netdev - [PATCH][next] xdp: Replace zero-length array with flexible-array member

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Message-ID: <20200228131907.GA17911@embeddedor>
Date:   Fri, 28 Feb 2020 07:19:07 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Björn Töpel <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] xdp: 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>
---
 net/xdp/xsk_queue.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
index 89a01ac4e079..b50bb5c76da5 100644
--- a/net/xdp/xsk_queue.h
+++ b/net/xdp/xsk_queue.h
@@ -19,13 +19,13 @@ struct xdp_ring {
 /* Used for the RX and TX queues for packets */
 struct xdp_rxtx_ring {
 	struct xdp_ring ptrs;
-	struct xdp_desc desc[0] ____cacheline_aligned_in_smp;
+	struct xdp_desc desc[] ____cacheline_aligned_in_smp;
 };
 
 /* Used for the fill and completion queues for buffers */
 struct xdp_umem_ring {
 	struct xdp_ring ptrs;
-	u64 desc[0] ____cacheline_aligned_in_smp;
+	u64 desc[] ____cacheline_aligned_in_smp;
 };
 
 struct xsk_queue {
-- 
2.25.0