Re: [PATCH] Bluetooth: L2CAP: don't check for out-of-bounds value

[Dan Carpenter <dan.carpenter@linaro.org>]

On Fri, May 26, 2023 at 05:19:52PM +0200, Simon Horman wrote:
> On Fri, May 26, 2023 at 05:34:30PM +0300, Dan Carpenter wrote:
> > On Fri, May 26, 2023 at 04:16:54PM +0200, Simon Horman wrote:
> > > @@ -4161,8 +4161,12 @@ static struct l2cap_chan *l2cap_connect(struct l2cap_conn *conn,
> > >  
> > >  	result = L2CAP_CR_NO_MEM;
> > >  
> > > -	/* Check for valid dynamic CID range (as per Erratum 3253) */
> > > -	if (scid < L2CAP_CID_DYN_START || scid > L2CAP_CID_DYN_END) {
> > > +	/* Check for valid dynamic CID range (as per Erratum 3253).
> > > +	 * As scid is an unsigned 16bit variable it's maximum
> > > +	 * value is L2CAP_CID_DYN_END (0xffff): there is no need to check
> > > +	 * if scid exceeds that value here.
> > > +	 */
> > > +	if (scid < L2CAP_CID_DYN_START) {
> > 
> > This is a false positive.  To me the warning looks reasonable.  But one
> > way we could silence it would be to keep a list of macros where the
> > check is impossible but we still want to have it.
> 
> Hi Dan,
> 
> I do agree that the existing code is harmless.
> Is this why you feel it is a false positive?
> 

Actually I was thinking of something else, but the other reason why this
is harmless is because it's part of a "clamp both upper and lower
bounds" condition.  Linus doesn't like these warnings because it's clear
to a human reader what the intent is. I re-wrote this code last week to
avoid this kind of warning.  I will push that so now it won't warn.  I
still need to tweak the re-written code a bit.

> > I could create something where we do:
> > 
> > echo "L2CAP_CID_DYN_END" >> smatch_data/kernel.allowed_impossible_limits
> > 
> > I'd do the same for unsigned comparisons with zero like:
> > 
> > 
> >         if (dpmcp_dev->obj_desc.ver_major < DPMCP_MIN_VER_MAJOR ||
> >             (dpmcp_dev->obj_desc.ver_major == DPMCP_MIN_VER_MAJOR &&
> >              dpmcp_dev->obj_desc.ver_minor < DPMCP_MIN_VER_MINOR)) {
> >              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >                 dev_err(&dpmcp_dev->dev,
> >                         "ERROR: Version %d.%d of DPMCP not supported.\n",
> > 
> > echo "DPMCP_MIN_VER_MINOR" >> smatch_data/kernel.allowed_impossible_limits
> 
> FWIIW, I've noticed problems with comparisons to enums. Which, f.e., may in
> practice are unsigned values of a particular width for a given build.
> But in theory could be any type.

Enum types are undefined in C but I think Sparse (and thus Smatch)
follow the same rules as GCC basically so they should catch these bugs.

	int x;

	if (x < ENUM_ZERO)  <-- x is negative but type promoted to uint

regards,
dan carpenter