Re: [PATCH] RISC-V: Fix disassembly of partial instructions

[Charlie Jenkins <charlie@rivosinc.com>]

On Sat, Dec 14, 2024 at 03:51:10PM +0800, Jiawei wrote:
> 在 2024/12/14 4:50, Charlie Jenkins 写道:
> > As of commit e43d8768d909 ("RISC-V: Fix disassemble fetch fail return
> > value.") partial instructions are no longer disassembled. While that
> > commit fixed the behavior of print_insn_riscv() returning the arbitrary
> > status value upon failure, it caused the behavior of dumping
> > instructions to change. Allow partial instructions to be disassembled
> > once again and only return -1 if no part of the instruction was able to
> > be disassembled.
> > 
> > Fixes: e43d8768d909 ("RISC-V: Fix disassemble fetch fail return value.")
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > ---
> > When testing linux perf, I noticed that this behavior of objdump has
> > changed. Before this patch and running `perf test` on riscv the
> > following test fails due to objdump not returning all of the expected
> > bytes.
> > 
> > Bytes read differ from those read by objdump
> > buf1 (dso):
> > 0x97 0xf7 0x11 0x00 0x93 0x87 0xc7 0x7c 0x22 0x85 0x7c 0xec 0xef 0x50 0x80 0x12
> > 0xa6 0x85 0xce 0x86 0x4a 0x86 0x22 0x85 0xef 0x50 0x40 0x40 0xa2 0x84 0x1d 0xc9
> > 0x7c 0x58 0x85 0x8b 0x85 0xc3 0x1c 0x40 0xa1 0x8b 0x89 0xcf 0x83 0x27 0x04 0x0c
> > 0x63 0x51 0xf0 0x04 0x97 0xf7 0x11 0x00 0x93 0x87 0x07 0x45 0xbe 0x86 0x58 0x70
> > 0x74 0xec 0x7c 0xf3 0xa2 0x70 0x02 0x74 0x42 0x69 0xa2 0x69 0x26 0x85 0xe2 0x64
> > 0x45 0x61 0x82 0x80 0x22 0x85 0xef 0x50 0x50 0x52 0x22 0x85 0xef 0x00 0xb1 0x39
> > 0xa2 0x70 0x02 0x74 0x81 0x44 0x42 0x69 0xa2 0x69 0x26 0x85 0xe2 0x64 0x45 0x61
> > 0x82 0x80 0x97 0x06 0x12 0x00 0x93 0x86 0xa6 0x8a 0x97 0xf7 0x11 0x00 0x93 0x87
> > 
> > buf2 (objdump):
> > 0x97 0xf7 0x11 0x00 0x93 0x87 0xc7 0x7c 0x22 0x85 0x7c 0xec 0xef 0x50 0x80 0x12
> > 0xa6 0x85 0xce 0x86 0x4a 0x86 0x22 0x85 0xef 0x50 0x40 0x40 0xa2 0x84 0x1d 0xc9
> > 0x7c 0x58 0x85 0x8b 0x85 0xc3 0x1c 0x40 0xa1 0x8b 0x89 0xcf 0x83 0x27 0x04 0x0c
> > 0x63 0x51 0xf0 0x04 0x97 0xf7 0x11 0x00 0x93 0x87 0x07 0x45 0xbe 0x86 0x58 0x70
> > 0x74 0xec 0x7c 0xf3 0xa2 0x70 0x02 0x74 0x42 0x69 0xa2 0x69 0x26 0x85 0xe2 0x64
> > 0x45 0x61 0x82 0x80 0x22 0x85 0xef 0x50 0x50 0x52 0x22 0x85 0xef 0x00 0xb1 0x39
> > 0xa2 0x70 0x02 0x74 0x81 0x44 0x42 0x69 0xa2 0x69 0x26 0x85 0xe2 0x64 0x45 0x61
> > 0x82 0x80 0x97 0x06 0x12 0x00 0x93 0x86 0xa6 0x8a 0x97 0xf7 0x11 0x00 0xad 0x00
> > 
> > ---- end(-1) ----
> >   24: Object code reading                              : FAILED!
> > 
> > After this patch, this test case no longer fails, as objdump returns the
> > expected values.
> > ---
> >   opcodes/riscv-dis.c | 50 +++++++++++++++++++++++++++++++++++++++++++++-----
> >   1 file changed, 45 insertions(+), 5 deletions(-)
> > 
> > diff --git a/opcodes/riscv-dis.c b/opcodes/riscv-dis.c
> > index 101380f93aafbd528ba0020371f0c43a85f41bd1..b0dc67c3a18caf7437a0a6d6229108299e8514a7 100644
> > --- a/opcodes/riscv-dis.c
> > +++ b/opcodes/riscv-dis.c
> > @@ -1308,6 +1308,14 @@ riscv_disassemble_data (bfd_vma memaddr ATTRIBUTE_UNUSED,
> >         (*info->fprintf_styled_func)
> >   	(info->stream, dis_style_immediate, "0x%04x", (unsigned) data);
> >         break;
> > +    case 3:
> > +      info->bytes_per_line = 7;
> > +      (*info->fprintf_styled_func)
> > +	(info->stream, dis_style_assembler_directive, ".word");
> > +      (*info->fprintf_styled_func) (info->stream, dis_style_text, "\t");
> > +      (*info->fprintf_styled_func)
> > +	(info->stream, dis_style_immediate, "0x%06x", (unsigned) data);
> > +      break;
> >       case 4:
> >         info->bytes_per_line = 8;
> >         (*info->fprintf_styled_func)
> > @@ -1360,13 +1368,28 @@ riscv_init_disasm_info (struct disassemble_info *info)
> >     return true;
> >   }
> > +/* Fetch an instruction. If only a partial instruction is able to be fetched,
> > +   return the number of accessible bytes. */
> 
> And a hint there should be two spaces before the end .
> 
> > +
> > +static bfd_vma
> > +fetch_insn (bfd_vma memaddr, bfd_byte *packet, bfd_vma dump_size, struct disassemble_info *info, volatile int *status)
> > +{
> > +  do
> > +    {
> > +      *status = (*info->read_memory_func) (memaddr, packet, dump_size, info);
> > +    }
> > +  while(*status != 0 && dump_size-- > 1);
> > +
> > +  return dump_size;
> > +}
> > +
> >   int
> >   print_insn_riscv (bfd_vma memaddr, struct disassemble_info *info)
> >   {
> >     bfd_byte packet[RISCV_MAX_INSN_LEN];
> >     insn_t insn = 0;
> > -  bfd_vma dump_size;
> > -  int status;
> > +  volatile bfd_vma dump_size, bytes_fetched;
> > +  volatile int status;
> >     enum riscv_seg_mstate mstate;
> >     int (*riscv_disassembler) (bfd_vma, insn_t, const bfd_byte *,
> >   			     struct disassemble_info *);
> > @@ -1398,24 +1421,41 @@ print_insn_riscv (bfd_vma memaddr, struct disassemble_info *info)
> >     else
> >       {
> >         /* Get the first 2-bytes to check the lenghth of instruction.  */
> > -      status = (*info->read_memory_func) (memaddr, packet, 2, info);
> > +      bytes_fetched = fetch_insn(memaddr, packet, 2, info, &status);
> >         if (status != 0)
> >   	{
> >   	  (*info->memory_error_func) (status, memaddr, info);
> >   	  return -1;
> >   	}
> > +      else if (bytes_fetched != 2)
> > +       {
> > +	  /* Only the first byte was able to be read. Dump the partial instruction. */
> 
> Same case at here, besides that LGTM:)

