linux-kernel - Re: [PATCH] iio: gyro: adis16136: use scnprintf instead of snprintf

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Message-ID: <CAHp75VddtJs1ZCk1XAZ2WJLhWQDcVwiiN5gDpK9oYPEOS=c_ZQ@mail.gmail.com>
Date:   Sun, 22 Mar 2020 12:27:27 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Rohit Sarkar <rohitsarkar5398@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...el.com>,
        linux-iio <linux-iio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jonathan Cameron <jic23@...nel.org>, dragos.bogdan@...log.com,
        Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Stefan Popa <stefan.popa@...log.com>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald <pmeerw@...erw.net>
Subject: Re: [PATCH] iio: gyro: adis16136: use scnprintf instead of snprintf

On Sun, Mar 22, 2020 at 8:11 AM Rohit Sarkar <rohitsarkar5398@...il.com> wrote:
>
> On Sun, Mar 22, 2020 at 02:25:42AM +0200, Andy Shevchenko wrote:
> > On Wed, Mar 18, 2020 at 08:25:22PM +0530, Rohit Sarkar wrote:
> > > scnprintf returns the actual number of bytes written into the buffer as
> > > opposed to snprintf which returns the number of bytes that would have
> > > been written if the buffer was big enough. Using the output of snprintf
> > > may lead to difficult to detect bugs.
> >
> > Nice. Have you investigate the code?
> >
> > > @@ -96,7 +96,7 @@ static ssize_t adis16136_show_serial(struct file *file,
> > >     if (ret)
> > >             return ret;
> > >
> > > -   len = snprintf(buf, sizeof(buf), "%.4x%.4x%.4x-%.4x\n", lot1, lot2,
> > > +   len = scnprintf(buf, sizeof(buf), "%.4x%.4x%.4x-%.4x\n", lot1, lot2,
> > >             lot3, serial);
> > >
> > >     return simple_read_from_buffer(userbuf, count, ppos, buf, len);
> >
> > The buffer size is 20, the pattern size I count to 19. Do you think snprintf()
> > can fail?
> That might be the case, but IMO using scnprintf can be considered as a
> best practice. There is no overhead with this change and further if the
> pattern is changed by someone in the future they might overlook the
> buffersize

If we cut the string above we will give wrong information to the user space.
I think scnprintf() change is a noise and does not improve the situation anyhow.

So, when anybody modifying such code the test should be performed.


-- 
With Best Regards,
Andy Shevchenko