Re: [PATCH v7 09/26] x86/insn-eval: Add utility function to identify string instructions

[Borislav Petkov]

On Fri, May 05, 2017 at 11:17:07AM -0700, Ricardo Neri wrote:
> String instructions are special because in protected mode, the linear
> address is always obtained via the ES segment register in operands that
> use the (E)DI register.

 ... and DS for rSI.

If we're going to account for both operands of string instructions with
two operands.

Btw, LODS and OUTS use only DS:rSI as a source operand. So we have to be
careful with the generalization here. So if ES:rDI is the only seg. reg
we want, then we don't need to look at those insns... (we assume DS by
default).

...

> +/**
> + * is_string_instruction - Determine if instruction is a string instruction
> + * @insn:	Instruction structure containing the opcode
> + *
> + * Return: true if the instruction, determined by the opcode, is any of the
> + * string instructions as defined in the Intel Software Development manual.
> + * False otherwise.
> + */
> +static bool is_string_instruction(struct insn *insn)
> +{
> +	insn_get_opcode(insn);
> +
> +	/* all string instructions have a 1-byte opcode */
> +	if (insn->opcode.nbytes != 1)
> +		return false;
> +
> +	switch (insn->opcode.bytes[0]) {
> +	case INSB:
> +		/* fall through */
> +	case INSW_INSD:
> +		/* fall through */
> +	case OUTSB:
> +		/* fall through */
> +	case OUTSW_OUTSD:
> +		/* fall through */
> +	case MOVSB:
> +		/* fall through */
> +	case MOVSW_MOVSD:
> +		/* fall through */
> +	case CMPSB:
> +		/* fall through */
> +	case CMPSW_CMPSD:
> +		/* fall through */
> +	case STOSB:
> +		/* fall through */
> +	case STOSW_STOSD:
> +		/* fall through */
> +	case LODSB:
> +		/* fall through */
> +	case LODSW_LODSD:
> +		/* fall through */
> +	case SCASB:
> +		/* fall through */

That "fall through" for every opcode is just too much. Also, you can use
the regularity of the x86 opcode space and do:

	case 0x6c ... 0x6f:	/* INS/OUTS */
	case 0xa4 ... 0xa7:	/* MOVS/CMPS */
	case 0xaa ... 0xaf:	/* STOS/LODS/SCAS */
		return true;
	default:
		return false;
}

And voila, there's your compact is_string_insn() function! :^)

(Modulo the exact list, as I mentioned above).

Thanks.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix ImendÃrffer, Jane Smithard, Graham Norton, HRB 21284 (AG NÃrnberg)
--