net/sunrpc: Fix return value for sysctl sunrpc.transports

mainline inclusion
from mainline-5.10-rc1
commit c09f56b8
category: bugfix
bugzilla: 51817
CVE: NA

-------------------------------------------------
Fix returning value for sysctl sunrpc.transports.
Return error code from sysctl proc_handler function proc_do_xprt instead of number of the written bytes.
Otherwise sysctl returns random garbage for this key.

Since v1:
- Handle negative returned value from memory_read_from_buffer as an error
Signed-off-by: NArtur Molchanov <arturmolchanov@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>
(cherry picked from commit c09f56b8)
Signed-off-by: NBaisong Zhong <zhongbaisong@huawei.com>
Reviewed-by: NYue Haibing <yuehaibing@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

net/sunrpc/sysctl.c

@@ -69,7 +69,13 @@ static int proc_do_xprt(struct ctl_table *table, int write,
 		return 0;
 	}
 	len = svc_print_xprts(tmpbuf, sizeof(tmpbuf));
-	return simple_read_from_buffer(buffer, *lenp, ppos, tmpbuf, len);
+	*lenp = simple_read_from_buffer(buffer, *lenp, ppos, tmpbuf, len);
+
+	if (*lenp < 0) {
+		*lenp = 0;
+		return -EINVAL;
+	}
+	return 0;
 }
 
 static int