[PATCH v3] [next] pcmcia: synclink_cs: replace 1-element array with flex-array member

[Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>]

One-element arrays are deprecated, and we are replacing them with
flexible array members instead. So, replace one-element array with
flexible-array member in struct RXBUF and refactor the rest of the code
accordingly. While at it, fix an edge case which could cause
rx_buf_count to be 0 when max_frame_size was set to the maximum
allowed value (65535).

It's worth mentioning that struct RXBUF was allocating 1 byte "too much"
for what is required (ignoring bytes added by padding).

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

Link: https://github.com/KSPP/linux/issues/79
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
---
Changelog:

- v3:
  fix size calculation mistakes using overflow.h macros: (Req: Andy
  Shevchenko, Kees Cook)
  add notes struct RXBUF size (Kees Cook)

- v2: removed changes to how the size of RXBUF was calculated. I
  changed my mind after thinking about the existing padding in the
  struct. Happy to discuss it if anyone sees it differently.

- v1: https://lore.kernel.org/lkml/Y5mMWEtHWKOiPVU+@mail.google.com/
---
 drivers/char/pcmcia/synclink_cs.c | 33 +++++++++++++++++++------------
 1 file changed, 20 insertions(+), 13 deletions(-)

diff --git a/drivers/char/pcmcia/synclink_cs.c b/drivers/char/pcmcia/synclink_cs.c
index b2735be81ab2..eee6772a0978 100644
--- a/drivers/char/pcmcia/synclink_cs.c
+++ b/drivers/char/pcmcia/synclink_cs.c
@@ -105,7 +105,7 @@ static MGSL_PARAMS default_params = {
 typedef struct {
 	int count;
 	unsigned char status;
-	char data[1];
+	char data[];
 } RXBUF;
 
 /* The queue of BH actions to be performed */
@@ -229,12 +229,18 @@ typedef struct _mgslpc_info {
 } MGSLPC_INFO;
 
 #define MGSLPC_MAGIC 0x5402
+#define MGSLPC_MAX_FRAME_SIZE 65535
+#define MGSLPC_MIN_FRAME_SIZE 4096
 
 /*
  * The size of the serial xmit buffer is 1 page, or 4096 bytes
  */
 #define TXBUFSIZE 4096
 
+/*
+ * RXBUF accommodates at least 1 buffer (header+data) of MGSLPC_MAX_FRAME_SIZE
+ */
+#define RXBUF_MAX_SIZE (sizeof(RXBUF) + MGSLPC_MAX_FRAME_SIZE)
 
 #define CHA     0x00   /* channel A offset */
 #define CHB     0x40   /* channel B offset */
@@ -529,7 +535,7 @@ static int mgslpc_probe(struct pcmcia_device *link)
 	tty_port_init(&info->port);
 	info->port.ops = &mgslpc_port_ops;
 	INIT_WORK(&info->task, bh_handler);
-	info->max_frame_size = 4096;
+	info->max_frame_size = MGSLPC_MIN_FRAME_SIZE;
 	init_waitqueue_head(&info->status_event_wait_q);
 	init_waitqueue_head(&info->event_wait_q);
 	spin_lock_init(&info->lock);
@@ -2611,19 +2617,20 @@ static int mgslpc_proc_show(struct seq_file *m, void *v)
 static int rx_alloc_buffers(MGSLPC_INFO *info)
 {
 	/* each buffer has header and data */
-	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
+	if (check_add_overflow(sizeof(RXBUF), info->max_frame_size, &info->rx_buf_size))
+		return -EINVAL;
 
-	/* calculate total allocation size for 8 buffers */
-	info->rx_buf_total_size = info->rx_buf_size * 8;
+	/* try to alloc as many buffers that can fit within RXBUF_MAX_SIZE (up to 8) */
+	if (check_mul_overflow(info->rx_buf_size, 8, &info->rx_buf_total_size))
+		return -EINVAL;
 
-	/* limit total allocated memory */
-	if (info->rx_buf_total_size > 0x10000)
-		info->rx_buf_total_size = 0x10000;
+	if (info->rx_buf_total_size > RXBUF_MAX_SIZE)
+		info->rx_buf_total_size = RXBUF_MAX_SIZE;
 
 	/* calculate number of buffers */
 	info->rx_buf_count = info->rx_buf_total_size / info->rx_buf_size;
 
-	info->rx_buf = kmalloc(info->rx_buf_total_size, GFP_KERNEL);
+	info->rx_buf = kcalloc(info->rx_buf_count, info->rx_buf_size, GFP_KERNEL);
 	if (info->rx_buf == NULL)
 		return -ENOMEM;
 
@@ -2695,10 +2702,10 @@ static int mgslpc_add_device(MGSLPC_INFO *info)
 		current_dev->next_device = info;
 	}
 
-	if (info->max_frame_size < 4096)
-		info->max_frame_size = 4096;
-	else if (info->max_frame_size > 65535)
-		info->max_frame_size = 65535;
+	if (info->max_frame_size < MGSLPC_MIN_FRAME_SIZE)
+		info->max_frame_size = MGSLPC_MIN_FRAME_SIZE;
+	else if (info->max_frame_size > MGSLPC_MAX_FRAME_SIZE)
+		info->max_frame_size = MGSLPC_MAX_FRAME_SIZE;
 
 	printk("SyncLink PC Card %s:IO=%04X IRQ=%d\n",
 		info->device_name, info->io_base, info->irq_level);
-- 
2.38.1