RE: [PATCH v3 1/3] elf: Allow dlopen of filter object to work [BZ #16272]

[David Kilroy <David.Kilroy@arm.com>]

> -----Original Message-----
> From: David Kilroy
> > -----Original Message-----
> > From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> >
> > > diff --git a/elf/dl-deps.c b/elf/dl-deps.c
> > > index c29b988..bb85c83 100644
> > > --- a/elf/dl-deps.c
> > > +++ b/elf/dl-deps.c
> > > @@ -550,13 +550,14 @@ Filters not supported with
> > LD_TRACE_PRELINKING"));
> > >      }
> > >
> > >    /* Maybe we can remove some relocation dependencies now.  */
> > > -  assert (map->l_searchlist.r_list[0] == map);
> >
> > Ok, the first entry is the filtee object.
> >
> > >    struct link_map_reldeps *l_reldeps = NULL;
> > >    if (map->l_reldeps != NULL)
> > >      {
> > > -      for (i = 1; i < nlist; ++i)
> > > +      for (i = 0; i < nlist; ++i)
> > >  	map->l_searchlist.r_list[i]->l_reserved = 1;
> > >
> > > +      /* Avoid removing relocation dependencies of the main binary.
> > */
> > > +      map->l_reserved = 0;
> > >        struct link_map **list = &map->l_reldeps->list[0];
> > >        for (i = 0; i < map->l_reldeps->act; ++i)
> > >  	if (list[i]->l_reserved)
> > > @@ -581,16 +582,32 @@ Filters not supported with
> > LD_TRACE_PRELINKING"));
> > >  	      }
> > >  	  }
> > >
> > > -      for (i = 1; i < nlist; ++i)
> > > +      for (i = 0; i < nlist; ++i)
> > >  	map->l_searchlist.r_list[i]->l_reserved = 0;
> > >      }
> >
> > I am trying to understand why we can't skip first element here.
> Neither
> > of the tests actually exercise this code patch (they won't add a
> > dependency on l_reldeps), so could you provide an example/testcase
> > where it requires such change?
> 
> I haven't observed the use case that this code handles. I just tried to
> maintain the existing behaviour that we avoid doing this for the main
> object.
> 
> I've been trying to understand what triggers this, but not getting very
> far.
> Does anyone have any hints as to how I need to setup the test so that
> we do
> trigger relocation removals?

OK, so I git blamed this code to commit c4bb124a75 from 2001. I adapted
elf/relmod5 to include a filter library, and that is working. But it's not
hitting the relocation removal code (checked by adding _dlerror_printfs).
I then ran all tests in the elf directory. Only elf/tst-libc_dlvsym-static and
elf/tst-dlmopen1 enter the for loop, but the l_reserved check is always false.
In both of the above cases map is libc.so.6 and list[i] is libdl.so.2

I verified the same is the case without any of my commits applied.

Have changes elsewhere superseded this? Suggestions as to what I should do
here appreciated :)


Dave.