linux-kernel - [PATCH] NFS: Replace zero-length array with flexible-array

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Date:   Thu, 7 May 2020 14:02:23 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>
Cc:     linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] NFS: Replace zero-length array with flexible-array

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@...nel.org>
---
 include/linux/nfs4.h    |    2 +-
 include/linux/nfs_xdr.h |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/nfs4.h b/include/linux/nfs4.h
index 82d8fb422092..7c4b63561035 100644
--- a/include/linux/nfs4.h
+++ b/include/linux/nfs4.h
@@ -38,7 +38,7 @@ struct nfs4_ace {

 struct nfs4_acl {
 	uint32_t	naces;
-	struct nfs4_ace	aces[0];
+	struct nfs4_ace	aces[];
 };

 #define NFS4_MAXLABELLEN	2048
diff --git a/include/linux/nfs_xdr.h b/include/linux/nfs_xdr.h
index 440230488025..6aad9b0a5062 100644
--- a/include/linux/nfs_xdr.h
+++ b/include/linux/nfs_xdr.h
@@ -1227,7 +1227,7 @@ struct nfs4_secinfo4 {

 struct nfs4_secinfo_flavors {
 	unsigned int		num_flavors;
-	struct nfs4_secinfo4	flavors[0];
+	struct nfs4_secinfo4	flavors[];
 };

 struct nfs4_secinfo_arg {