[PATCH] elf: Support DT_RELR relative relocation format [BZ #27924]

[H.J. Lu]

On Tue, Oct 12, 2021 at 1:18 AM Jan Beulich via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> On 11.10.2021 20:43, Fāng-ruì Sòng wrote:
> > On Mon, Oct 11, 2021 at 12:48 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 08.10.2021 08:57, Fangrui Song via Binutils wrote:
> >>> --- a/elf/dynamic-link.h
> >>> +++ b/elf/dynamic-link.h
> >>> @@ -192,6 +192,33 @@ elf_machine_lazy_rel (struct link_map *map, struct r_scope_elem *scope[],
> >>>  #  define ELF_DYNAMIC_DO_RELA(map, scope, lazy, skip_ifunc) /* Nothing to do.  */
> >>>  # endif
> >>>
> >>> +# define ELF_DYNAMIC_DO_RELR(map)                                          \
> >>> +  do {                                                                             \
> >>> +    ElfW(Addr) l_addr = (map)->l_addr, base = 0, start;                            \
> >>> +    const ElfW(Relr) *r = 0, *end = 0;                                             \
> >>> +    if (!(map)->l_info[DT_RELR])                                           \
> >>> +      break;                                                               \
> >>> +    start = D_PTR((map), l_info[DT_RELR]);                                 \
> >>> +    r = (const ElfW(Relr) *)start;                                         \
> >>> +    end = (const ElfW(Relr) *)(start + (map)->l_info[DT_RELRSZ]->d_un.d_val); \
> >>> +    for (; r < end; ++r) {                                                 \
> >>> +      ElfW(Relr) entry = *r;                                               \
> >>> +      if ((entry & 1) == 0) {                                                      \
> >>> +     *((ElfW(Addr) *)(l_addr + entry)) += l_addr;                          \
> >>> +     base = entry + sizeof(ElfW(Addr));                                    \
> >>> +     continue;                                                             \
> >>> +      }                                                                            \
> >>> +      ElfW(Addr) offset = base;                                                    \
> >>> +      do {                                                                 \
> >>> +     entry >>= 1;                                                          \
> >>> +     if ((entry & 1) != 0)                                                 \
> >>> +       *((ElfW(Addr) *)(l_addr + offset)) += l_addr;                       \
> >>> +     offset += sizeof(ElfW(Addr));                                         \
> >>> +      } while (entry != 0);                                                \
> >>> +      base += (8 * sizeof(ElfW(Relr)) - 1) * sizeof(ElfW(Addr));           \
> >>
> >> While in line with the proposed spec additions I'm afraid the uses of
> >> ElfW(Addr) here aren't universally correct: You assume that ELF
> >> container type (size) expresses an aspect of the ABI. While this is
> >> indeed the case for several arch-es, I think this has been a mistake.
> >> IA-64, while meanwhile mostly dead, is (was) an example where 64-bit
> >> code can validly live in a 32-bit ELF container (at least as far as
> >> the psABI is concerned; I have no idea whether glibc actually
> >> followed the spec). There's a separate ELF header flag indicating the
> >> ABI, and hence the size of a pointer.
> >
> > Thanks for chiming in.
> >
> > As of ia64 buildability, it works for me:
> >
> > scripts/build-many-glibcs.py /tmp/glibc-many compilers ia64-linux-gnu
> > mkdir -p out/ia64; cd out/ia64
> > ../../configure --prefix=/tmp/glibc/ia64 --host=ia64-linux-gnu
> > CC=/tmp/glibc-many/install/compilers/ia64-linux-gnu/bin/ia64-glibc-linux-gnu-gcc
> > CXX=/tmp/glibc-many/install/compilers/ia64-linux-gnu/bin/ia64-glibc-linux-gnu-g++
> > make -j 50
>
> I didn't suggest the build would fail. What I said is that I don't
> think the code is correct there.
>
> > As of the actual functionality, ugh, I cannot find ia64 in my Debian
> > testing's qemu-user-static package:( So I cannot test.
> >
> > That said, gold and LLD don't support ia64.
> > If we have a concern that ia64 may not work, the GNU ld maintainers
> > can simply not add ia64 support:)
>
> But you realize that I took ia64 only as example, as that's where
> I know ABI (pointer size) and ELF container size aren't connected.
> As per my looking at merely EF_MIPS_* in context of reading
> Joseph's reply, it might be that MIPS is another such example. But
> I lack sufficient knowledge of MIPS ...
>

The new code should be tested and verified on all supported
targets.  That is another reason to implement this in binutils
ld first.

-- 
H.J.