Re: [PULL 19/92] hw/char/serial: Assert serial_ioport_read/write offset fits 8 bytes

[Paolo Bonzini]

On 18/11/20 16:40, Peter Maydell wrote:
> On Thu, 24 Sep 2020 at 10:40, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > From: Philippe Mathieu-Daudé <f4bug@amsat.org>
> >
> > The serial device has 8 registers, each 8-bit. The MemoryRegionOps
> > 'serial_io_ops' is initialized with max_access_size=1, and all
> > memory_region_init_io() callers correctly set the region size to
> > 8 bytes:
> > - serial_io_realize
> > - serial_isa_realizefn
> > - serial_pci_realize
> > - multi_serial_pci_realize
> >
> > It is safe to assert the offset argument of serial_ioport_read()
> > and serial_ioport_write() is always less than 8.
> >
> > Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> > Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> > Message-Id: <20200907015535.827885-2-f4bug@amsat.org>
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> >   hw/char/serial.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/hw/char/serial.c b/hw/char/serial.c
> > index fd80ae5592..840da89de7 100644
> > --- a/hw/char/serial.c
> > +++ b/hw/char/serial.c
> > @@ -344,7 +344,7 @@ static void serial_ioport_write(void *opaque, hwaddr addr, 
> > uint64_t val,
> >   {
> >       SerialState *s = opaque;
> >
> > -    addr &= 7;
> > +    assert(size == 1 && addr < 8);
> >       trace_serial_ioport_write(addr, val);
> >       switch(addr) {
> >       default:
>
> Bug report https://bugs.launchpad.net/qemu/+bug/1904331
> points out that the addition of this assert() makes obvious
> that either the assert is wrong or some code later in the
> function which is looking at size must be dead:
>              if (size == 1) {
>                  s->divider = (s->divider & 0xff00) | val;
>              } else {
>                  s->divider = val;
>              }
>
> Presumably it's the if() that should be fixed ?

It can be dropped, because serial_io_ops has

    .impl = {
        .min_access_size = 1,
        .max_access_size = 1,
    },

Therefore, a 16-bit write to addr==0 is automatically split into an 
8-byte write to addr==0 and one to addr=1.  Together, the two set the 
full 16 bits of s->divider.

Thanks,

Paolo