dmaengine Archive on lore.kernel.org
 help / color / Atom feed
* [PATCH] dmaengine: at_hdmac: Replace zero-length array with flexible-array
@ 2020-05-07 19:00 Gustavo A. R. Silva
  2020-05-11  5:47 ` Ludovic Desroches
  2020-05-13 14:56 ` Vinod Koul
  0 siblings, 2 replies; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-05-07 19:00 UTC (permalink / raw)
  To: Ludovic Desroches; +Cc: linux-arm-kernel, dmaengine, linux-kernel

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@kernel.org>
---
 drivers/dma/at_hdmac_regs.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/dma/at_hdmac_regs.h b/drivers/dma/at_hdmac_regs.h
index 397692e937b3..80fc2fe8c77e 100644
--- a/drivers/dma/at_hdmac_regs.h
+++ b/drivers/dma/at_hdmac_regs.h
@@ -331,7 +331,7 @@ struct at_dma {
 	struct dma_pool		*dma_desc_pool;
 	struct dma_pool		*memset_pool;
 	/* AT THE END channels table */
-	struct at_dma_chan	chan[0];
+	struct at_dma_chan	chan[];
 };
 
 #define	dma_readl(atdma, name) \


^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [PATCH] dmaengine: at_hdmac: Replace zero-length array with flexible-array
  2020-05-07 19:00 [PATCH] dmaengine: at_hdmac: Replace zero-length array with flexible-array Gustavo A. R. Silva
@ 2020-05-11  5:47 ` Ludovic Desroches
  2020-05-13 14:56 ` Vinod Koul
  1 sibling, 0 replies; 3+ messages in thread
From: Ludovic Desroches @ 2020-05-11  5:47 UTC (permalink / raw)
  To: Gustavo A. R. Silva; +Cc: linux-arm-kernel, dmaengine, linux-kernel

On Thu, May 07, 2020 at 02:00:38PM -0500, Gustavo A. R. Silva wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> 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@kernel.org>
Acked-by: Ludovic Desroches<ludovic.desroches@microchip.com>

Thanks
> ---
>  drivers/dma/at_hdmac_regs.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma/at_hdmac_regs.h b/drivers/dma/at_hdmac_regs.h
> index 397692e937b3..80fc2fe8c77e 100644
> --- a/drivers/dma/at_hdmac_regs.h
> +++ b/drivers/dma/at_hdmac_regs.h
> @@ -331,7 +331,7 @@ struct at_dma {
>         struct dma_pool         *dma_desc_pool;
>         struct dma_pool         *memset_pool;
>         /* AT THE END channels table */
> -       struct at_dma_chan      chan[0];
> +       struct at_dma_chan      chan[];
>  };
> 
>  #define        dma_readl(atdma, name) \
> 

^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [PATCH] dmaengine: at_hdmac: Replace zero-length array with flexible-array
  2020-05-07 19:00 [PATCH] dmaengine: at_hdmac: Replace zero-length array with flexible-array Gustavo A. R. Silva
  2020-05-11  5:47 ` Ludovic Desroches
@ 2020-05-13 14:56 ` Vinod Koul
  1 sibling, 0 replies; 3+ messages in thread
From: Vinod Koul @ 2020-05-13 14:56 UTC (permalink / raw)
  To: Gustavo A. R. Silva
  Cc: Ludovic Desroches, linux-arm-kernel, dmaengine, linux-kernel

On 07-05-20, 14:00, 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]
> 
> 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")

Applied, thanks

-- 
~Vinod

^ permalink raw reply	[flat|nested] 3+ messages in thread

end of thread, back to index

Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-05-07 19:00 [PATCH] dmaengine: at_hdmac: Replace zero-length array with flexible-array Gustavo A. R. Silva
2020-05-11  5:47 ` Ludovic Desroches
2020-05-13 14:56 ` Vinod Koul

dmaengine Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/dmaengine/0 dmaengine/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 dmaengine dmaengine/ https://lore.kernel.org/dmaengine \
		dmaengine@vger.kernel.org
	public-inbox-index dmaengine

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.dmaengine


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git