Re: [PATCH] RISC-V: Support PCREL_* relocations agaist weak undefined symbols

[Palmer Dabbelt <palmer at dabbelt dot com>]

Committed.

On Wed, 06 Sep 2017 11:04:54 PDT (-0700), Palmer Dabbelt wrote:
> I recently modified our Linux port's base address such the absolute
> address 0 is no longer addressable as a 32-bit PC-relative offset.
> Since Linux links a weak undefined symbol in an intermediate binary, it
> needs to be able to reference absolute address 0.
>
> This patch changes R_RISCV_PCREL_* relocations to absolute relocations
> while resolving them in order to allow these symbols to be referenced in
> PC-relative programs linked at high addresses.  Note that this doesn't
> apply to PIC, which also uses PC-relative relocations, just to
> position-dependent objects, which we use to allow programs to be linked
> at high addresses.
>
> In case some of our embedded users are using R_RISCV_PCREL_* as a hacked
> up method of getting position-independent binaries (which can work if
> you have very simple programs), we only convert the relocations when the
> PC-relative version would overflow.
>
> bfd/ChangeLog:
>
> 2017-08-16  Palmer Dabbelt  <palmer@dabbelt.com>
>
>         * elfnn-riscv.c (riscv_zero_pcrel_hi_reloc): New function.
>         (riscv_record_pcrel_hi_reloc): Add absolute argument.
>         (riscv_elf_relocate_section): Call riscv_zero_pcrel_hi_reloc for
>         R_RISCV_PCREL_HI20 relocs, and pass the result to
>         riscv_record_pcrel_hi_reloc.
> ---
>  bfd/elfnn-riscv.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 68 insertions(+), 5 deletions(-)
>
> diff --git a/bfd/elfnn-riscv.c b/bfd/elfnn-riscv.c
> index 973f471e0529..fdb151be827f 100644
> --- a/bfd/elfnn-riscv.c
> +++ b/bfd/elfnn-riscv.c
> @@ -1651,9 +1651,50 @@ riscv_free_pcrel_relocs (riscv_pcrel_relocs *p)
>  }
>
>  static bfd_boolean
> -riscv_record_pcrel_hi_reloc (riscv_pcrel_relocs *p, bfd_vma addr, bfd_vma value)
> +riscv_zero_pcrel_hi_reloc (Elf_Internal_Rela *rel,
> +			   struct bfd_link_info *info,
> +			   bfd_vma pc,
> +			   bfd_vma addr,
> +			   bfd_byte *contents,
> +			   const reloc_howto_type *howto,
> +			   bfd *input_bfd)
>  {
> -  riscv_pcrel_hi_reloc entry = {addr, value - addr};
> +  /* We may need to reference low addreses in PC-relative modes even when the
> +   * PC is far away from these addresses.  For example, undefweak references
> +   * need to produce the address 0 when linked.  As 0 is far from the arbitrary
> +   * addresses that we can link PC-relative programs at, the linker can't
> +   * actually relocate references to those symbols.  In order to allow these
> +   * programs to work we simply convert the PC-relative auipc sequences to
> +   * 0-relative lui sequences.  */
> +  if (bfd_link_pic (info))
> +    return FALSE;
> +
> +  /* If it's possible to reference the symbol using auipc we do so, as that's
> +   * more in the spirit of the PC-relative relocations we're processing.  */
> +  bfd_vma offset = addr - pc;
> +  if (ARCH_SIZE == 32 || VALID_UTYPE_IMM (RISCV_CONST_HIGH_PART (offset)))
> +    return FALSE;
> +
> +  /* If it's impossible to reference this with a LUI-based offset then don't
> +   * bother to convert it at all so users still see the PC-relative relocation
> +   * in the truncation message.  */
> +  if (ARCH_SIZE > 32 && !VALID_UTYPE_IMM (RISCV_CONST_HIGH_PART (addr)))
> +    return FALSE;
> +
> +  rel->r_info = ELFNN_R_INFO(addr, R_RISCV_HI20);
> +
> +  bfd_vma insn = bfd_get(howto->bitsize, input_bfd, contents + rel->r_offset);
> +  insn = (insn & ~MASK_AUIPC) | MATCH_LUI;
> +  bfd_put(howto->bitsize, input_bfd, insn, contents + rel->r_offset);
> +  return TRUE;
> +}
> +
> +static bfd_boolean
> +riscv_record_pcrel_hi_reloc (riscv_pcrel_relocs *p, bfd_vma addr,
> +			     bfd_vma value, bfd_boolean absolute)
> +{
> +  bfd_vma offset = absolute ? value : value - addr;
> +  riscv_pcrel_hi_reloc entry = {addr, offset};
>    riscv_pcrel_hi_reloc **slot =
>      (riscv_pcrel_hi_reloc **) htab_find_slot (p->hi_relocs, &entry, INSERT);
>
> @@ -1758,6 +1799,7 @@ riscv_elf_relocate_section (bfd *output_bfd,
>    Elf_Internal_Shdr *symtab_hdr = &elf_symtab_hdr (input_bfd);
>    struct elf_link_hash_entry **sym_hashes = elf_sym_hashes (input_bfd);
>    bfd_vma *local_got_offsets = elf_local_got_offsets (input_bfd);
> +  bfd_boolean absolute;
>
>    if (!riscv_init_pcrel_relocs (&pcrel_relocs))
>      return FALSE;
> @@ -1931,7 +1973,17 @@ riscv_elf_relocate_section (bfd *output_bfd,
>  		}
>  	    }
>  	  relocation = sec_addr (htab->elf.sgot) + off;
> -	  if (!riscv_record_pcrel_hi_reloc (&pcrel_relocs, pc, relocation))
> +	  absolute = riscv_zero_pcrel_hi_reloc (rel,
> +						info,
> +						pc,
> +						relocation,
> +						contents,
> +						howto,
> +						input_bfd);
> +	  r_type = ELFNN_R_TYPE (rel->r_info);
> +	  howto = riscv_elf_rtype_to_howto (r_type);
> +	  if (!riscv_record_pcrel_hi_reloc (&pcrel_relocs, pc,
> +					    relocation, absolute))
>  	    r = bfd_reloc_overflow;
>  	  break;
>
> @@ -2017,8 +2069,18 @@ riscv_elf_relocate_section (bfd *output_bfd,
>  	  }
>
>  	case R_RISCV_PCREL_HI20:
> +	  absolute = riscv_zero_pcrel_hi_reloc (rel,
> +						info,
> +						pc,
> +						relocation,
> +						contents,
> +						howto,
> +						input_bfd);
> +	  r_type = ELFNN_R_TYPE (rel->r_info);
> +	  howto = riscv_elf_rtype_to_howto (r_type);
>  	  if (!riscv_record_pcrel_hi_reloc (&pcrel_relocs, pc,
> -					    relocation + rel->r_addend))
> +					    relocation + rel->r_addend,
> +					    absolute))
>  	    r = bfd_reloc_overflow;
>  	  break;
>
> @@ -2214,7 +2276,8 @@ riscv_elf_relocate_section (bfd *output_bfd,
>
>  	  BFD_ASSERT (off < (bfd_vma) -2);
>  	  relocation = sec_addr (htab->elf.sgot) + off + (is_ie ? ie_off : 0);
> -	  if (!riscv_record_pcrel_hi_reloc (&pcrel_relocs, pc, relocation))
> +	  if (!riscv_record_pcrel_hi_reloc (&pcrel_relocs, pc,
> +					    relocation, FALSE))
>  	    r = bfd_reloc_overflow;
>  	  unresolved_reloc = FALSE;
>  	  break;