comedi: Fix use of uninitialized data in insn_rw_emulate_bits()

In the Linux kernel, the following vulnerability has been resolved:

comedi: Fix use of uninitialized data in insn_rw_emulate_bits()

For Comedi INSN_READ and INSN_WRITE instructions on "digital" subdevices (subdevice types COMEDI_SUBD_DI, COMEDI_SUBD_DO, and COMEDI_SUBD_DIO), it is common for the subdevice driver not to have insn_read and insn_write handler functions, but to have an insn_bits handler function for handling Comedi INSN_BITS instructions. In that case, the subdevice's insn_read and/or insn_write function handler pointers are set to point to the insn_rw_emulate_bits() function by __comedi_device_postconfig().

For INSN_WRITE, insn_rw_emulate_bits() currently assumes that the supplied data[0] value is a valid copy from user memory. It will at least exist because do_insnlist_ioctl() and do_insn_ioctl() in "comedi_fops.c" ensure at lease MIN_SAMPLES (16) elements are allocated. However, if insn->n is 0 (which is allowable for INSN_READ and INSN_WRITE instructions, then data[0] may contain uninitialized data, and certainly contains invalid data, possibly from a different instruction in the array of instructions handled by do_insnlist_ioctl(). This will result in an incorrect value being written to the digital output channel (or to the digital input/output channel if configured as an output), and may be reflected in the internal saved state of the channel.

Fix it by returning 0 early if insn->n is 0, before reaching the code that accesses data[0]. Previously, the function always returned 1 on success, but it is supposed to be the number of data samples actually read or written up to insn->n, which is 0 in this case.