Thanks! I sent out a new version:

https://sourceware.org/pipermail/binutils/2024-December/138139.html

- Charlie


> 
> Jiawei
> 
> > +	  dump_size = bytes_fetched;
> > +	  info->bytes_per_chunk = dump_size;
> > +	  riscv_disassembler = riscv_disassemble_data;
> > +	 goto print;
> > +       }
> >         insn = (insn_t) bfd_getl16 (packet);
> >         dump_size = riscv_insn_length (insn);
> >         riscv_disassembler = riscv_disassemble_insn;
> >       }
> > -  /* Fetch the instruction to dump.  */
> > -  status = (*info->read_memory_func) (memaddr, packet, dump_size, info);
> > +  bytes_fetched = fetch_insn(memaddr, packet, dump_size, info, &status);
> > +
> >     if (status != 0)
> >       {
> >         (*info->memory_error_func) (status, memaddr, info);
> >         return -1;
> >       }
> > +  else if (bytes_fetched != dump_size)
> > +    {
> > +      dump_size = bytes_fetched;
> > +      info->bytes_per_chunk = dump_size;
> > +      riscv_disassembler = riscv_disassemble_data;
> > +    }
> > +
> > +print:
> > +
> >     insn = (insn_t) bfd_get_bits (packet, dump_size * 8, false);
> >     return (*riscv_disassembler) (memaddr, insn, packet, info);
> > 
> > ---
> > base-commit: 978324718990b6b371d4eeeba02cfe13a0ebf120
> > change-id: 20241121-fix_objdump_partial_insn-94e236f3db38
>