[PATCH net-next] net: sched: cls_u32: Use flexible-array member

[PATCH net-next] net: sched: cls_u32: Use flexible-array member

[Gustavo A. R. Silva]

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
unadvertenly introduced[3] to the codebase from now on.

[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")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
 net/sched/cls_u32.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index a0e6fac613de..5537b5b72ad5 100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c
@@ -79,7 +79,7 @@ struct tc_u_hnode {
 	/* The 'ht' field MUST be the last field in structure to allow for
 	 * more entries allocated at end of structure.
 	 */
-	struct tc_u_knode __rcu	*ht[1];
+	struct tc_u_knode __rcu	*ht[];
 };
 
 struct tc_u_common {
-- 
2.23.0

Re: [PATCH net-next] net: sched: cls_u32: Use flexible-array member

[Cong Wang]

On Thu, Jan 16, 2020 at 1:36 PM Gustavo A. R. Silva
<gustavo@embeddedor.com> wrote:
>
> 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);

Hmm, your patch is not correct, for u32 it is length+1, so you have
to allocate size+1 after you switch to zero-length array.

Take a look at u32_walk() if you have any doubt.

Thanks.