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

[Jan Beulich]

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