lkml.org 
[lkml]   [2020]   [Mar]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH] iio: gyro: adis16136: use scnprintf instead of snprintf
On Mon, Mar 23, 2020 at 03:04:23PM +0000, David Laight wrote:
> From: Andy Shevchenko
> > Sent: 22 March 2020 10:27
> > On Sun, Mar 22, 2020 at 8:11 AM Rohit Sarkar <rohitsarkar5398@gmail.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.
>
> If, for any reason, any of the values are large the user will get
> corrupt data.

> But you don't want to leak random kernel memory to the user.

How? Kernel already got crashed at this point.

>
> So while you may be able to prove that this particular snprintf()
> can't overflow, in general checking it requires knowledge of the code.

Here it's still a noise.

> With scnprintf() you know nothing odd will happen.

...and quite likely hide a lot of issues.

Really any "micro" / "small" correction / optimization to be very carefully
thought through.
> FWIW I suspect the 'standard' return value from snprintf() comes
> from the return value of the original user-space implementations
> which faked-up a FILE structure on stack and just silently discarded
> the output bytes that wouldn't fit in the buffer (they'd usually
> by flushed to a real file).
> The original sprintf() just specified a very big length so the
> flush would never be requested.

--
With Best Regards,
Andy Shevchenko


\
 
 \ /
  Last update: 2020-03-23 17:07    [W:0.046 / U:1.256 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site