[PATCH] slab: Replace zero-length array with flexible-array

From: "Gustavo A. R. Silva" <gustavoars@kernel.org>
To: Andrew Morton <akpm@linux-foundation.org>,
	Christoph Lameter <cl@linux.com>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: [PATCH] slab: Replace zero-length array with flexible-array
Date: Thu, 7 May 2020 14:22:04 -0500	[thread overview]
Message-ID: <20200507192204.GA16270@embeddedor> (raw)

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

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

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 mm/slab.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/slab.h b/mm/slab.h
index 207c83ef6e06..815e4e9a94cd 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[];
 };
 
 /*

