[PATCH v2] tty: n_hdlc: Use flexible-array member and struct_size() helper

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
presence of a "variable length array":

struct something {
    int length;
    u8 data[1];
};

struct something *instance;

instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
instance->length = size;
memcpy(instance->data, source, size);

There is also 0-byte arrays. Both cases pose confusion for things like
sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
to declare variable-length types such as the one above is a flexible array
member[2] which need to be the last member of a structure and empty-sized:

struct something {
        int stuff;
        u8 data[];
};

Also, by making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertenly introduced[3] to the codebase from now on.

Lastly, make use of the struct_size() helper to safely calculate the
allocation size for instances of struct n_hdlc_buf and avoid any potential
type mistakes[4][5].

[1] https://github.com/KSPP/linux/issues/21
[2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
[4] https://lore.kernel.org/lkml/60e14fb7-8596-e21c-f4be-546ce39e7bdb@embeddedor.com/
[5] commit 553d66cb1e86 ("iommu/vt-d: Use struct_size() helper")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
Changes in v2:
 - Make use of struct_size() helper.
 - Update subject and changelog text.

 drivers/tty/n_hdlc.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
index 98361acd3053..27b506bf03ce 100644
--- a/drivers/tty/n_hdlc.c
+++ b/drivers/tty/n_hdlc.c
@@ -115,11 +115,9 @@
 struct n_hdlc_buf {
 	struct list_head  list_item;
 	int		  count;
-	char		  buf[1];
+	char		  buf[];
 };
 
-#define	N_HDLC_BUF_SIZE	(sizeof(struct n_hdlc_buf) + maxframe)
-
 struct n_hdlc_buf_list {
 	struct list_head  list;
 	int		  count;
@@ -524,7 +522,8 @@ static void n_hdlc_tty_receive(struct tty_struct *tty, const __u8 *data,
 		/* no buffers in free list, attempt to allocate another rx buffer */
 		/* unless the maximum count has been reached */
 		if (n_hdlc->rx_buf_list.count < MAX_RX_BUF_COUNT)
-			buf = kmalloc(N_HDLC_BUF_SIZE, GFP_ATOMIC);
+			buf = kmalloc(struct_size(buf, buf, maxframe),
+				      GFP_ATOMIC);
 	}
 	
 	if (!buf) {
@@ -853,7 +852,7 @@ static struct n_hdlc *n_hdlc_alloc(void)
 
 	/* allocate free rx buffer list */
 	for(i=0;i<DEFAULT_RX_BUF_COUNT;i++) {
-		buf = kmalloc(N_HDLC_BUF_SIZE, GFP_KERNEL);
+		buf = kmalloc(struct_size(buf, buf, maxframe), GFP_KERNEL);
 		if (buf)
 			n_hdlc_buf_put(&n_hdlc->rx_free_buf_list,buf);
 		else if (debuglevel >= DEBUG_LEVEL_INFO)	
@@ -862,7 +861,7 @@ static struct n_hdlc *n_hdlc_alloc(void)
 	
 	/* allocate free tx buffer list */
 	for(i=0;i<DEFAULT_TX_BUF_COUNT;i++) {
-		buf = kmalloc(N_HDLC_BUF_SIZE, GFP_KERNEL);
+		buf = kmalloc(struct_size(buf, buf, maxframe), GFP_KERNEL);
 		if (buf)
 			n_hdlc_buf_put(&n_hdlc->tx_free_buf_list,buf);
 		else if (debuglevel >= DEBUG_LEVEL_INFO)	
-- 
2.23.0