Re: [Qemu-devel] [PATCH v2 04/12] i2c: Add a length check to the SMBus write handling

[Corey Minyard]

On 11/20/18 9:33 AM, Peter Maydell wrote:
> On 15 November 2018 at 19:24,  <address@hidden> wrote:
> > From: Corey Minyard <address@hidden>
> >
> > Avoid an overflow.
> >
> > Signed-off-by: Corey Minyard <address@hidden>
> > ---
> >   hw/i2c/smbus_slave.c | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/hw/i2c/smbus_slave.c b/hw/i2c/smbus_slave.c
> > index 83ca041b5d..fa988919d8 100644
> > --- a/hw/i2c/smbus_slave.c
> > +++ b/hw/i2c/smbus_slave.c
> > @@ -182,7 +182,11 @@ static int smbus_i2c_send(I2CSlave *s, uint8_t data)
> >       switch (dev->mode) {
> >       case SMBUS_WRITE_DATA:
> >           DPRINTF("Write data %02x\n", data);
> > -        dev->data_buf[dev->data_len++] = data;
> > +        if (dev->data_len >= sizeof(dev->data_buf)) {
> > +            BADF("Too many bytes sent\n");
> > +        } else {
> > +            dev->data_buf[dev->data_len++] = data;
> > +        }
> >           break;
> Reviewed-by: Peter Maydell <address@hidden>
>
> What happens on a real device in this situation ?

It's device specific.  Some devices (most eeproms, I suspect) will just keep
taking data, wrapping around when you hit the end of the memory. Some
devices (IPMI BMCs, generally) will ignore the extra data.  Some devices
may return a NAK on a byte if it gets too much data.  The specification
says:

   The slave device detects an invalid command or invalid data. In this
   case the slave
   device must NACK the received byte. The master upon detection of
   this condition
   must generate a STOP condition and retry the transaction.

So a NAK may be appropriate here, but it's kind of fuzzy.  Since generating
a NAK is more complicated, I would guess that most devices don't.

-corey


> thanks
> -- PMM