linux-kernel - Re: [PATCH 2/2] firmware: arm_scmi: Fix pointers arithmetic in Base protocol

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Date:   Tue, 31 May 2022 12:36:07 +0100
From:   Cristian Marussi <cristian.marussi@....com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH 2/2] firmware: arm_scmi: Fix pointers arithmetic in Base
 protocol

On Tue, May 31, 2022 at 12:02:53PM +0100, Robin Murphy wrote:
> On 2022-05-30 12:52, Cristian Marussi wrote:
> > Fix a possible undefined behaviour involving pointer arithmetic in Base
> > protocol scmi_base_implementation_list_get().
> > 
> > cppcheck complains with:
> > 
> > drivers/firmware/arm_scmi/base.c:190:19: warning: 't->rx.buf' is of type 'void *'. When using void pointers in calculations, the behaviour is undefined. [arithOperationsOnVoidPointer]
> >   list = t->rx.buf + sizeof(*num_ret);
> 
> Except we use GNU C, where it is well-defined[1]. We use void pointer
> arithmetic *all over* Linux, so there really isn't any valid argument that
> it could be problematic. If this was a common SCMI library intended to be
> portable then the patch would seem more reasonable, but in Linux-specific
> driver code it's just pointless churn.
> 

Hi Robin,

thanks for the correction, I'll drop this.

Thanks,
Cristian

> Cheers,
> Robin.
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html
> 
> > Fixes: b6f20ff8bd94 ("firmware: arm_scmi: add common infrastructure and support for base protocol")
> > Cc: Sudeep Holla <sudeep.holla@....com>
> > Signed-off-by: Cristian Marussi <cristian.marussi@....com>
> > ---
> >   drivers/firmware/arm_scmi/base.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/firmware/arm_scmi/base.c b/drivers/firmware/arm_scmi/base.c
> > index 20fba7370f4e..6d6214d9e68c 100644
> > --- a/drivers/firmware/arm_scmi/base.c
> > +++ b/drivers/firmware/arm_scmi/base.c
> > @@ -187,7 +187,7 @@ scmi_base_implementation_list_get(const struct scmi_protocol_handle *ph,
> >   	num_skip = t->tx.buf;
> >   	num_ret = t->rx.buf;
> > -	list = t->rx.buf + sizeof(*num_ret);
> > +	list = ((u8 *)t->rx.buf) + sizeof(*num_ret);
> >   	do {
> >   		size_t real_list_sz;