Re: [PATCH 4/7] hw/ide/atapi: Be explicit that assigning to s->lcyl truncates

[Peter Maydell <peter.maydell@linaro.org>]

On Wed, 31 Jul 2024 at 15:47, Markus Armbruster <armbru@redhat.com> wrote:
>
> Peter Maydell <peter.maydell@linaro.org> writes:
>
> > In ide_atapi_cmd_reply_end() we calculate a 16-bit size, and then
> > assign its two halves to s->lcyl and s->hcyl like this:
> >
> >            s->lcyl = size;
> >            s->hcyl = size >> 8;
> >
> > Coverity warns that the first line here can overflow the
> > 8-bit s->lcyl variable. This is true, and in this case we're
> > deliberately only after the low 8 bits of the value. The
> > code is clearer to both humans and Coverity if we're explicit
> > that we only wanted the low 8 bits, though.
> >
>
> Resolves?

Whoops.

Resolves: Coverity CID 1547621

> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> > ---
> >  hw/ide/atapi.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/hw/ide/atapi.c b/hw/ide/atapi.c
> > index fcb6cca1573..e82959dc2d3 100644
> > --- a/hw/ide/atapi.c
> > +++ b/hw/ide/atapi.c
> > @@ -265,7 +265,7 @@ void ide_atapi_cmd_reply_end(IDEState *s)
>                /* a new transfer is needed */
>                s->nsector = (s->nsector & ~7) | ATAPI_INT_REASON_IO;
>                ide_bus_set_irq(s->bus);
>                byte_count_limit = atapi_byte_count_limit(s);
>                trace_ide_atapi_cmd_reply_end_bcl(s, byte_count_limit);
>                size = s->packet_transfer_size;
>                if (size > byte_count_limit) {
>                    /* byte count limit must be even if this case */
>                    if (byte_count_limit & 1)
> >                      byte_count_limit--;
> >                  size = byte_count_limit;
> >              }
> > -            s->lcyl = size;
> > +            s->lcyl = size & 0xff;
> >              s->hcyl = size >> 8;
> >              s->elementary_transfer_size = size;
> >              /* we cannot transmit more than one sector at a time */
>
> @size is int.  I wonder why it's fine with size >> 8...  ah,
> atapi_byte_count_limit() returns uint16_t, and Coverity is smart enough
> to data-flow this via @byte_count_limit into @size.
>
> Reviewed-by: Markus Armbruster <armbru@redhat.com>

thanks
-- PMM