Re: [PATCH 3/4] Add run-time chesk for single global definition

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Jun 21, 2021 at 12:16 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * H. J. Lu via Libc-alpha:
>
> > +static inline void __attribute__ ((always_inline))
> > +_dl_check_protected_symbol (const char *undef_name,
> > +                         const struct link_map *undef_map,
> > +                         const ElfW(Sym) *ref,
> > +                         const struct link_map *map,
> > +                         int type_class)
> > +{
> > +  if (undef_map != NULL
> > +      && !(undef_map->l_1_needed
> > +        & GNU_PROPERTY_1_NEEDED_SINGLE_GLOBAL_DEFINITION)
> > +      && (map->l_1_needed
> > +       & GNU_PROPERTY_1_NEEDED_SINGLE_GLOBAL_DEFINITION))
> > +    {
> > +      if ((type_class & ELF_RTYPE_CLASS_COPY))
> > +     /* Disallow copy relocations against protected data symbols in
> > +        an object with single global definition.  */
> > +     _dl_fatal_printf ("copy relocation against non-copyable protected symbol=%s in file=%s\n",
> > +                       undef_name, DSO_FILENAME (map->l_name));
> > +      else if (ref->st_value != 0
> > +            && ref->st_shndx == SHN_UNDEF
> > +            && (type_class & ELF_RTYPE_CLASS_PLT))
> > +     /* Disallow non-zero symbol values of undefined symbols, which
> > +        are used as the function pointer, against protected function
> > +        symbols in an object with single global definition.  */
> > +     _dl_fatal_printf ("non-canonical reference to canonical protected function symbol=%s in file=%s\n",
> > +                       undef_name, DSO_FILENAME (map->l_name));
> > +    }
> > +}
>
> Why are those fatal errors?

2 copies of the data symbol can be out of sync between executable and
shared library.  We can make them as warnings with tunable to control
it.

> I have trouble understanding the second comment (for the
> ELF_RTYPE_CLASS_PLT).

If st_value is the undefined symbol in executable is not zero, it
is the PLT address in executable and ld.so will use it for function
pointer which is different from the function address in shared
library.

-- 
H.J.