Re: [PATCH 01/] firewire-net: Use kmalloc_array() in fwnet_broadcast_start()

Stefan Richter
2016-09-24T11:49:26Z

On Sep 18 SF Markus Elfring wrote:
> From: Markus Elfring <elfring@users.sourceforge.net>
> Date: Sat, 17 Sep 2016 21:55:42 +0200
>
> * A multiplication for the size determination of a memory allocation
> indicated that an array data structure should be processed.
> Thus use the corresponding function "kmalloc_array".
>
> This issue was detected by using the Coccinelle software.
>
> * Replace the specification of a data type by a pointer dereference
> to make the corresponding size determination a bit safer according to
> the Linux coding style convention.
>
> Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
> ---
> drivers/firewire/net.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/firewire/net.c b/drivers/firewire/net.c
> index 309311b..7911f13 100644
> --- a/drivers/firewire/net.c
> +++ b/drivers/firewire/net.c
> @@ -1103,8 +1103,7 @@ static int fwnet_broadcast_start(struct fwnet_device *dev)
>
> max_receive = 1U << (dev->card->max_receive + 1);
> num_packets = (FWNET_ISO_PAGE_COUNT * PAGE_SIZE) / max_receive;
> -
> - ptrptr = kmalloc(sizeof(void *) * num_packets, GFP_KERNEL);
> + ptrptr = kmalloc_array(num_packets, sizeof(*ptrptr), GFP_KERNEL);
> if (!ptrptr) {
> retval = -ENOMEM;
> goto failed;

Coccinelle enabled you to determine that kmalloc_array /could/ be used
here. But whether it /should/ be used here is another question, and it is
not addressed in your changelog. (You state that there is an "issue" but
do not explain.)

kmalloc_array is a kmalloc wrapper which adds an inline check for integer
overflow. So, can sizeof(void *) * num_packets ever overflow size_t?

If yes,
do we want a runtime check here (which kmalloc_array provides),
or do we want a compile-time check?

If no,
then the remaining benefit of the patch is that it is more obvious
to the reader that dev->broadcast_rcv_buffer_ptrs is an array,
but possibly at the cost of superfluous code. Is gcc's optimizer
able to resolve kmalloc_array's check at compile time as always
false, such that the superfluous code is eliminated as dead code?

I believe I know answers to this but prefer to hear what you as the patch
author think about it.
--
Stefan Richter
-======----- =--= ==---