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* [PATCH] dmaengine: tegra210-adma: Replace zero-length array with flexible-array member
@ 2020-02-14 17:16 Gustavo A. R. Silva
  2020-02-19 12:19 ` Vinod Koul
  0 siblings, 1 reply; 2+ messages in thread
From: Gustavo A. R. Silva @ 2020-02-14 17:16 UTC (permalink / raw)
  To: Laxman Dewangan, Jon Hunter, Dan Williams, Vinod Koul, Thierry Reding
  Cc: dmaengine, linux-tegra, linux-kernel, 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]

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/dma/tegra210-adma.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/dma/tegra210-adma.c b/drivers/dma/tegra210-adma.c
index 6e1268552f74..c4ce5dfb149b 100644
--- a/drivers/dma/tegra210-adma.c
+++ b/drivers/dma/tegra210-adma.c
@@ -164,7 +164,7 @@ struct tegra_adma {
 	const struct tegra_adma_chip_data *cdata;
 
 	/* Last member of the structure */
-	struct tegra_adma_chan		channels[0];
+	struct tegra_adma_chan		channels[];
 };
 
 static inline void tdma_write(struct tegra_adma *tdma, u32 reg, u32 val)
-- 
2.25.0


^ permalink raw reply	[flat|nested] 2+ messages in thread

* Re: [PATCH] dmaengine: tegra210-adma: Replace zero-length array with flexible-array member
  2020-02-14 17:16 [PATCH] dmaengine: tegra210-adma: Replace zero-length array with flexible-array member Gustavo A. R. Silva
@ 2020-02-19 12:19 ` Vinod Koul
  0 siblings, 0 replies; 2+ messages in thread
From: Vinod Koul @ 2020-02-19 12:19 UTC (permalink / raw)
  To: Gustavo A. R. Silva
  Cc: Laxman Dewangan, Jon Hunter, Dan Williams, Thierry Reding,
	dmaengine, linux-tegra, linux-kernel

On 14-02-20, 11:16, Gustavo A. R. Silva wrote:
> 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]

Applied, thanks

-- 
~Vinod

^ permalink raw reply	[flat|nested] 2+ messages in thread

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2020-02-14 17:16 [PATCH] dmaengine: tegra210-adma: Replace zero-length array with flexible-array member Gustavo A. R. Silva
2020-02-19 12:19 ` Vinod Koul

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