tty: serial: pch_uart: Use scnprintf() for avoiding potential buffer overflow

Since snprintf() returns the would-be-output size instead of the
actual output size, the succeeding calls may go beyond the given
buffer limit.  Fix it by replacing with scnprintf().
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20200311092930.24433-1-tiwai@suse.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/pch_uart.c

@@ -310,32 +310,32 @@ static ssize_t port_show_regs(struct file *file, char __user *user_buf,
 	if (!buf)
 		return 0;
 
-	len += snprintf(buf + len, PCH_REGS_BUFSIZE - len,
+	len += scnprintf(buf + len, PCH_REGS_BUFSIZE - len,
 			"PCH EG20T port[%d] regs:\n", priv->port.line);
 
-	len += snprintf(buf + len, PCH_REGS_BUFSIZE - len,
+	len += scnprintf(buf + len, PCH_REGS_BUFSIZE - len,
 			"=================================\n");
-	len += snprintf(buf + len, PCH_REGS_BUFSIZE - len,
+	len += scnprintf(buf + len, PCH_REGS_BUFSIZE - len,
 			"IER: \t0x%02x\n", ioread8(priv->membase + UART_IER));
-	len += snprintf(buf + len, PCH_REGS_BUFSIZE - len,
+	len += scnprintf(buf + len, PCH_REGS_BUFSIZE - len,
 			"IIR: \t0x%02x\n", ioread8(priv->membase + UART_IIR));
-	len += snprintf(buf + len, PCH_REGS_BUFSIZE - len,
+	len += scnprintf(buf + len, PCH_REGS_BUFSIZE - len,
 			"LCR: \t0x%02x\n", ioread8(priv->membase + UART_LCR));
-	len += snprintf(buf + len, PCH_REGS_BUFSIZE - len,
+	len += scnprintf(buf + len, PCH_REGS_BUFSIZE - len,
 			"MCR: \t0x%02x\n", ioread8(priv->membase + UART_MCR));
-	len += snprintf(buf + len, PCH_REGS_BUFSIZE - len,
+	len += scnprintf(buf + len, PCH_REGS_BUFSIZE - len,
 			"LSR: \t0x%02x\n", ioread8(priv->membase + UART_LSR));
-	len += snprintf(buf + len, PCH_REGS_BUFSIZE - len,
+	len += scnprintf(buf + len, PCH_REGS_BUFSIZE - len,
 			"MSR: \t0x%02x\n", ioread8(priv->membase + UART_MSR));
-	len += snprintf(buf + len, PCH_REGS_BUFSIZE - len,
+	len += scnprintf(buf + len, PCH_REGS_BUFSIZE - len,
 			"BRCSR: \t0x%02x\n",
 			ioread8(priv->membase + PCH_UART_BRCSR));
 
 	lcr = ioread8(priv->membase + UART_LCR);
 	iowrite8(PCH_UART_LCR_DLAB, priv->membase + UART_LCR);
-	len += snprintf(buf + len, PCH_REGS_BUFSIZE - len,
+	len += scnprintf(buf + len, PCH_REGS_BUFSIZE - len,
 			"DLL: \t0x%02x\n", ioread8(priv->membase + UART_DLL));
-	len += snprintf(buf + len, PCH_REGS_BUFSIZE - len,
+	len += scnprintf(buf + len, PCH_REGS_BUFSIZE - len,
 			"DLM: \t0x%02x\n", ioread8(priv->membase + UART_DLM));
 	iowrite8(lcr, priv->membase + UART_LCR);