[PATCH] elf: Support DT_RELR relative relocation format [BZ #27924]
Fāng-ruì Sòng
maskray@google.com
Wed Oct 13 06:13:55 GMT 2021
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.
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