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

Borislav Petkov
2017-05-29T21:48:16Z

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 Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
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