[PATCH v2] RISC-V: Make R_RISCV_SUB6 conforms to riscv ABI standard

[Nelson Chu]

Thanks for fixing this.  We also need to add the out-of-range check
for R_RISCV_SUB6 in the riscv_elf_add_sub_reloc, and the overflow
checks for all ADD/SUB/SET relocations, but since they can be added in
the later patches, so I committed this one after passing the
riscv-gnu-toolchain regressions.

Thanks
Nelson

On Mon, Nov 21, 2022 at 8:02 PM <zengxiao@eswincomputing.com> wrote:
>
> From: Xiao Zeng <zengxiao@eswincomputing.com>
>
> The R_RISCV_SUB6 only the lower 6 bits of the code are valid, which
> can be found in 8.5. Relocations of:
> https://github.com/riscv-non-isa/riscv-elf-psabi-doc/releases/download/v1.0-rc4/riscv-abi.pdf
>
> bfd/ChangeLog:
>
>         * elfnn-riscv.c (riscv_elf_relocate_section): Take the R_RISCV_SUB6
>         lower 6 bits as the significant bit.
>         * elfxx-riscv.c (riscv_elf_add_sub_reloc): Likewise.
> ---
>  bfd/elfnn-riscv.c | 4 ++++
>  bfd/elfxx-riscv.c | 4 ++++
>  2 files changed, 8 insertions(+)
>
> diff --git a/bfd/elfnn-riscv.c b/bfd/elfnn-riscv.c
> index 0570a971b5a..a02aa64786e 100644
> --- a/bfd/elfnn-riscv.c
> +++ b/bfd/elfnn-riscv.c
> @@ -2427,6 +2427,10 @@ riscv_elf_relocate_section (bfd *output_bfd,
>           break;
>
>         case R_RISCV_SUB6:
> +         relocation = (old_value & ~howto->dst_mask)
> +                      | (((old_value & howto->dst_mask) - relocation)
> +                         & howto->dst_mask);
> +         break;

The old_value needs to be defined, but it's easy enough to add, so I fixed it.

>         case R_RISCV_SUB8:
>         case R_RISCV_SUB16:
>         case R_RISCV_SUB32:
> diff --git a/bfd/elfxx-riscv.c b/bfd/elfxx-riscv.c
> index afbde56b9e5..2db24acf7a5 100644
> --- a/bfd/elfxx-riscv.c
> +++ b/bfd/elfxx-riscv.c
> @@ -994,6 +994,10 @@ riscv_elf_add_sub_reloc (bfd *abfd,
>        relocation = old_value + relocation;
>        break;
>      case R_RISCV_SUB6:
> +      relocation = (old_value & ~howto->dst_mask)
> +                   | (((old_value & howto->dst_mask) - relocation)
> +                     & howto->dst_mask);
> +      break;
>      case R_RISCV_SUB8:
>      case R_RISCV_SUB16:
>      case R_RISCV_SUB32:
> --
> 2.34.1
>