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Subject[PATCH][next] net/sched: cls_u32: Replace one-element array with flexible-array member
There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code according to the use of a flexible-array member in
struct tc_u_hnode and use the struct_size() helper to calculate the
size for the allocations. Commit 5778d39d070b ("net_sched: fix struct
tc_u_hnode layout in u32") makes it clear that the code is expected to
dynamically allocate divisor + 1 entries for ->ht[] in tc_uhnode. Also,
based on other observations, as the piece of code below:

1232 for (h = 0; h <= ht->divisor; h++) {
1233 for (n = rtnl_dereference(ht->ht[h]);
1234 n;
1235 n = rtnl_dereference(n->next)) {
1236 if (tc_skip_hw(n->flags))
1237 continue;
1238
1239 err = u32_reoffload_knode(tp, n, add, cb,
1240 cb_priv, extack);
1241 if (err)
1242 return err;
1243 }
1244 }

we can assume that, in general, the code is actually expecting to allocate
that extra space for the one-element array in tc_uhnode, everytime it
allocates memory for instances of tc_uhnode or tc_u_common structures.
That's the reason for passing '1' as the last argument for struct_size()
in the allocation for _root_ht_ and _tp_c_, and 'divisor + 1' in the
allocation code for _ht_.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arrays

Tested-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/5f7062af.z3T9tn9yIPv6h5Ny%25lkp@intel.com/
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
net/sched/cls_u32.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index 7b69ab1993ba..54209a18d7fe 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 {
@@ -353,7 +353,7 @@ static int u32_init(struct tcf_proto *tp)
void *key = tc_u_common_ptr(tp);
struct tc_u_common *tp_c = tc_u_common_find(key);

- root_ht = kzalloc(sizeof(*root_ht), GFP_KERNEL);
+ root_ht = kzalloc(struct_size(root_ht, ht, 1), GFP_KERNEL);
if (root_ht == NULL)
return -ENOBUFS;

@@ -364,7 +364,7 @@ static int u32_init(struct tcf_proto *tp)
idr_init(&root_ht->handle_idr);

if (tp_c == NULL) {
- tp_c = kzalloc(sizeof(*tp_c), GFP_KERNEL);
+ tp_c = kzalloc(struct_size(tp_c, hlist->ht, 1), GFP_KERNEL);
if (tp_c == NULL) {
kfree(root_ht);
return -ENOBUFS;
@@ -933,7 +933,7 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
NL_SET_ERR_MSG_MOD(extack, "Divisor can only be used on a hash table");
return -EINVAL;
}
- ht = kzalloc(sizeof(*ht) + divisor*sizeof(void *), GFP_KERNEL);
+ ht = kzalloc(struct_size(ht, ht, divisor + 1), GFP_KERNEL);
if (ht == NULL)
return -ENOBUFS;
if (handle == 0) {
--
2.27.0
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