linux-kernel - RE: [PATCH] mei: bus: type promotion bug in mei_nfc_if

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Date:   Wed, 4 Jul 2018 13:57:44 +0000
From:   "Winkler, Tomas" <tomas.winkler@...el.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        Julia Lawall <julia.lawall@...6.fr>
CC:     "Usyskin, Alexander" <alexander.usyskin@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>
Subject: RE: [PATCH] mei: bus: type promotion bug in mei_nfc_if_version()

> 
> On Wed, Jul 04, 2018 at 01:59:14PM +0200, Julia Lawall wrote:
> >
> >
> > On Wed, 4 Jul 2018, Dan Carpenter wrote:
> >
> > > We accidentally removed the check for negative returns without
> > > considering the issue of type promotion.  The "if_version_length"
> > > variable is type size_t so if __mei_cl_recv() returns a negative
> > > then "bytes_recv" is type promoted to a high positive value and
> > > treated as success.
> > >
> > > Fixes: 582ab27a063a ("mei: bus: fix received data size check in NFC
> > > fixup")
> > > Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> > >
> > > diff --git a/drivers/misc/mei/bus-fixup.c
> > > b/drivers/misc/mei/bus-fixup.c index 0208c4b027c5..fa0236a5e59a
> > > 100644
> > > --- a/drivers/misc/mei/bus-fixup.c
> > > +++ b/drivers/misc/mei/bus-fixup.c
> > > @@ -267,7 +267,7 @@ static int mei_nfc_if_version(struct mei_cl *cl,
> > >
> > >  	ret = 0;
> > >  	bytes_recv = __mei_cl_recv(cl, (u8 *)reply, if_version_length, 0);
> > > -	if (bytes_recv < if_version_length) {
> > > +	if (bytes_recv < 0 || bytes_recv < if_version_length) {
> >
> > Is this preferred to adding an int cast?
> 
> I don't think it matters.  I kind of like explicitly testing for negative but
> maybe later people will just remove the check like we did here?  You could
> do it a bunch of different ways:
> 
> 1: if (ret < 0 || ret < ARRAY_SIZE(xxx))
> 2: if (ret < (int)ARRAY_SIZE(xxx))
> 3: if (ret != ARRAY_SIZE(xxx))
> 
> They're all equivalent.  I guess I don't like casting too much.  My first
> approach to fixing this was just to declare if_version_length as an int, but
> then I saw that originally there was a "bytes_recv < 0"
> check and decided to go that way instead.

Actually bytes_recv should be probably of ssize_t type,  so could be the if_version_length. 

How did you find this, I haven't seen it in reported by sparse, smatch and I believe -Wsign-compare is suppressed in compilation warnings.
Thanks
Tomas