Re: [PATCH] nptl: Open libgcc.so with RTLD_NOW during pthread_cancel

[Florian Weimer <fweimer at redhat dot com>]

On 01/10/2018 08:09 PM, Carlos O'Donell wrote:
> On 01/10/2018 02:49 AM, Florian Weimer wrote:
> > Disabling lazy binding reduces stack usage during unwinding.
> >
> > Note that RTLD_NOW only makes a difference if libgcc.so has not
> > already been loaded, so this is only a partial fix.
> >
> > 2018-01-10  Florian Weimer  <fweimer@redhat.com>
> >
> > 	[BZ #22636]
> > 	* sysdeps/nptl/unwind-forcedunwind.c (pthread_cancel_init): Open
> > 	libgcc.so with RTLD_NOW, to avoid lazy binding during unwind.
> >
> > diff --git a/sysdeps/nptl/unwind-forcedunwind.c b/sysdeps/nptl/unwind-forcedunwind.c
> > index ab4350de99..67b8e74b53 100644
> > --- a/sysdeps/nptl/unwind-forcedunwind.c
> > +++ b/sysdeps/nptl/unwind-forcedunwind.c
> > @@ -49,7 +49,7 @@ pthread_cancel_init (void)
> >         return;
> >       }
> >   
> > -  handle = __libc_dlopen (LIBGCC_S_SO);
> > +  handle = __libc_dlopen_mode (LIBGCC_S_SO, RTLD_NOW | __RTLD_DLOPEN);
> >   
> >     if (handle == NULL
> >         || (resume = __libc_dlsym (handle, "_Unwind_Resume")) == NULL
>
> Should we not also do this for sysdeps/gnu/unwind-resume.c ?

Interesting.

_Unwind_Resume is only called from a landing pad after unwinding has 
already begun, so the code never actually loads libgcc_s, and RTLD_NOW 
does not have any effect at this point (even with lazy binding, most of 
the unwinder will already have been bound by this point).  In contrast, 
the nptl unwinding code used by pthread_exit/pthread_cancel actually 
initiates unwinding.

There is certainly a cleanup opportunity here, but changing the dlopen 
flags is not needed at this point.

Thanks,
Florian