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Date:   Mon, 24 Feb 2020 18:06:47 -0600
From:   "Gustavo A. R. Silva" <gustavo@embeddedor.com>
To:     Solarflare linux maintainers <linux-net-drivers@solarflare.com>,
        Edward Cree <ecree@solarflare.com>,
        Martin Habets <mhabets@solarflare.com>,
        "David S. Miller" <davem@davemloft.net>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Subject: [PATCH][next] sfc: Replace zero-length array with flexible-array
 member
Message-ID: <20200225000647.GA17795@embeddedor>
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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@embeddedor.com>
---
 drivers/net/ethernet/sfc/falcon/net_driver.h | 2 +-
 drivers/net/ethernet/sfc/net_driver.h        | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/sfc/falcon/net_driver.h b/drivers/net/ethernet/sfc/falcon/net_driver.h
index a49ea2e719b6..a529ff395ead 100644
--- a/drivers/net/ethernet/sfc/falcon/net_driver.h
+++ b/drivers/net/ethernet/sfc/falcon/net_driver.h
@@ -288,7 +288,7 @@ struct ef4_rx_buffer {
 struct ef4_rx_page_state {
 	dma_addr_t dma_addr;
 
-	unsigned int __pad[0] ____cacheline_aligned;
+	unsigned int __pad[] ____cacheline_aligned;
 };
 
 /**
diff --git a/drivers/net/ethernet/sfc/net_driver.h b/drivers/net/ethernet/sfc/net_driver.h
index 9f9886f222c8..392bd5b7017e 100644
--- a/drivers/net/ethernet/sfc/net_driver.h
+++ b/drivers/net/ethernet/sfc/net_driver.h
@@ -336,7 +336,7 @@ struct efx_rx_buffer {
 struct efx_rx_page_state {
 	dma_addr_t dma_addr;
 
-	unsigned int __pad[0] ____cacheline_aligned;
+	unsigned int __pad[] ____cacheline_aligned;
 };
 
 /**
-- 
2.25.0