Re: [PATCH v3 3/3] capstone: Add s390x skipdata callback

[Thomas Huth]

On 15/10/2019 19.51, Richard Henderson wrote:
> Capstone assumes any unknown instruction is 2 bytes.
> Instead, use the ilen field in the first two bits of
> the instruction to stay in sync with the insn stream.
> 
> Signed-off-by: Richard Henderson <address@hidden>
> ---
>  disas.c | 37 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 37 insertions(+)
> 
> diff --git a/disas.c b/disas.c
> index 51c71534a3..2a000cbeb0 100644
> --- a/disas.c
> +++ b/disas.c
> @@ -178,6 +178,39 @@ static int print_insn_od_target(bfd_vma pc, 
> disassemble_info *info)
>     to share this across calls and across host vs target disassembly.  */
>  static __thread cs_insn *cap_insn;
>  
> +/*
> + * The capstone library always skips 2 bytes for S390X.
> + * This is less than ideal, since we can tell from the first two bits
> + * the size of the insn and thus stay in sync with the insn stream.
> + */
> +static size_t CAPSTONE_API
> +cap_skipdata_s390x_cb(const uint8_t *code, size_t code_size,
> +                      size_t offset, void *user_data)
> +{
> +    size_t ilen;
> +
> +    /* See get_ilen() in target/s390x/internal.h.  */
> +    switch (code[offset] >> 6) {
> +    case 0:
> +        ilen = 2;
> +        break;
> +    case 1:
> +    case 2:
> +        ilen = 4;
> +        break;
> +    default:
> +        ilen = 6;
> +        break;
> +    }
> +
> +    return ilen;
> +}

The kernel has also a nice function to calculate this:

static inline int insn_length(unsigned char code)
{
        return ((((int) code + 64) >> 7) + 1) << 1;
}

... but the switch-case is likely easier to read, so anyway:

Reviewed-by: Thomas Huth <address@hidden>