[bug report] net/sunrpc: Fix return value for sysctl sunrpc.transports

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Artur Molchanov,

The patch c09f56b8f68d: "net/sunrpc: Fix return value for sysctl
sunrpc.transports" from Oct 12, 2020, leads to the following static
checker warning:

	net/sunrpc/sysctl.c:75 proc_do_xprt()
	warn: unsigned '*lenp' is never less than zero.

net/sunrpc/sysctl.c
    62  static int proc_do_xprt(struct ctl_table *table, int write,
    63                          void *buffer, size_t *lenp, loff_t *ppos)
    64  {
    65          char tmpbuf[256];
    66          size_t len;
    67  
    68          if ((*ppos && !write) || !*lenp) {
                              ^^^^^^^
It's weird that we don't just return -EINVAL for writes or something.

    69                  *lenp = 0;
    70                  return 0;
    71          }
    72          len = svc_print_xprts(tmpbuf, sizeof(tmpbuf));
    73          *lenp = memory_read_from_buffer(buffer, *lenp, ppos, tmpbuf, len);
    74  
    75          if (*lenp < 0) {

"*lenp" is unsigned so it can't be less than zero.  memory_read_from_buffer()
only returns an error if ppos is negative but that's impossible because
this is a proc file so negatives are prevented in rw_verify_area().

In other words this bug can't affect runtime.

    76                  *lenp = 0;
    77                  return -EINVAL;
    78          }
    79          return 0;
    80  }

regards,
dan carpenter