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@xxxxxxxxx> 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
