linux-kernel - [PATCH] ceph: cache: Replace zero-length array with flexible-array member

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Message-ID: <20200213160004.GA4334@embeddedor>
Date:   Thu, 13 Feb 2020 10:00:04 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Jeff Layton <jlayton@...nel.org>, Sage Weil <sage@...hat.com>,
        Ilya Dryomov <idryomov@...il.com>
Cc:     ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] ceph: cache: 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/ceph/cache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ceph/cache.c b/fs/ceph/cache.c
index 270b769607a2..2f5cb6bc78e1 100644
--- a/fs/ceph/cache.c
+++ b/fs/ceph/cache.c
@@ -32,7 +32,7 @@ struct ceph_fscache_entry {
 	size_t uniq_len;
 	/* The following members must be last */
 	struct ceph_fsid fsid;
-	char uniquifier[0];
+	char uniquifier[];
 };
 
 static const struct fscache_cookie_def ceph_fscache_fsid_object_def = {
-- 
2.25.0