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

["Fāng-ruì Sòng" <maskray@google.com>]

On Tue, Oct 12, 2021 at 11:13 PM Fāng-ruì Sòng <maskray@google.com> wrote:
>
> On Tue, Oct 12, 2021 at 11:00 PM Jan Beulich <jbeulich@suse.com> wrote:
> >
> > On 12.10.2021 18:07, Fāng-ruì Sòng wrote:
> > > On Tue, Oct 12, 2021 at 7:10 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> > >>
> > >> 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.
> > >
> > > --pack-dyn-relocs=relr is well tested on arm, aarch64, and x86, and
> > > works on popular arches like ppc64 as well.
> > > For mips, it is no harm to keep the DT_RELR code path. Its
> > > elf_machine_rel_relative is empty and it has no relative relocation
> > > anyway.
> > > I wish that our reasonable design decisions are not restricted by the
> > > few retrocomputing architectures, especially when the concern is still
> > > at the theoretical stage.
> >
> > For ia64 it's not theoretical at all, as long as you leave aside the
> > fact the deprecation state of that architecture. I also have to admit
> > that I have trouble seeing why the design can't be adjusted to fit
> > original ELF intentions rather than (as said, imo bad) decisions
> > taken by a few (popular) architectures. Besides adjusting the wording
> > accordingly, all it takes for your implementation is to parameterize
> > word (pointer) size.
> >
> > Jan
> >
>
> I re-read your first reply. Did you mean 64-bit small code model code
> in an ELFCLASS32 ET_EXEC/ET_DYN file?
> OK. I considered it as a size optimization for a different task.
> "AArch64 and x86-64 define ILP32 ABIs and use ELFCLASS32, but
> technically they can use ELFCLASS32 for small code model with regular
> ABIs, if the kernel allows." (from
> https://maskray.me/blog/2021-01-31-metadata-sections-comdat-and-shf-link-order)
>
> I assume that switching *((ElfW(Addr) *)(l_addr + entry)) += l_addr;
> to `unsigned long` makes this use case work, though I doubt glibc's
> ia64 implementation supports this as sysdeps/ia64/dl-machine.h uses
> Elf64.

It won't. sysdeps/ia64/dl-machine.h:elf_machine_rela_relative writes
to an Elf64_Addr.
l_addr and many other representations assume that ElfW can not be an
Elf32 container for a 64-bit architecture.