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

[Fāng-ruì Sòng]

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

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:)
I hope that means we don't need ELF_MACHINE_NO_RELR.

> > --- /dev/null
> > +++ b/elf/tst-relr.c
> > @@ -0,0 +1,20 @@
> > +static int o, x;
> > +void *arr[] = {
> > +  &o, &o, &o, &o, &o, &o, &o, &o, &o, &o, &o, &o, &o, &o, &o, &o,
> > +  0,
> > +  &x, &x, &x, &x, &x, &x, &x, &x, &x, &x, &x, &x, &x, &x, &x, &x,
> > +};
>
> Personally I consider this overly simplistic a testcase. The two
> sequences are fully identical, have no gaps except for the one in
> the middle, and there's also no interleaving / mixing of pointers.
> None of this should matter as the specific symbol the relocation
> is for is not supposed to be of interest, but in a testcase you
> want to make sure none of this has an effect.
>
> I also wonder whether it is a good idea to have a testcase with
> "fundamentally" different behavior on 32-bit vs 64-bit: The former
> will require two RELR entries afaict, while the latter will get
> away with one here.

Thanks for the test suggestion. I can use some fancy patterns.

> Jan
>
> > +static int
> > +do_test (void)
> > +{
> > +  for (int i = 0; i < 16; i++)
> > +    if (arr[i] != &o)
> > +      return 1;
> > +  for (int i = 17; i < 33; i++)
> > +    if (arr[i] != &x)
> > +      return 1;
> > +  return 0;
> > +}
> > +
> > +#include <support/test-driver.c>
> >
>


-- 
宋方睿