll keep this in my TODO, even though it's
> not likely to bubble up very soon.  Thanks for the discussion/ideas!

So I played with this a bit more on the plane back from Cauldron,
to try to see if we'd hit some major roadblock.  I also chatted
with Carlos a bit about this back at the Cauldron, and seemingly
there's no major reason this can't be made to work,
TLS-internals-wise.

Seems like that it's mainly a case of moving libthread_db.so-related
symbols from libpthread.so elsewhere.  More below.

I hacked libthread_db.so to disable the nptl_version check, so that
it always successfully loads with non-threaded programs.  And then
I tweaked GDB enough to make it actually reach libthread_db.so's
td_thr_tls_get_addr in that scenario too.  That's when I hit
another snag: the symbols that libthread_db.so needs which describe
the necessary offsets of internal data structures for getting at the
TLS blocks are also in libpthread.so...  In particular, the first
we stumble on is "_thread_db_link_map_l_tls_modid".  I made GDB
print the symbol lookups to make it easier to debug.  Vis:

 (gdb) p errno
 ps_pglobal_lookup: name="__stack_user" => PS_NOSYM
 warning: Cannot find user-level thread for LWP 31772: generic error
 ps_pglobal_lookup: name="_thread_db_link_map_l_tls_modid" => PS_NOSYM
 Cannot find thread-local storage for process 31772, shared library /lib64/libc.so.6:
 operation not applicable to

The lookup is coming from here:

 (top-gdb) bt
 #0  ps_pglobal_lookup (ph=0x1f65fe0, obj=0x7fffe58f93ae "libpthread.so.0", name=0x7fffe58f9e48 "_thread_db_link_map_l_tls_modid", sym_addr=0x7fffffffc428) at src/gdb/proc-service.c:115
 #1  0x00007fffe58f88a8 in td_mod_lookup (ps=<optimized out>, mod=mod@entry=0x7fffe58f93ae "libpthread.so.0", idx=<optimized out>, sym_addr=sym_addr@entry=0x7fffffffc428)
     at td_symbol_list.c:48
 #2  0x00007fffe58f8f45 in _td_locate_field (ta=ta@entry=0x1f84df0, desc=desc@entry=0x1f84fbc, descriptor_name=descriptor_name@entry=43, idx=idx@entry=0x0, 
     address=address@entry=0x7fffffffc458) at fetch-value.c:54
 #3  0x00007fffe58f8ff0 in _td_fetch_value (ta=0x1f84df0, desc=0x1f84fbc, descriptor_name=descriptor_name@entry=43, idx=idx@entry=0x0, address=0x7ffff7ff7658, 
     result=result@entry=0x7fffffffc498) at fetch-value.c:94
 #4  0x00007fffe58f8ddf in td_thr_tls_get_addr (th=0x7fffffffc4e0, map_address=<optimized out>, offset=16, address=0x7fffffffc4f8) at td_thr_tls_get_addr.c:31
 ...

So we'd need to move that symbol (and maybe others) to one of ld.so/libc.so
instead.  AFAICT, those magic symbols are described in nptl_db/structs.def.
I haven't looked enough to figure out what ends up expanding those macros
in libpthread.so.  This is where I stopped.

I'm attaching the gdb and libthread_db.so patches I used, both against current
master in their respective projects.  See comments within the patches.
I've also pushed the gdb patch to a "users/palves/tls-nonthreaded" branch.
(I don't think I have write access to glibc's git.)

Thanks,
Pedro Alves

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