Subject: [PATCH] bpf: Replace zero-length array with flexible-array member

From: "Gustavo A. R. Silva" <[email-masked]>
Date: Wed, 26 Feb 2020 18:17:44 -0600

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]

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 <[email-masked]>
---
 include/linux/bpf-cgroup.h  | 2 +-
 include/linux/bpf.h         | 2 +-
 include/uapi/linux/bpf.h    | 2 +-
 kernel/bpf/bpf_struct_ops.c | 2 +-
 kernel/bpf/hashtab.c        | 2 +-
 kernel/bpf/lpm_trie.c       | 2 +-
 6 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
index a11d5b7dbbf3..a7cd5c7a2509 100644
--- a/include/linux/bpf-cgroup.h
+++ b/include/linux/bpf-cgroup.h
@@ -36,7 +36,7 @@ struct bpf_cgroup_storage_map;
 
 struct bpf_storage_buffer {
 	struct rcu_head rcu;
-	char data[0];
+	char data[];
 };
 
 struct bpf_cgroup_storage {
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 1acd5bf70350..9aa33b8f3d55 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -859,7 +859,7 @@ struct bpf_prog_array_item {
 
 struct bpf_prog_array {
 	struct rcu_head rcu;
-	struct bpf_prog_array_item items[0];
+	struct bpf_prog_array_item items[];
 };
 
 struct bpf_prog_array *bpf_prog_array_alloc(u32 prog_cnt, gfp_t flags);
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 906e9f2752db..8e98ced0963b 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -73,7 +73,7 @@ struct bpf_insn {
 /* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */
 struct bpf_lpm_trie_key {
 	__u32	prefixlen;	/* up to 32 for AF_INET, 128 for AF_INET6 */
-	__u8	data[0];	/* Arbitrary size */
+	__u8	data[];	/* Arbitrary size */
 };
 
 struct bpf_cgroup_storage_key {
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 042f95534f86..c498f0fffb40 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -23,7 +23,7 @@ enum bpf_struct_ops_state {
 
 struct bpf_struct_ops_value {
 	BPF_STRUCT_OPS_COMMON_VALUE;
-	char data[0] ____cacheline_aligned_in_smp;
+	char data[] ____cacheline_aligned_in_smp;
 };
 
 struct bpf_struct_ops_map {
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 53d9483fee10..d541c8486c95 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -118,7 +118,7 @@ struct htab_elem {
 		struct bpf_lru_node lru_node;
 	};
 	u32 hash;
-	char key[0] __aligned(8);
+	char key[] __aligned(8);
 };
 
 static inline bool htab_is_prealloc(const struct bpf_htab *htab)
diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
index 3b3c420bc8ed..65c236cf341e 100644
--- a/kernel/bpf/lpm_trie.c
+++ b/kernel/bpf/lpm_trie.c
@@ -25,7 +25,7 @@ struct lpm_trie_node {
 	struct lpm_trie_node __rcu	*child[2];
 	u32				prefixlen;
 	u32				flags;
-	u8				data[0];
+	u8				data[];
 };
 
 struct lpm_trie {
-- 
2.25.0