[PATCH glibc] nptl_db: different libpthread/ld.so load orders (bug 27744)

[Simon Marchi]

On 2021-04-16 1:18 p.m., Pedro Alves wrote:
> On 16/04/21 17:53, Simon Marchi wrote:
> 
>> Do we need / want to fix GDB if this goes in glibc then?  
> 
> I think so.  We may need to discuss more the "needs_setup" hack, but we
> can do that in that thread.

In the new version I add a new inferior flag "in_initial_library_scan".
It's perhaps not ideal but the result is better than what I had in v1.
But yeah let's discuss that in that thread.

> Given that libpthread.so is going away in the future, we should be thinking about
> addressing that as well.  Does your patch fix that as side effect?

No, I think of that as a separate problem.

> 
> If not, GDB should be keying the loading of libthread_db.so on something else.
> Making GDB try to load libthread_db in reaction to processing ld.so instead
> libthread_db.so would fix that, I think.  And, it would fix this ordering problem at
> hand as well.  So if we do that, maybe we don't need the other changes.

Is it possible to have a completely static executable that doesn't use
ld.so but uses pthreads?

I don't think that would work well for the "run" case, where ld-linux
arrives before libpthreads.so (and before libc.so, for when pthreads is
moved to libc.so).  If we try to load libthread_db when ld-linux
appears, the symbols provided by libpthreads (or libc) won't be found.

We would have to key the loading on when both libpthread and ld-linux
are there (or both libc and ld-linux).

Another problem with the current state (but that would be fixed with
Florian's patch I think) is that gdb has this setting "set
auto-solib-add".  If off, GDB won't load the symbols of shared
libraries.  _Except_ if the library if libpthread:

  https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gdb/solib.c;h=d32b2210005fdd6a5013cfd566620d6afeae9cf6;hb=HEAD#l968

But it means that if auto-solib-add is off, we won't load ld.so's
symbols, and we fall back on the original problem, even if the ordering
is right.  Since Florian's patch makes libthread_db only access symbols
of libpthread.so, I think that avoids this issue as well.

But then when libpthreads is moved into libc, we'll have the problem
that libthread_db simply won't get loaded...

Simon