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

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Date:   Wed, 4 Jul 2018 15:16:00 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     Tomas Winkler <tomas.winkler@...el.com>,
        Alexander Usyskin <alexander.usyskin@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...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.


regards,
dan carpenter