linux-kernel - Re: [PATCH] usb: gadget: function: Fix returning incorrect PNP string

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Date:   Fri, 7 Jan 2022 10:37:19 +0800
From:   陳偉銘 <jj251510319013@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        balbi@...nel.org,
        Maciej Żenczykowski <maze@...gle.com>
Subject: Re: [PATCH] usb: gadget: function: Fix returning incorrect PNP string

Greg KH <gregkh@...uxfoundation.org> 於 2022年1月6日 週四 下午10:35寫道：
>
> On Wed, Jan 05, 2022 at 12:04:39PM +0800, Wei Ming Chen wrote:
> > There will be 2 leading bytes indicating the total length of
> > the PNP string, so I think we should add value by 2, otherwise
> > the PNP string copied to user will not contain the last 2 bytes
> >
> > Signed-off-by: Wei Ming Chen <jj251510319013@...il.com>
> > ---
> >  drivers/usb/gadget/function/f_printer.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/usb/gadget/function/f_printer.c b/drivers/usb/gadget/function/f_printer.c
> > index abec5c58f525..3fb00fd0b5ee 100644
> > --- a/drivers/usb/gadget/function/f_printer.c
> > +++ b/drivers/usb/gadget/function/f_printer.c
> > @@ -1005,9 +1005,11 @@ static int printer_func_setup(struct usb_function *f,
> >                               break;
> >                       }
> >                       value = strlen(dev->pnp_string);
> > +                     memcpy(buf + 2, dev->pnp_string, value);
> > +
> > +                     value += 2;
> >                       buf[0] = (value >> 8) & 0xFF;
> >                       buf[1] = value & 0xFF;
> > -                     memcpy(buf + 2, dev->pnp_string, value);
> >                       DBG(dev, "1284 PNP String: %x %s\n", value,
> >                           dev->pnp_string);
> >                       break;
> > --
> > 2.25.1
> >
>
> Are you sure this is correct?
>
> How is this related to this recent thread:
>         https://lore.kernel.org/all/CAKjGFBUdjXcZoVV4jdrgTz4rKThTfZAK4CqreKmBZ4KHE+K1GA@mail.gmail.com/#t
>
> your change is different from what is proposed there, why?

I didn’t notice this thread before I send this patch, I think the
concept of my change is similar to Volodymyr Lisivka’s change, he/she
introduced a separate variable for the PNP string length, I think it
may be clearer and more readable than just “value += 2”


Another thing that I am not too sure whether I am correct is this line of code

DBG(dev, "1284 PNP String: %x %s\n", value,
                             dev->pnp_string);

What Volodymyr Lisivka changed is like this

DBG(dev, "1284 PNP String: %x %s\n", pnp_length,
                             dev->pnp_string);

In my change, “value” equals to “pnp_length + 2” in Volodymyr
Lisivka’s change, and I think we should print “the PNP string length +
2” instead of “the PNP string length”?


>
> thanks,
>
> greg k-h

thanks,

Wei Ming Chen