linux-kernel - Re: [PATCH v3] media: si2168: Refactor command setup code

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Date:   Fri, 12 Jul 2019 14:45:37 -0300
From:   Mauro Carvalho Chehab <mchehab+samsung@...nel.org>
To:     Brad Love <brad@...tdimension.cc>
Cc:     Marc Gonzalez <marc.w.gonzalez@...e.fr>,
        Antti Palosaari <crope@....fi>,
        Jonathan Neuschäfer <j.neuschaefer@....net>,
        linux-media <linux-media@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        MSM <linux-arm-msm@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Subject: Re: [PATCH v3] media: si2168: Refactor command setup code

Em Fri, 12 Jul 2019 10:47:17 -0500
Brad Love <brad@...tdimension.cc> escreveu:

> Hi Marc,
> 
> Replying inline.
> 
> 
> On 04/07/2019 05.33, Marc Gonzalez wrote:
> > Refactor the command setup code, and let the compiler determine
> > the size of each command.
> >
> > Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@....net>
> > Signed-off-by: Marc Gonzalez <marc.w.gonzalez@...e.fr>
> > ---
> > Changes from v1:
> > - Use a real function to populate struct si2168_cmd *cmd, and a trivial
> > macro wrapping it (macro because sizeof).
> > Changes from v2:
> > - Fix header mess
> > - Add Jonathan's tag
> > ---
> >  drivers/media/dvb-frontends/si2168.c | 146 +++++++++------------------
> >  1 file changed, 45 insertions(+), 101 deletions(-)
> >
> > diff --git a/drivers/media/dvb-frontends/si2168.c b/drivers/media/dvb-frontends/si2168.c
> > index c64b360ce6b5..5e81e076369c 100644
> > --- a/drivers/media/dvb-frontends/si2168.c
> > +++ b/drivers/media/dvb-frontends/si2168.c
> > @@ -12,6 +12,16 @@
> >  
> >  static const struct dvb_frontend_ops si2168_ops;
> >  
> > +static void cmd_setup(struct si2168_cmd *cmd, char *args, int wlen, int rlen)
> > +{
> > +	memcpy(cmd->args, args, wlen);
> > +	cmd->wlen = wlen;
> > +	cmd->rlen = rlen;
> > +}
> > +  
> 
> 
> struct si2168_cmd.args is u8, not char. I also think const should apply
> to the pointer.
> 
> 
> > +#define CMD_SETUP(cmd, args, rlen) \
> > +	cmd_setup(cmd, args, sizeof(args) - 1, rlen)
> > +  
> 
> 
> This is only a valid helper if args is a null terminated string. It just
> so happens that every instance in this driver is, but that could be a
> silent pitfall if someone used a u8 array with this macro.

Actually, it is uglier than that. Of one writes something like:

	char buf[20];

	buf[0] = 0x20;
	buf[1] = 0x03;

	CMD_SETUP(cmd, buf, 0);

	// some other init, up to 5 values, then another CMD_SETUP()


sizeof() will evaluate to 20, and not to 2, with would be the
expected buffer size, and it will pass 18 random values.

IMHO, using sizeof() here is a very bad idea.

Regards,
Mauro