static bool __kprobes decode_regs()

in probes/decode.c [258:331]

• 50 lines of code
• 20 McCabe index (conditional complexity)

static bool __kprobes decode_regs(probes_opcode_t *pinsn, u32 regs, bool modify)
{
	probes_opcode_t insn = *pinsn;
	probes_opcode_t mask = 0xf; /* Start at least significant nibble */

	for (; regs != 0; regs >>= 4, mask <<= 4) {

		probes_opcode_t new_bits = INSN_NEW_BITS;

		switch (regs & 0xf) {

		case REG_TYPE_NONE:
			/* Nibble not a register, skip to next */
			continue;

		case REG_TYPE_ANY:
			/* Any register is allowed */
			break;

		case REG_TYPE_SAMEAS16:
			/* Replace register with same as at bit position 16 */
			new_bits = INSN_SAMEAS16_BITS;
			break;

		case REG_TYPE_SP:
			/* Only allow SP (R13) */
			if ((insn ^ 0xdddddddd) & mask)
				goto reject;
			break;

		case REG_TYPE_PC:
			/* Only allow PC (R15) */
			if ((insn ^ 0xffffffff) & mask)
				goto reject;
			break;

		case REG_TYPE_NOSP:
			/* Reject SP (R13) */
			if (((insn ^ 0xdddddddd) & mask) == 0)
				goto reject;
			break;

		case REG_TYPE_NOSPPC:
		case REG_TYPE_NOSPPCX:
			/* Reject SP and PC (R13 and R15) */
			if (((insn ^ 0xdddddddd) & 0xdddddddd & mask) == 0)
				goto reject;
			break;

		case REG_TYPE_NOPCWB:
			if (!is_writeback(insn))
				break; /* No writeback, so any register is OK */
			fallthrough;
		case REG_TYPE_NOPC:
		case REG_TYPE_NOPCX:
			/* Reject PC (R15) */
			if (((insn ^ 0xffffffff) & mask) == 0)
				goto reject;
			break;
		}

		/* Replace value of nibble with new register number... */
		insn &= ~mask;
		insn |= new_bits & mask;
	}

	if (modify)
		*pinsn = insn;

	return true;

reject:
	return false;
}