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

[Jan Beulich <jbeulich@suse.com>]

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 ...

Jan