[PATCH] dmaengine: Replace zero-length array with flexible-array

[Gustavo A. R. Silva]

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@xxxxxxxxxx>
---
 drivers/dma/milbeaut-hdmac.c |    2 +-
 drivers/dma/milbeaut-xdmac.c |    2 +-
 drivers/dma/moxart-dma.c     |    2 +-
 drivers/dma/ti/edma.c        |    2 +-
 drivers/dma/ti/k3-udma.c     |    2 +-
 drivers/dma/timb_dma.c       |    2 +-
 include/linux/dmaengine.h    |    4 ++--
 7 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/drivers/dma/milbeaut-hdmac.c b/drivers/dma/milbeaut-hdmac.c
index 8853d442430b..a8cfb59f6efe 100644
--- a/drivers/dma/milbeaut-hdmac.c
+++ b/drivers/dma/milbeaut-hdmac.c
@@ -77,7 +77,7 @@ struct milbeaut_hdmac_device {
 	struct dma_device ddev;
 	struct clk *clk;
 	void __iomem *reg_base;
-	struct milbeaut_hdmac_chan channels[0];
+	struct milbeaut_hdmac_chan channels[];
 };
 
 static struct milbeaut_hdmac_chan *
diff --git a/drivers/dma/milbeaut-xdmac.c b/drivers/dma/milbeaut-xdmac.c
index ab3d2f395378..85a597228fb0 100644
--- a/drivers/dma/milbeaut-xdmac.c
+++ b/drivers/dma/milbeaut-xdmac.c
@@ -74,7 +74,7 @@ struct milbeaut_xdmac_chan {
 struct milbeaut_xdmac_device {
 	struct dma_device ddev;
 	void __iomem *reg_base;
-	struct milbeaut_xdmac_chan channels[0];
+	struct milbeaut_xdmac_chan channels[];
 };
 
 static struct milbeaut_xdmac_chan *
diff --git a/drivers/dma/moxart-dma.c b/drivers/dma/moxart-dma.c
index e04499c1f27f..2a77fa319d78 100644
--- a/drivers/dma/moxart-dma.c
+++ b/drivers/dma/moxart-dma.c
@@ -127,7 +127,7 @@ struct moxart_desc {
 	unsigned int			dma_cycles;
 	struct virt_dma_desc		vd;
 	uint8_t				es;
-	struct moxart_sg		sg[0];
+	struct moxart_sg		sg[];
 };
 
 struct moxart_chan {
diff --git a/drivers/dma/ti/edma.c b/drivers/dma/ti/edma.c
index c4a5c170c1f9..35d81bd857f1 100644
--- a/drivers/dma/ti/edma.c
+++ b/drivers/dma/ti/edma.c
@@ -211,7 +211,7 @@ struct edma_desc {
 	u32				residue;
 	u32				residue_stat;
 
-	struct edma_pset		pset[0];
+	struct edma_pset		pset[];
 };
 
 struct edma_cc;
diff --git a/drivers/dma/ti/k3-udma.c b/drivers/dma/ti/k3-udma.c
index a9c0251adf1a..eb76d546931b 100644
--- a/drivers/dma/ti/k3-udma.c
+++ b/drivers/dma/ti/k3-udma.c
@@ -170,7 +170,7 @@ struct udma_desc {
 	void *metadata; /* pointer to provided metadata buffer (EPIP, PSdata) */
 
 	unsigned int hwdesc_count;
-	struct udma_hwdesc hwdesc[0];
+	struct udma_hwdesc hwdesc[];
 };
 
 enum udma_chan_state {
diff --git a/drivers/dma/timb_dma.c b/drivers/dma/timb_dma.c
index 39382694fdfc..68e48bf54d78 100644
--- a/drivers/dma/timb_dma.c
+++ b/drivers/dma/timb_dma.c
@@ -88,7 +88,7 @@ struct timb_dma {
 	struct dma_device	dma;
 	void __iomem		*membase;
 	struct tasklet_struct	tasklet;
-	struct timb_dma_chan	channels[0];
+	struct timb_dma_chan	channels[];
 };
 
 static struct device *chan2dev(struct dma_chan *chan)
diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index 21065c04c4ac..18b8813fafb2 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h
@@ -153,7 +153,7 @@ struct dma_interleaved_template {
 	bool dst_sgl;
 	size_t numf;
 	size_t frame_size;
-	struct data_chunk sgl[0];
+	struct data_chunk sgl[];
 };
 
 /**
@@ -537,7 +537,7 @@ struct dmaengine_unmap_data {
 	struct device *dev;
 	struct kref kref;
 	size_t len;
-	dma_addr_t addr[0];
+	dma_addr_t addr[];
 };
 
 struct dma_async_tx_descriptor;