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From:   Qianli Zhao <zhaoqianligood@gmail.com>
To:     cl@linux.com, penberg@kernel.org, rientjes@google.com,
        iamjoonsoo.kim@lge.com, akpm@linux-foundation.org
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com, zhaoqianli@xiaomi.com
Subject: [PATCH] mm: slab: Avoid the use of one-element array and use struct_size() helper
Date:   Wed, 29 Jul 2020 22:50:14 +0800
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From: Qianli Zhao <zhaoqianli@xiaomi.com>

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].

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://github.com/KSPP/linux/issues/21

Signed-off-by: Qianli Zhao <zhaoqianli@xiaomi.com>
---
 mm/slab.h        | 2 +-
 mm/slab_common.c | 7 ++-----
 2 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index 74f7e09..c12fb65 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -34,7 +34,7 @@ struct kmem_cache {
 
 struct memcg_cache_array {
 	struct rcu_head rcu;
-	struct kmem_cache *entries[0];
+	struct kmem_cache *entries[];
 };
 
 /*
diff --git a/mm/slab_common.c b/mm/slab_common.c
index fe8b684..56f4818 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -166,9 +166,7 @@ static int init_memcg_params(struct kmem_cache *s,
 	if (!memcg_nr_cache_ids)
 		return 0;
 
-	arr = kvzalloc(sizeof(struct memcg_cache_array) +
-		       memcg_nr_cache_ids * sizeof(void *),
-		       GFP_KERNEL);
+	arr = kvzalloc(struct_size(arr, entries, memcg_nr_cache_ids), GFP_KERNEL);
 	if (!arr)
 		return -ENOMEM;
 
@@ -199,8 +197,7 @@ static int update_memcg_params(struct kmem_cache *s, int new_array_size)
 {
 	struct memcg_cache_array *old, *new;
 
-	new = kvzalloc(sizeof(struct memcg_cache_array) +
-		       new_array_size * sizeof(void *), GFP_KERNEL);
+	new = kvzalloc(struct_size(new, entries, new_array_size), GFP_KERNEL);
 	if (!new)
 		return -ENOMEM;
 
-- 
2.7.4