linux-kernel - [PATCH][next] nfs: Replace zero-length array with flexible-array member

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Message-ID: <20200309182442.GA5422@embeddedor>
Date:   Mon, 9 Mar 2020 13:24:42 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>
Cc:     linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] nfs: Replace zero-length array with flexible-array
 member

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 <gustavo@...eddedor.com>
---
 fs/nfs/dir.c      | 2 +-
 fs/nfs/nfs4proc.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index d4b839b6cf89..a551a30047f9 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -141,7 +141,7 @@ struct nfs_cache_array {
 	int size;
 	int eof_index;
 	u64 last_cookie;
-	struct nfs_cache_array_entry array[0];
+	struct nfs_cache_array_entry array[];
 };
 
 typedef int (*decode_dirent_t)(struct xdr_stream *, struct nfs_entry *, bool);
diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 69b7ab7a5815..cc0960558284 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -5550,7 +5550,7 @@ static int buf_to_pages_noslab(const void *buf, size_t buflen,
 struct nfs4_cached_acl {
 	int cached;
 	size_t len;
-	char data[0];
+	char data[];
 };
 
 static void nfs4_set_cached_acl(struct inode *inode, struct nfs4_cached_acl *acl)
-- 
2.25.0