Re: [PATCH] tuners: tda8290: reduce stack usage with kasan

Joe Perches
2017-12-12T16:02:51Z

On Tue, 2017-12-12 at 15:21 +0100, Arnd Bergmann wrote:
> On Tue, Dec 12, 2017 at 1:45 PM, Mauro Carvalho Chehab
> <mchehab@kernel.org> wrote:
> > Em Tue, 12 Dec 2017 03:42:32 -0800
> > Joe Perches <joe@perches.com> escreveu:
> >
> > > > I actually thought about marking them 'const' here before sending
> > > > (without noticing the changelog text) and then ran into what must
> > > > have led me to drop the 'const' originally: tuner_i2c_xfer_send()
> > > > takes a non-const pointer. This can be fixed but it requires
> > > > an ugly cast:
> > >
> > > Casting away const is always a horrible hack.
> > >
> > > Until it could be changed, my preference would
> > > be to update the changelog and perhaps add to
> > > the changelog the reason why it can not be const
> > > as detailed below.
> > >
> > > ie: xfer_send and xfer_xend_recv both take a
> > > non-const unsigned char *
>
> Ok.
>
> > Perhaps, on a separate changeset, we could change I2C routines to
> > accept const unsigned char pointers. This is unrelated to tda8290
> > KASAN fixes. So, it should go via I2C tree, and, once accepted
> > there, we can change V4L2 drivers (and other drivers) accordingly.
>
> I don't see how that would work unfortunately. i2c_msg contains
> a pointer to the data, and that is used for both input and output,
> including arrays like
>
> struct i2c_msg msgs[] = {
> {
> .addr = dvo->slave_addr,
> .flags = 0,
> .len = 1,
> .buf = &addr,
> },
> {
> .addr = dvo->slave_addr,
> .flags = I2C_M_RD,
> .len = 1,
> .buf = val,
> }
> };
>
> that have one constant output pointer and one non-constant
> input pointer. We could add an anonymous union for 'buf'
> to make that two separate pointers, but that's barely any
> better than the cast, and it would break the named initializers
> in the example above, at least on older compilers. Adding
> a second pointer to i2c_msg would add a bit of bloat and
> also require tree-wide changes or ugly hacks.

Perhaps add something like

struct i2c_msg_set {
__u16 addr; /* slave address */
__u16 flags;
__u16 len; /* msg length */
const __u8 *buf; /* pointer to read-only msg data */
};

struct i2c_msg_get {
__u16 addr; /* slave address */
__u16 flags;
__u16 len; /* msg length */
__u8 *buf; /* pointer to writeable msg data */
};

to the uapi include and use that where appropriate
but where a write then read is done via a single
i2c_msg array, it's not really feasible either.

Probably better to avoid any churn and just mark
all these as static rather than static const.