[PATCH v3 2/5] Make libc symbols hidden in static PIE

[H.J. Lu]

On Fri, Jan 15, 2021 at 6:27 AM Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>
> The 01/15/2021 05:43, H.J. Lu wrote:
> > On Fri, Jan 15, 2021 at 3:25 AM Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
> > > options:
> > >
> > > (1) fix extern hidden pie calls on i686 (by making
> > >     them the same as default vis pie calls so ifuncs
> > >     work).
> > >
> > > (2) annotate ifuncs (avoid hidden for them, ifuncs
> > >     cannot appear in early start code anyway because
> > >     of IRELATIVE): this can be difficult to maintain.
> > >
> > > (3) annotate early object accesses to be hidden so
> > >     RELATIVE relocs are avoided. (most targets want
> > >     all objects to be hidden, but this solves bug
> > >     27072 without causing problems on i686)
> > >
> > > (4) make pie always use copy relocations on i686.
> > >     (and then no hidden annotation is needed, object
> > >     access is always local in pie).
> ...
> > > > --- a/elf/dl-reloc-static-pie.c
> > > > +++ b/elf/dl-reloc-static-pie.c
> > > > @@ -17,6 +17,7 @@
> > > >     <https://www.gnu.org/licenses/>.  */
> > > >
> > > >  #if ENABLE_STATIC_PIE
> > > > +#pragma GCC visibility push(hidden)
> > >
> > > yes, this is option (3). you will also need it in _dl_aux_init
> > > and __libc_init_secure and __tunables_init.
> >
> > I will try it.
>
> the naive way does not seem to work:
>
> _dl_support.c has _dl_aux_init as well as _dl_non_dynamic_init,
> the former needs hidden the latter does not and calls rawmemchr
> which is ifunc on i686.
>
> i think the easiest fix is to move those two functions into
> separate files. (ideally we would have a small set of files
> that are involved in the start code before self relocation)
>
> now i realized that there is another option:
>
> (5) remove all ifuncs from i686 libc.
>
> i assume there are not many users who care about i686 performance.

I don't know if this will work on i686.  Since i386 doesn't have IP relative
addressing, we can't remove all RELATIVE relocations.  We need to
call _dl_aux_init again after relocating PIE.  I don't know what other symbols
are affected.  My current patches are on users/hjl/pr27072/master branch:

https://gitlab.com/x86-glibc/glibc/-/commits/users/hjl/pr27072/master

-- 
H.J.