Re: [PATCH 1/1] RISC-V: Make R_RISCV_SUB6 conforms to riscv abi standard

["Xiao Zeng" <zengxiao@eswincomputing.com>]

On Fri, Nov 18, 2022 at 12:00:00 AM Nelson Chu <nelson@rivosinc.com> wrote:
>
>On Fri, Nov 18, 2022 at 1:00 PM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>>
>> On Fri, 11 Nov 2022 07:32:49 PST (-0800), shihua@iscas.ac.cn wrote:
>> > LGTM,and I think it would be better to have a test example.
>> >
>> >
>> >
>> >
>> >
>> >> From: zengxiao <zengxiao@eswincomputing.com>
>> >>
>> >> This patch makes R_RISCV_SUB6 conforms to riscv abi standard.
>> >> R_RISCV_SUB6 only the lower 6 bits of the code are valid.
>> >> The proposed specification 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:
>> >>
>> >> * elfxx-riscv.c (riscv_elf_add_sub_reloc): Take the lower
>> >> 6 bits as the significant bit
>> >>
>> >> reviewed-by: gaofei@eswincomputing.com
>> >>              jinyanjiang@eswincomputing.com
>>
>> Is this trying to say that both of you reviewed it?
>>
>> >> Signed-off-by: zengxiao <zengxiao@eswincomputing.com>
>>
>>
>> >> ---
>> >>  bfd/elfxx-riscv.c | 7 +++++++
>> >>  1 file changed, 7 insertions(+)
>> >>
>> >> diff --git a/bfd/elfxx-riscv.c b/bfd/elfxx-riscv.c
>> >> index f0c91cc97f7..0fbfedd17fe 100644
>> >> --- a/bfd/elfxx-riscv.c
>> >> +++ b/bfd/elfxx-riscv.c
>> >> @@ -994,6 +994,13 @@ riscv_elf_add_sub_reloc (bfd *abfd,
>> >>        relocation = old_value + relocation;
>> >>        break;
>> >>      case R_RISCV_SUB6:
>> >> +      {
>> >> +        bfd_vma six_bit_valid_value = old_value & howto->dst_mask;
>> >> +        six_bit_valid_value -= relocation;
>> >> +        relocation = (six_bit_valid_value & howto->dst_mask) |
>> >> +              (old_value & ~howto->dst_mask);
>> >> +      }
>> >> +      break;
>>
>> Unless I'm missing something here, this just just silently truncates the
>> relocation to 6 bits because the range check still assumes an 8-bit
>> relocation range.  I'm not sure if there's a way to massage the howto
>> entry to make bfd_reloc_offset_in_range() understand this is a 6-bit
>> relocation, if that's not viable then we should just check for the
>> overflow here and return bfd_reloc_outofrange.
>
>Yeah agreed, we shouldn't call bfd_reloc_offset_in_range for
>R_RISCV_SUB6 since we are assuming it is an 8-bit relocation.  That
>means if the R_RISCV_SUB6 is used to relocate the last 6-bit of the
>section, then ld will always report bfd_reloc_outofrange since it
>assumes at least 8-bit is needed, although the case seems minor.
>

Thanks Nelson for catching the point of my patch: the R_RISCV_SUB6 is 
incorrectly assumed an 8-bit relocation.

Therefore, the overflow check of bfd_reloc_offset_in_range() may not 
help objdump correctly resolve the R_ RISCV_ SUB6.

>As for the overflow of relocation values, we don't have any checks of
>them, so I think we can just ignore them in the short-term.
>
>Thanks
>Nelson
>
>> That also means there should be at least two test cases, on within range
>> and one outside of it.
>> 

Thanks
Xiao
>> >>      case R_RISCV_SUB8:
>> >>      case R_RISCV_SUB16:
>> >>      case R_RISCV_SUB32:
>> >> --
>> >> 2.34.1