[PATCH] powernv/opal-prd: Silence memcpy() run-time false positive warnings

[Mahesh J Salgaonkar]

On 2023-07-05 11:06:46 Wed, Jordan Niethe wrote:
> 
> 
> On 26/6/23 5:04 pm, Mahesh Salgaonkar wrote:
> > opal_prd_msg_notifier extracts the opal prd message size from the message
> > header and uses it for allocating opal_prd_msg_queue_item that includes
> > the correct message size to be copied. However, while running under
> > CONFIG_FORTIFY_SOURCE=y, it triggers following run-time warning:
> > 
> > [ 6458.234352] memcpy: detected field-spanning write (size 32) of single field "&item->msg" at arch/powerpc/platforms/powernv/opal-prd.c:355 (size 4)
> > [ 6458.234390] WARNING: CPU: 9 PID: 660 at arch/powerpc/platforms/powernv/opal-prd.c:355 opal_prd_msg_notifier+0x174/0x188 [opal_prd]
> > [...]
> > [ 6458.234709] NIP [c00800000e0c0e6c] opal_prd_msg_notifier+0x174/0x188 [opal_prd]
> > [ 6458.234723] LR [c00800000e0c0e68] opal_prd_msg_notifier+0x170/0x188 [opal_prd]
> > [ 6458.234736] Call Trace:
> > [ 6458.234742] [c0000002acb23c10] [c00800000e0c0e68] opal_prd_msg_notifier+0x170/0x188 [opal_prd] (unreliable)
> > [ 6458.234759] [c0000002acb23ca0] [c00000000019ccc0] notifier_call_chain+0xc0/0x1b0
> > [ 6458.234774] [c0000002acb23d00] [c00000000019ceac] atomic_notifier_call_chain+0x2c/0x40
> > [ 6458.234788] [c0000002acb23d20] [c0000000000d69b4] opal_message_notify+0xf4/0x2c0
> > [...]
> > 
> > Add a flexible array member to avoid false positive run-time warning.
> > 
> > Reported-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
> > Signed-off-by: Mahesh Salgaonkar <mahesh at linux.ibm.com>
> > ---
> >   arch/powerpc/platforms/powernv/opal-prd.c |    7 +++++--
> >   1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/platforms/powernv/opal-prd.c b/arch/powerpc/platforms/powernv/opal-prd.c
> > index 113bdb151f687..9e2c4775f75f5 100644
> > --- a/arch/powerpc/platforms/powernv/opal-prd.c
> > +++ b/arch/powerpc/platforms/powernv/opal-prd.c
> > @@ -30,7 +30,10 @@
> >    */
> >   struct opal_prd_msg_queue_item {
> >   	struct list_head		list;
> > -	struct opal_prd_msg_header	msg;
> > +	union {
> > +		struct opal_prd_msg_header	msg;
> > +		DECLARE_FLEX_ARRAY(__u8, msg_flex);
> > +	};
> >   };
> >   static struct device_node *prd_node;
> > @@ -352,7 +355,7 @@ static int opal_prd_msg_notifier(struct notifier_block *nb,
> >   	if (!item)
> >   		return -ENOMEM;
> > -	memcpy(&item->msg, msg->params, msg_size);
> > +	memcpy(&item->msg_flex, msg->params, msg_size);
> 
> This does silence the warning but it seems like we might be able to go a
> little further.
> 
> What about not adding that flex array to struct opal_prd_msg_queue_item, but
> adding one to struct opal_prd_msg. That is what the data format actually is.
> 
> So we'd have something like:
> 
> 
>     struct opal_prd_msg  {
> 
>         struct opal_prd_msg_header hdr;
> 
>         char msg[];
> 
>     }
> 
> 
> and change things to use that instead?
> 
> But that might be more trouble than it is worth,

Yup, it would cause more changes to the code elsewhere as well.

> alternatively we can just
> do:
> 
> 	item->msg = *hdr;
> 	memcpy((char *)item->msg + sizeof(*hdr), (char *)hdr + sizeof(*hdr),
> msg_size - sizeof(*hdr));

Agree, this helps, I tried with small change to above code which
also works fine.

	item->msg = *hdr;
	hdr++;
	memcpy((char *)&item->msg + sizeof(*hdr), hdr, msg_size - sizeof(*hdr));

Will send out v2.

Thanks,
-Mahesh.